U.S. patent application number 12/188258 was filed with the patent office on 2008-12-04 for femtosecond laser pulse surface structuring methods and materials resulting therefrom.
This patent application is currently assigned to UNIVERSITY OF ROCHESTER. Invention is credited to Chunlei Guo, Anatoliy Y. Vorobyev.
Application Number | 20080299408 12/188258 |
Document ID | / |
Family ID | 40088609 |
Filed Date | 2008-12-04 |
United States Patent
Application |
20080299408 |
Kind Code |
A1 |
Guo; Chunlei ; et
al. |
December 4, 2008 |
Femtosecond Laser Pulse Surface Structuring Methods and Materials
Resulting Therefrom
Abstract
Embodiments of the present invention are generally directed to
materials processing methods using femtosecond duration laser
pulses, and to the altered materials obtained by such methods. The
resulting nanostructured (with or without macro- and
micro-structuring) materials have a variety of applications,
including, for example, aesthetic applications for jewelry or
ornamentation; biomedical applications related to biocompatibility;
catalysis applications; and modification of, for example, the
optical and hydrophilic properties of materials including selective
coloring.
Inventors: |
Guo; Chunlei; (Rochester,
NY) ; Vorobyev; Anatoliy Y.; (St. Catharines,
CA) |
Correspondence
Address: |
BOND, SCHOENECK & KING, PLLC
10 BROWN ROAD, SUITE 201
ITHACA
NY
14850-1248
US
|
Assignee: |
UNIVERSITY OF ROCHESTER
Rochester
NY
|
Family ID: |
40088609 |
Appl. No.: |
12/188258 |
Filed: |
August 8, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11862449 |
Sep 27, 2007 |
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12188258 |
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60847916 |
Sep 29, 2006 |
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Current U.S.
Class: |
428/573 ;
219/121.72 |
Current CPC
Class: |
C21D 1/09 20130101; B23K
2103/52 20180801; B23K 26/0006 20130101; C22F 3/00 20130101; B23K
2103/14 20180801; B23K 26/355 20180801; B23K 26/362 20130101; Y10T
428/12201 20150115; B23K 2103/05 20180801; B82Y 40/00 20130101;
B23K 2103/50 20180801; C21D 8/0294 20130101; B23K 2103/08 20180801;
Y10T 428/12993 20150115; B23K 2103/42 20180801; B23K 26/361
20151001; B23K 2103/10 20180801; B23K 2103/54 20180801; B23K
2103/56 20180801; B23K 2103/12 20180801; B23K 2103/16 20180801;
B82Y 30/00 20130101; B23K 26/0624 20151001; B23K 26/352 20151001;
B44C 1/228 20130101 |
Class at
Publication: |
428/573 ;
219/121.72 |
International
Class: |
B32B 3/02 20060101
B32B003/02; B23K 26/38 20060101 B23K026/38 |
Goverment Interests
FEDERALLY SPONSORED RESEARCH
[0002] This invention was made with government support under
Contract No. CTS-042506 sponsored by the National Science
Foundation. The government may have certain rights in the
invention.
Claims
1. A surface-structured metal, comprising: a base metal having at
least one of a femtosecond laser pulse-induced nanostructured
surface and a nanostructure-covered laser-induced periodic surface
structure (NC-LIPSS).
2. The surface-structured metal of claim 1, wherein the
nanostructure-covered laser-induced periodic surface structure has
a period that is less than a period of a non-nanostructure-covered
laser-induced periodic surface structure (LIPSSs) in the metal
surface.
3. The surface-structured metal of claim 1, wherein the base metal
has a polished surface.
4. The surface-structured metal of claim 1, wherein the base metal
includes substantially any metal and metal alloy.
5. The surface-structured metal of claim 1, wherein the base metal
is one of aluminum, gold, titanium, copper, platinum, tungsten,
silver, titanium alloy, aluminum alloy, brass, and stainless
steel.
6. The surface-structured metal of claim 1, wherein the
surface-structured metal has a colored appearance.
7. The surface-structured metal of claim 6, wherein the
surface-structured metal has at least one of a black color, a blue
color, a gold color, a yellow color, a gray color, a red color, and
a combination thereof.
8. The surface-structured metal of claim 6, wherein the
surface-structured metal has an iridescence.
9. The surface-structured metal of claim 1, wherein the base metal
is aluminum and the color is black.
10. The surface-structured metal of claim 1, wherein the base metal
is aluminum and the color is gold.
11. The surface-structured metal of claim 1, wherein the base metal
is titanium and the color is blue.
12. The surface-structured metal of claim 1, wherein the base metal
is platinum and the color is gold.
13. The surface-structured metal of claim 6, having a color that
changes as a function of viewing angle.
14. The surface-structured metal of claim 6, having a color that is
substantially constant as a function of viewing angle.
15. The surface-structured metal of claim 1, having an absorptance
equal to or greater than 0.8 for a wavelength that is equal to or
greater than about 200 nanometers.
16. The surface-structured metal of claim 1, having an absorptance
equal to or greater than 0.2 for a wavelength that is equal to or
greater than about 50 micrometers.
17. The surface-structured metal of claim 1, having an absorptance
equal to or greater than 0.9.
18. The surface-structured metal of claim 1, having an absorptance
equal to or greater than 0.5, wherein the base metal has an
absorptance substantially less than 0.5 at normal incidence.
19. The surface-structured metal of claim 1, wherein the surface
structure further includes a plurality of microscale
aggregates.
20. The surface-structured metal of claim 19, wherein the
microscale aggregates further comprise at least one of micropores,
microgrooves, and microchannels.
21. The surface-structured metal of claim 19, wherein the surface
structure further includes a periodic plurality of elongate
grooves.
22. The surface-structured metal of claim 1, wherein the surface
structure comprises a plurality of nanobranches.
23. The surface-structured metal of claim 22, wherein the surface
structure further comprises a plurality of spherical
nanoparticles.
24. The surface-structured metal of claim 1, wherein the surface
structure comprises nanostructures having a shape of nanovoids.
25. The surface-structured metal of claim 1, wherein the surface
structure comprises redeposited nanoparticles of ablated base
metal.
26. The surface-structured metal of claim 1, wherein the base metal
is a bulk metal.
27. The surface-structured metal of claim 1, wherein the base metal
is a thin film.
28. A surface-structured metal having an induced absorptance that
is equal to or greater than 0.8 for a wavelength that is equal to
or greater than about 200 nanometers.
29. The surface-structured metal of claim 28, comprising an induced
nanostructure in a surface of the metal.
30. The surface-structured metal of claim 28, comprising
substantially any metal and metal alloy.
31. The surface-structured metal of claim 29, wherein the induced
nanostructure is a nanostructure-covered laser-induced periodic
surface structures (NC-LIPSS).
32. The surface-structured metal of claim 29, wherein the induced
nanostructure comprises a plurality of nanobranches.
33. The surface-structured metal of claim 32, wherein the induced
nanostructure further comprises a plurality of spherical
nanoparticles.
34. The surface-structured metal of claim 28, wherein the metal is
a bulk metal.
35. The surface-structured metal of claim 28, wherein the metal is
a thin film.
36. A method for treating a base metal, comprising: exposing a
surface region of the base metal to a femtosecond duration laser
pulse having a fluence, F, that is sufficient to alter a surface
structure of the base metal; and altering the surface structure of
the base metal by creating a nanostructure in the surface.
37. The method of claim 36, comprising exposing the surface region
of the base metal to a single femtosecond duration laser pulse.
38. The method of claim 36, comprising creating a plurality of
nanobranch structures.
39. The method of claim 38, further comprising creating a plurality
of spherical nanoparticle structures.
40. The method of claim 38, further comprising creating a plurality
of nanovoids.
41. The method of claim 38, further comprising creating surface
microstructures.
42. The method of claim 36, further comprising creating at least
one of micropores, arcuate microgrooves, and central
microchannels.
43. The method of claim 36, comprising creating a plurality of
nano-covered laser-induced periodic surface structures
(NC-LIPSSs).
44. The method of claim 43, further comprising creating a plurality
of surface macrostructures.
45. The method of claim 44, comprising scanning a femtosecond
duration pulsed laser beam across the surface of the base metal at
a selected velocity.
46. The method of claim 45, comprising creating a periodic
plurality of grooves.
47. The method of claim 46, wherein the periodic plurality of
grooves has a period equal to a translation step between two
adjacent laser scanning lines.
48. The method of claim 36, further comprising ablating material
from a region of the surface and redepositing the ablated material
on the surface.
49. The method of claim 37, comprising exposing the surface region
of the base metal to between 1-10 additional femtosecond duration
laser pulses.
50. The method of claim 49, comprising exposing the surface region
of the base metal to between 10-100 additional femtosecond duration
laser pulses.
51. The method of claim 50, comprising exposing the surface region
of the base metal to between 100-300 additional femtosecond
duration laser pulses.
52. The method of claim 51, comprising exposing the surface region
of the base metal to greater than 300 additional femtosecond
duration laser pulses.
53. The method of claim 44, wherein the NC-LIPSS has a period, d,
that is less than the laser wavelength, .lamda..
54. The method of claim 53, comprising controlling the period of
the NC-LIPSS by controlling at least one of .lamda., the incidence
angle, .theta., and the real part of the effective refractive index
of an air-metal interface, .eta., according to the equation
d=.lamda./(.eta..+-.sin .theta.).
55. The method of claim 36, comprising polishing the base metal
surface prior to exposing the surface to the femtosecond duration
laser pulse.
Description
RELATED APPLICATION DATA
[0001] This patent application is a continuation-in-part of, and
claims priority to, U.S. application Ser. No. 11/862,449 filed on
Sep. 27, 2007, as well as to U.S. Provisional Application Ser. No.
60/847,916, filed Sep. 29, 2006, the subject matters of which are
herein incorporated by reference in their entireties.
BACKGROUND
[0003] 1. Field of the Invention
[0004] Embodiments of the invention generally pertain to the field
of materials processing and associates processed materials. More
particularly, embodiments of the invention are directed to methods
for processing materials using femtosecond duration laser pulses,
applications of such methods, and materials and/or material
properties resulting from such methods. Even more particularly,
embodiments of the invention are directed to methods for altering
the surface structure of metal materials using femtosecond duration
laser pulses, applications of such methods, and materials and/or
material properties resulting from such methods, including
blackened and colored metals.
[0005] 2. Description of Related Art
[0006] Although materials may be shaped or otherwise altered in a
large variety of ways including milling, machining, grinding, etc.,
in recent years, laser-based alteration of materials has become a
common method for a variety of materials processing applications.
For example, laser alteration of materials by high energy laser
pulses has been used to both create precise hole patterns in metals
or metal films as well as for more subtle material alterations such
as texturing of metals or metal films by the intense
heating/melting/vaporization effects of such high energy laser
beams.
[0007] Reported methods of laser alteration of materials involves
the use of `short-duration` (i.e., nanosecond (ns) and picosecond
(ps)) laser pulses. See, for example, U.S. Pat. Nos. 5,635,089 and
4,972,061. U.S. Pat. No. 6,979,798 describes the use of laser
pulses of preferably less than 130 femtoseconds (fs) to
specifically burn metal links on integrated circuits. Thus the use
of ultrashort (ns) duration laser pulses for laser processing of
materials may achieve results that are different than those from
longer duration (i.e., `short`) laser pulses. The duration of a
nanosecond laser pulse is long enough for the pulse to interact
with the material as it is ejected from the surface. Ultra-short
duration, femtosecond (fs) laser pulses, by comparison, are not
long enough in duration to interact with the material ejected from
the surface of the irradiated substance, since the pulse ends long
before the hydrodynamic expansion of the ejected material. Another
difference between different laser pulse timescales is that the
laser-supported combustion and detonation waves that are commonly
generated in a nanosecond duration laser pulse do not occur in an
ultra-short fs laser pulse, again offering up the possibility of
materials processing effects and resulting material parameters that
may be difficult or impossible to obtain with longer duration laser
irradiation.
[0008] In light of the above observations, advantageous benefits
may be obtained from the use of ultra-short, femtosecond pulses in
the processing of certain materials and the altered materials or
material characteristics resulting from processing with one or more
fs laser pulses. Certain advantageous benefits may also be realized
by the ability to controllably modify optical properties of a
metal.
SUMMARY
[0009] Embodiments of the invention are directed to methods for
processing materials using femtosecond duration (i.e., 1-999 fs)
laser pulses, applications of such methods, and altered materials
and/or material properties resulting from such methods.
Particularly advantageous aspects of the invention are directed to
methods for altering the surface structure of metal and other
materials by application of one or more femtosecond duration laser
pulses, applications of such methods, and altered materials and/or
material properties resulting from such methods. According to
non-limiting aspects, methods for uniformly coloring, non-uniformly
coloring, and blackening a metal or other material, increasing a
material's absorptance (up to nearly 100%), increasing a material's
surface area, as well as altered materials exhibiting these
characteristics are disclosed.
[0010] As used herein, the term "colored metal" will mean a metal
having a color that is due to a femtosecond laser pulse(s)-induced
nanostructuring of the surface of the metal, but not due to
artificial or applied coloring, painting, dying, etching,
polishing, anodizing, or other non-femtosecond laser
pulse(s)-induced nanostructuring of the surface of the metal. The
term "color" will have its typical meaning as well as the practical
consideration that color is observable over the visible light
spectrum. In that regard, however, one skilled in the art will also
recognize the relationship between color and the
reflectance/absorptance of light by a material, where `white` would
correspond to high reflectivity and `black` to high absorptivity.
As further used herein, the term "base metal" will mean the metal
prior to femtosecond laser surface structuring as it is ready to
undergo femtosecond laser pulse(s)-induced nanostructuring of the
surface of the metal, in which case it may or may not be polished.
According to the embodiments of the invention, the absorptance of
the base metal will always be less than the corresponding
absorptance of the fs laser-treated material.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 is a graph that plots the absorptance of a gold (Au)
surface against number of fs laser pulses of varying fluence from a
Ti:sapphire laser having a central wavelength of 0.8 .mu.m and a
pulse duration of 60 fs, and further shows four regimes for
absorptance change: AB, BC, CD, and DE, according to an
illustrative embodiment of the invention;
[0012] FIGS. 2A, 2B are scanning electron micrograph (SEM) images
of a gold surface (a) before irradiation and (b) after one shot
from the laser described in the legend to FIG. 1, showing nanoscale
roughness (2B) corresponding to the region AB in FIG. 1, according
to an illustrative embodiment of the invention;
[0013] FIGS. 3A, 3B are SEM images of nanoscale surface structural
features produced on a gold surface (region BC of FIG. 1) from the
laser described in the legend to FIG. 1: (a) nanobranches after two
shot ablation; (b) spherical nanoparticles after five shot
ablation, according to an illustrative embodiment of the
invention;
[0014] FIGS. 4A, 4B are SEM images showing nanostructure-covered,
laser induced periodic surface structures (NC-LIPSS) in an
irradiated area of a sample after 20,000 shots at a fluence of
F=0.17 J/cm.sup.2 (region CD in FIG. 1) from the laser described in
the legend to FIG. 1: (a) SEM micrograph showing the period of the
NC-LIPSS; (b) nanobranches and supported spherical nanoparticles in
the NC-LIPSS, according to an illustrative embodiment of the
invention;
[0015] FIGS. 5A, 5B are SEM images showing (a) NC-LIPSS on the
periphery of the irradiated area and gold-black deposit outside the
irradiated area after 10,000 shots at a fluence of F=1.1 J/cm.sup.2
(region DE in FIG. 1) from the laser described in the legend to
FIG. 1; (b) gold-black deposit after 20,000 shots at F=0.17
J/cm.sup.2 (region CD in FIG. 1) consisting of spherical aggregates
with a mean diameter that decreases as the distance from the crater
increases, according to an illustrative embodiment of the
invention;
[0016] FIG. 6 is an SEM image of spherical nanoparticles in a
spherical aggregate of the gold-black deposit shown in FIG. 5;
[0017] FIGS. 7A, 7B are SEM images of (A) a crater produced by
5,000 shots at F=0.17 J/cm.sup.2; (B) SEM image of a crater
produced by 5,000 shots at F=1.1 J/cm.sup.2, according to an
illustrative embodiment of the invention;
[0018] FIG. 8 is a graph of the residual energy coefficients of
aluminum (Al) versus laser fluence following ablation with a single
55 ns pulse of a Nd:YAG laser at various ambient gas conditions,
presented for illustrative effect;
[0019] FIG. 9 is a graph of the residual energy coefficients for
aluminum versus laser fluence following ablation with a single 45
ns pulse of a ruby laser at various ambient gas conditions,
presented for illustrative effect;
[0020] FIGS. 10A, 10B are open-shutter photographs of plasmas
produced by 55 ns Nd:YAG laser pulses in 1 atm air and in vacuum at
(A) F=4.7 J/cm.sup.2 and (B) F=19.5 J/cm.sup.2, where the laser
beam is normally incident on the sample from the left (the white
dashed lines indicate the front surface of the sample), according
to an illustrative embodiment of the invention;
[0021] FIG. 11A is a graph of estimates of surface temperatures of
Al samples for a Nd:YAG laser pulse at F.sub.abl approximately
equal to F.sub.pl=1.4 J/cm.sup.2 in 1 atm air (solid line) and at
F.sub.abl approximately equal to F.sub.pl=2.7 J/cm.sup.2 in vacuum
at a base pressure of 0.01 torr (dotted line); FIG. 11B is a graph
of estimated surface temperatures of Al samples for a ruby laser
pulse at F.sub.abl approximately equal to F.sub.pl=1.1 J/cm.sup.2
in 1 atm air (solid line) and at F.sub.abl approximately equal to
F.sub.pl=2.1 J/cm.sup.2 in vacuum at a base pressure of 0.01 torr
(dotted line), according to an illustrative embodiment of the
invention;
[0022] FIG. 12 is a graph of the residual energy coefficients of Al
in air at various pressures versus laser fluence following single
pulse fs laser ablation using a Ti:sapphire laser producing 60 fs
pulses with a central wavelength of about 0.8 .mu.m at a base
vacuum pressure of about 0.01 torr, according to an illustrative
embodiment of the invention;
[0023] FIG. 13A is a SEM image of a mechanically polished Al
surface before laser irradiation; FIG. 13B is a SEM image of a
typical surface modification of the Al after 1 shot at
F=F.sub.abl=0.053 J/cm.sup.2 in 1 atm air using the fs laser
described in the legend to FIG. 12, according to an illustrative
embodiment of the invention;
[0024] FIG. 14 is a SEM images of the Al surface after 1 shot at
F=F.sub.pl=0.086 J/cm.sup.2 in 1 atm air using the fs laser
described in the legend to FIG. 12, showing the number and size of
spherical nanoparticles on the surface being greater than those at
F=F.sub.abl (i.e., than in FIG. 13(B)), according to an
illustrative embodiment of the invention;
[0025] FIG. 15 shows open shutter photographs of plasma produced by
a single fs laser pulse at F=1.16 J/cm.sup.2 in vacuum (pressure of
about 0.01 torr) using the laser described in the legend to FIG.
12, where the laser beam is normally incident on the target from
the left (the white dashed line indicates the front surface of the
sample), according to an illustrative embodiment of the
invention;
[0026] FIGS. 16(A-D) are SEM images of nanoscale structures in the
center of the irradiated spot on a copper sample following ablation
at F=0.35 J/cm.sup.2 using a Ti:sapphire laser with a central
wavelength of 0.8 .mu.m and a pulse duration of 65 fs: (A) sample
surface before irradiation; (B) a different area of the copper
surface after one shot ablation showing random fine nanostructures
in the form of nanoprotrusions, nanocavities, and nanorims; (C)
after two shot ablation; (D) after 1,000 shot ablation, according
to an illustrative embodiment of the invention;
[0027] FIGS. 17(A-D) show SEM images of the central part of the
irradiated spot on copper following ablation at F=1.52 J/cm.sup.2
using the laser described in the legend to FIG. 16: (A) surface
after one shot exhibiting random nanostructures in the form of
nanoprotrusions and nanocavities; (B) surface after two shot
ablation showing random nanostructures in the form of spherical
nanoprotrusions and nanocavities; (C) surface after 10 shots
showing both nano- and microstructures; (D) surface after 1,000
shots showing predominantly microstructures, according to an
illustrative embodiment of the invention;
[0028] FIG. 18 shows a SEM image of copper following two shot
ablation at F=9.6 J/cm.sup.2 using the laser described in the
legend to FIG. 16, showing only microstructures in the central area
and nanostructures on the periphery of the ablated spot; the insert
shows microstructural details in the central area;
[0029] FIG. 19 shows a summary graphic of the different types of
structural features observed under a SEM on a copper surface as a
function of laser fluence and number of shots, derived using the fs
duration laser pulses obtained from the laser described in the
legend to FIG. 16, according to an illustrative embodiment of the
invention;
[0030] FIG. 20A shows an image of a copper sample surface before
irradiation; FIG. 20B shows an image of nascent nanostructures
formed on copper by ablation at F=0.35 J/cm.sup.2 with a single
laser pulse using the laser described in the legend to FIG. 16,
according to an illustrative embodiment of the invention;
[0031] FIGS. 21A, 21B are SEM images showing the evolution of
nanostructure-covered, laser induced periodic surface structures
(NC-LIPSS) in the central area of the irradiated spot on a platinum
(Pt) sample at F=0.16 J/cm.sup.2 delivered from a Ti:sapphire laser
system that generates 65 fs pulses with a central wavelength of 0.8
.mu.m: (A) initial random nanoroughness formed after 10 shots (the
inset shows a detailed view of the nanoroughness); (B)
nanostructure-covered LIPSS after 30 shots (the inset shows a
detailed view);
[0032] FIGS. 22(A-D) show SEM images illustrating the formation of
NC-LIPSS in the peripheral area of the irradiated spot on Pt at
F=0.16 J/cm.sup.2 with 100 shots using the laser described in the
legend to FIG. 21: (A) general view of ablated spot; (B) the
magnified details show that LIPSS disappears in the central area;
(C) nanostructure-covered LIPSS with a period of 0.62 .mu.m in the
peripheral area; (D) further magnified detail of (C), according to
an illustrative embodiment of the invention;
[0033] FIG. 23 is a graph showing Atomic Force Microscopy (AFM)
measurements of the surface profile following mechanical polishing
and 10 laser shots using the laser described in the legend to FIG.
21, according to an illustrative embodiment of the invention;
[0034] FIG. 24 is a graph showing a NC-LIPSS profile measured with
AFM following 30 laser shots using the laser described in the
legend to FIG. 21, according to an illustrative embodiment of the
invention;
[0035] FIG. 25 is a SEM image showing nanostructure-covered LIPSS
with a period of 0.58 .mu.m in the central area of the irradiated
spot on Au after 100 shots at a fluence of F=0.16 J/cm.sup.2 using
the laser described in the legend to FIG. 21, according to an
illustrative embodiment of the invention;
[0036] FIGS. 26(A-D) are SEM images of nanoroughness on a titanium
(Ti) sample following fs laser treatment at near damage threshold
fluence of F=0.067 J/cm.sup.2 using a Ti:sapphire laser system that
generates 65 fs pulses with a central wavelength of 0.8 .mu.m: (A)
sample surface before irradiation; (B) nanoroughness after two shot
laser treatment; (C) after 10 shot treatment; (D) a magnified view
of a section in (B) showing fine surface nanostructures in the
forms of nanopores and nanoprotrusions typically of spherical
shape, according to an illustrative embodiment of the
invention;
[0037] FIGS. 27(A-D) show SEM images illustrating the
nanotopography of Ti following femtosecond laser treatment at
F=0.084 J/cm.sup.2 using the laser described in the legend to FIG.
26: (A) nanoroughness after one shot; (B) nanoroughness after two
shots; (C) a magnified view of a section in (A) showing fine
details of surface nanoroughness; (D) magnified view of a section
in (B) showing fine details of surface nanoroughness;
[0038] FIGS. 28(A-D) show SEM images illustrating fs laser produced
periodic surface patterns on Ti following laser treatment at
F=0.067 J/cm.sup.2 using the laser described in the legend to FIG.
26: (A) periodic surface pattern after 40 shots; (B) periodic
surface pattern after 100 shots; (C) periodic surface pattern after
400 shots; (D) a magnified view of a section in (C) showing fine
details of the periodic pattern covered with nanostructural
features, according to an illustrative embodiment of the
invention;
[0039] FIGS. 29(A-D) show SEM images illustrating fs laser produced
periodic surface patterns on Ti following laser treatment at
F=0.084 J/Cm.sup.2 using the laser described in the legend to FIG.
26: (A) periodic surface pattern after 20 shots; (B) periodic
surface pattern after 400 shots; (C) periodic surface pattern after
800 shots; (D) a magnified view of a section in (C) showing fine
details of the periodic pattern covered with nanostructural
features, according to an illustrative embodiment of the
invention;
[0040] FIGS. 30(A-D) show SEM images illustrating the surface nano-
and microtopography of Ti following fs laser treatment at F=0.16
J/cm.sup.2 using the laser described in the legend to FIG. 26: (a)
nanoroughness after one shot; (b) nano- and microroughness after 20
shots; (c) typical microroughness covered with nanostructures after
40 shot treatment; (d) typical columnar microstructure after 200
shot treatment, according to an illustrative embodiment of the
invention;
[0041] FIG. 31(A-D) show SEM images illustrating the surface
topography of Ti following fs laser treatment at F=0.35 J/cm.sup.2
using the laser described in the legend to FIG. 26: (a) nano and
microroughness after one shot laser treatment; (b) typical random
microroughness covered with nanostructures after 40 shot treatment;
(c) typical columnar microstructures after 100 shot treatment; (d)
typical columnar microstructures after 200 shot treatment,
according to an illustrative embodiment of the invention;
[0042] FIGS. 32(A-D) show SEM images illustrating the surface
topography of Ti following fs laser treatment at F=0.48 J/cm.sup.2
using the laser described in the legend to FIG. 26: (a)
microroughness covered with nanoroughness after 40 shots; (b)
typical microstructures following 70 shot treatment; (c) typical
microstructures following 100 shot treatment; (d) a crater with a
diameter of about 350 .mu.m after a 1,500 shot treatment, according
to an illustrative embodiment of the invention;
[0043] FIGS. 33(A-D) show SEM images illustrating the surface
topography of Ti following fs laser treatment at F=2.9 J/cm.sup.2
using the laser described in the legend to FIG. 26: (a) smooth
surface with microinhomogeneities after a one shot laser treatment;
(b) smooth surface with some nanostructures after two shots; (c) a
magnified view of a section in (b) showing surface nanostructures;
(d) nanotopography of a smooth surface following 4 shot treatment
with observable spherical nanostructures as small as about 10 nm,
according to an illustrative embodiment of the invention;
[0044] FIG. 34 shows a plot of % reflectance versus wavelength in
nm for polished Al (open circles); "black" Al (black diamonds; see
also FIG. 35(A)); grayed Al (gray circles; see also FIG. 35(B));
"golden" Al (gray squares; see also FIG. 35(C)); and, Al colored by
NC-LIPSS (open squares; see also FIG. 36), according to an
illustrative embodiment of the invention;
[0045] FIGS. 35(A-C) show photographs of metals processed to have
different optical properties: (A) black Al; (B) grayed Al with two
gray shades; (C) golden Al, according to an illustrative embodiment
of the invention; and
[0046] FIGS. 36A, 36B show photographs of Al colored by NC-LIPSS,
where the color of the samples depend upon the viewing angle due to
a grating effect, according to an illustrative embodiment of the
invention.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0047] Embodiments of the present invention are generally directed
to laser-based materials processing using one or more femtosecond
duration (i.e., 1-999.99 fs) laser pulses, and to the altered
materials obtained by such materials processing. As used herein,
the term `materials processing` and `surface treatment` refer to
altering the surface structure or restructuring the surface of the
material being processed by creating various nanostructures that
may or may not be created in combination with additional micro- and
macrostructures. Non-limiting examples of nanostructured surfaces
in accordance with embodiments and aspects of the invention are
shown in the figures and will be described in detail below.
[0048] Material alterations obtained in accord with embodiments of
the present invention may be defined by a variety of experimental
methods for analyzing the alterations obtained (synonymously "the
materials processing outcome(s)"); for example, by electron
micrographic analysis, by spectroscopic analysis (e.g., absorption
of light or other electromagnetic energy by the altered surface),
and via other techniques recognized in the art. Material
alterations may also be additionally and/or separately defined in
terms of theoretical modeling of alterations and the mechanisms by
which alterations are generated, e.g., by post-ablation
redeposition of material, by the formation of
nanostructure-covered, laser induced periodic surface structures
(NC-LIPSS), and others described herein and known in the art.
[0049] In this regard, the term "ablation" is used to refer to
material alterations generally, rather than to any specific process
of material alteration. Specifically, "ablation" is defined as
occurring by experimental observation, i.e., by the onset of
surface damage or alteration to the material being processed, where
the surface damage or alteration is typically observed by eye or by
SEM analysis (see, e.g., Example 2). Thus the term "ablation" is
generic, and is not used to refer to a specific physical process of
material alteration, for example, the specific physical process of
vaporization or other form of removal of material from a surface,
etc.
[0050] The structural material alterations described herein below
may be defined more precisely as, e.g., "nanostructures,"
"nanoscale structure," "nanoscale roughness," or "nanoroughness"
obtained by femtosecond laser pulse nanostructuring of the
material. Other alterations that may or may not occur in the
presence of nanostructuring include, without limitation,
"microstructures," "microscale structure," "microscale roughness,"
or "microroughness" obtained by microstructuring effects obtained
by femtosecond laser pulse nanostructuring of the material, and
"macrostructures," "macroscale structure," "macroscale roughness,"
or "macroroughness" such as craters or other features obtained by
macrostructuring effects obtained by femtosecond laser pulse
nanostructuring of the material.
[0051] With further regard to nanostructures, terms including but
not limited to "nanobranches," "nanoparticles," "nanoprotrusions,"
"nanocavities," "nanorims," "nanopores," nanospheres" are used to
describe nanoscale dimension alterations having the visual
appearances under SEM analysis of branches, particles, protrusions,
cavities, spheres, channels, etc. With regard to microstructures,
for example, "columnar microstructures" is used to refer to
microstructures that appear visually under SEM analysis as columns
(see, e.g., FIGS. 30 and 31; FIGS. 3, 6, and 26 show illustrative
examples of these different micro- and nanostructures).
[0052] Further with regard to the above terms, SEM analysis may be
used to establish quantitative as well as qualitative definitions
for macro-, micro-, and nanostructures, and these definitions may
be used to define the materials obtained by the materials
processing methods according to embodiments and aspects of the
invention.
[0053] In some non-limiting aspects of the invention, it may be
desirable to create essentially a single kind of materials
structuring, while in other non-limiting aspects it may be
advantageous to create `mixed` structuring. In this context the
word "dominated" is used herein to refer to a situation where one
type of structuring is prevalent, i.e., where one type of
structuring occurs across, e.g. about 80% or more of the surface
area of the surface produced by the specified materials processing
regime. In general, however, when the surface is "dominated" by
nanostructures, for example, it will be understood that other
percentage values are explicitly contemplated; i.e., 70, 71, 72,
73, 74, 75 . . . 97, 98, 99% (i.e., counting by 1% intervals) of
the surface area is of the structure specified.
[0054] Although the above visually-based terms are used herein to
classify nanostructuring, microstructuring, and macrostructuring
effects according to embodiments of the invention, other methods
can be used to categorize these structures. For example, because
the absorptance of a material is a function of the intrinsic
absorptance, A.sub.INTR, and the surface roughness, A.sub.SR,
alterations to a material that manifest as alterations in surface
roughness may be described by absorptance changes rather than, or
in addition to, descriptions of macro-, micro-, or nanostructural
changes based on changes in the visual appearance. Thus Example 1
below shows in detail how different femtosecond pulse duration
laser processing regimes alter absorptance, and how these
alterations in absorptance correlate with macro-, micro- and
nanostructural changes in the surface of the material.
[0055] Further with regard to absorptance, as discussed in Example
1 and particularly shown in FIG. 1, the materials processing
regimes of the embodiments of the invention are capable of
producing alterations to materials resulting in extremely high
absorptivity; e.g., absorptivity for gold of close to 100%. Such
high absorptivity may have particular utility in, e.g., heat
absorption applications (e.g., heat exchange and heat absorption
for hot water heating from solar energy, etc.). However, as FIG. 1
shows, other absorptance values may also be obtained. Thus the
present invention is directed to producing materials having
absorptance values from 0.01, 0.02, 0.03, 0.04, 0.05, . . . , 1.0
(counting by 0.01), where the resulting absorptance of the material
is significantly greater than before processing.
[0056] The absorptance values determined in Example 1 are measured
calorimetrically; however, absorptance may also be measured by
other means, and specifically by methods that allow absorptance to
be determined as a function of the wavelength of the light
impinging on the sample. Reflectivity may also be measured in
addition to, or in substitution for, absorptance, especially in
situations where it is desirable to produce a material with
favorably altered reflectivity. Reflectivity may be measured by any
standard method used for such determinations; examples of
reflectivity measurements are provided in, for example, U.S. Pat.
No. 4,972,061, the contents of which are incorporated herein by
reference in their entirety.
[0057] Thus in a non-limiting, exemplary aspect, materials
processing methods and resulting altered or treated materials are
directed to the field of jewelry. The surface of virtually any
metal or metal alloy such as, but not limited to, gold, platinum,
silver, stainless steel, various precious metals, decorative
metals, and others may be decorated, initialed, patterned, colored,
blackened, or otherwise marked via femtosecond laser surface
structuring so as to have, for example, altered reflectivity
ranging from the reflectivity of the unmarked metal down to
essentially 0% reflectivity, depending upon the desired
application. In various non-limiting aspects, reflectivity may vary
with wavelength (producing different colors) and/or viewing
angle.
[0058] According to the various embodiments described herein, the
materials to be altered by femtosecond laser surface structuring
include most generally all metals and alloys thereof, including,
but not limited to, gold, aluminum, copper, platinum, titanium,
tungsten, stainless steel, and others. The alteration of
semiconductor materials and dielectrics are contemplated. Also
contemplated are ceramic, glass, and plastic materials.
[0059] According to non-limiting aspects, metal materials are
intended to include metal films (e.g., thin metal layers coated on
glass, silicon or other additional underlying layer) and bulk
metals. Bulk metals refer to non-thin films of more than a few
hundred nm, particularly more than 1 .mu.m, and more particularly
to more than 10 .mu.m in thickness. Thus "bulk metals" refers to
metals with the characteristics just recited, whereas "thin films"
refers to metals of less than a few hundred nm, including the thin
films described in the Examples below.
[0060] Further with regard to the materials of the present
invention, as shown in the Examples and in, e.g., FIGS. 13-14,
there is evidence that various of the alterations of the materials
obtained by the embodied materials processing methods occur
preferentially on surface defects of the materials being irradiated
by the femtosecond laser pulse(s). Thus in some aspects, highly
polished materials will be used (pre-processing) in order to reduce
the preferential formation of material alterations at material
defects; in other situations, it may be advantageous to leave the
material unpolished, to roughen the material, or even to introduce
inhomogeneities or other "defects" into the material in order to
facilitate certain alterations.
[0061] Pulse duration is a function of the laser system used. In
various non-limiting embodiments, the laser system is a Ti:sapphire
laser system generating 65 fs duration pulses at a central
wavelength of 0.8 .mu.m; however, other laser systems generating
different fs pulse durations are also contemplated. See, e.g., U.S.
Pat. No. 6,979,798 and U.S. Publication No. 2006/0207976A1, the
contents of which are incorporated herein by reference in their
entireties, for non-limiting descriptions of other such fs duration
laser systems, e.g., a Yb-doped fiber laser such as the FPCA uJewel
(available from IMRA America, Ann Arbor Mich.). Other such fs
duration lasers may include, e.g., dye lasers, Cr:LiSAF lasers, KrF
lasers, and others known in the art.
[0062] In addition to laser pulse duration, a number of other laser
parameters may be varied in various aspects of the present
invention in order to obtain the desired materials processing
effects, including but not limited to: the polarization of the
laser beam (typically horizontally polarized); the diameter of the
spot of laser irradiation on the surface of the material sample
(typically between 100 and 1200 .mu.m); the wavelength of the laser
beam; the energy density, F (fluence), of the laser beam; the
number of laser pulses (shots) applied to the material sample; the
extent of overlap between multiple laser pulses (shots) applied to
the particular region of the material being processed; whether the
shots are applied in vacuum or under higher pressure conditions,
and others.
[0063] According to various non-limiting exemplary embodiments, the
fs laser has a central wavelength (lambda) of 0.8 .mu.m. However,
other wavelengths in the IR, visible, ultraviolet, infrared, THz
frequency, etc., may be advantageously used.
[0064] With regard to laser fluence on the surface of the material
to be processed, as will be discussed below, contemplated fluences
will be sufficient to alter the surface structure of the metal as
described herein and will be generally below about 25 J/cm.sup.2 at
the material surface; i.e., below about 25, 24, 23, 22, 21, 20, 19,
18, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, or 1
J/cm.sup.2. The exact choice of fluence varies, however, depending
upon the desired materials processing effects. Thus for example,
the summary graphic of FIG. 19 shows that different materials
properties may be obtained for fs laser irradiation using different
combinations of laser fluence and number of laser pulses.
[0065] Further with regard to fluence, in an exemplary embodiment
the choice of fluence is expressed by reference to the threshold
laser fluence (synonymously, the "ablation threshold" or F.sub.abl)
required for visible material surface damage under SEM. Thus as
described in the Examples, materials processing effects can be
calibrated to the ablation threshold, e.g., the fluence specified
to obtain a particular effect may be given both in absolute terms
of J/cm.sup.2 or, alternatively, may be given as a percentage of
the ablation threshold, i.e., as 1, 2, 3, 4, 5, . . . , 100, 101,
102, 103, 104, 105, . . . 10,000% (counting by ones) of
F.sub.abl.
[0066] With regard to laser pulses, embodiments of the present
invention may use single- and multi-pulse exposures of materials to
obtain desired materials processing effects. Laser "pulse" or
synonymously, "shot", refers to a single laser pulse applied to the
sample material using for example an electromechanical shutter to
select a single pulse. Multi-pulse or multi-shot situations involve
more than a single shot, e.g., 2, 3, 4, 5, 6, 7, 8, 9, 10, etc.
(counting by ones) up to thousands, tens-of-thousands, or
hundreds-of-thousands of shots. The exact number of pulses or shots
chosen will depend upon the desired materials processing outcome,
as shown in for example in the summary graphic of FIG. 19 and as
discussed below.
[0067] The extent of overlap between shots in a multi-shot
situation may be varied in order to obtained desired effects, e.g.,
by specifying that at least x % of the area of an additional shot
or shots overlap with the first or previous shot, where x can be 1
to 100% counting by ones (i.e., 1, 2, 3, 4, 5, . . . , 100%). Such
variations may be particularly important when, for example, the
portion of the material in the center of the irradiation by the
laser pulse or pulses undergoes different alterations as a result
of the centrality of the beam than portions of the material at the
periphery of the pulse or pulses (see, e.g., FIGS. 18 and 22).
[0068] As a result of shot overlap or other controllable
parameters, a variable percentage of a surface may be altered to
have the desired structure or structures. For example, a precise
scanning pattern of a laser beam across the surface of the material
may be used to ensure that a variable percentage of the surface is
altered to possess the desired nanostructure(s), microstructure(s),
macrostructure(s), or combinations thereof. Contemplated
percentages of a surface to be modified range from 1 to 100%
counting by ones (i.e., 1, 2, 3, 4, 5, . . . , 100%). As shown in
FIG. 18, precise patterns of laser irradiation application, either
at one fluence alone or in a combination of fluences (e.g., high
fluence/low fluence) may influence the type of structuring of the
material obtained. FIG. 18, for example, shows that a two shot high
fluence regime at F=9.6 J/cm.sup.2 on copper will produce a mixed
materials processing result of a microstructured central area
surrounded by a nanostructured periphery.
[0069] In addition to specifying the percentage of the surface to
be modified, the materials processing effects may also be expressed
in terms of a total area modified, e.g., 0.1, 0.2, 0.3, 0.4, 0.5, .
. . 10,000 cm.sup.2 (counting by 0.1 cm.sup.2 units). In this
regard, it is advantageous that the embodied materials processing
methods produce sufficiently large amounts of altered materials,
where these amounts may be specified in terms of the total surface
area of the material that has been altered.
[0070] The exact surface area or range of surface areas required
for any particular application of the present invention will depend
upon the application; aesthetic applications such as jewelry, for
example, will require relatively small amounts of altered material.
In contrast, larger surface areas of altered materials may be
required for other applications, e.g., formation of heat absorptive
surfaces, or for applications for, e.g., catalysis or materials
implantation into the human body for, e.g., dental implants or
other situations where nanostructuring is advantageous for cellular
growth and penetration into the implant material.
[0071] According to non-limiting aspects of the invention,
advantageous nanostructured materials processing effects may be
obtained under ambient air/pressure conditions. Thus the pressure
conditions under which materials processing occurs affect both the
threshold laser fluence (synonymously, the "ablation threshold" or
F.sub.abl) required for visible material surface damage under SEM
and the plasma ignition threshold (F.sub.pl) as assayed by the
onset of bright violet radiation from the laser-irradiated spot as
measured either by a photomultiplier or an open-shutter camera
(see, e.g., FIG. 12). Therefore, various embodiments of the present
invention are directed to materials processing at: low-pressure
conditions (e.g., below 5 torr), where, for example, related
materials processing to produce "gold-black" is done; vacuum
conditions (i.e., below 0.1 torr); between 5 torr and 760 torr (1
atm), i.e., 5, 6, 7, 8, 9, 10, . . . 760 torr (counting by ones);
and, at atmospheric pressure, where the Examples provided below
show desirable materials processing effects can occur, contrary to
prior teachings that materials processing must be performed at low
pressure.
[0072] Additionally, Example 2 below discusses the effects of
ambient air versus a highly reactive gas (oxygen) versus an inert
gas (helium) on materials processing using ns duration pulses, and
concludes that these effects are dependent upon gas pressure,
rather than the type of gas environment used. While these effects
are expected to be applicable to fs duration pulses as well,
non-limiting aspects of the present invention nevertheless
contemplate the use of purified gases in addition to ambient air
for use with the materials processing regimes. Inert gases may have
particularly desirable effects, thus such gases or other purified
gas or mixtures of gases may be used in aspects of the present
invention.
Exemplary Method Embodiments
[0073] Embodiments of the invention use pulsed laser beams of
femtosecond (fs) duration to obtain nanostructuring of material
surfaces with or without microstructuring and/or macrostructuring
effects. The specific conditions for generating a particular
structure (macro-, micro-, or nano-) or combination of structures
is a function of a number of variables including laser pulse
duration, laser energy density or fluence (in J/cm.sup.2), and the
number of pulses or "shots" of the laser beam delivered to a
particular region of the material to be altered.
[0074] As shown in the Examples below, both on theoretical and
experimental grounds, fs pulses produce different materials
processing effects than do ps and ns duration pulses. It is also
observed that the materials processing effects obtained with fs
laser pulses are not a priori uniform; rather, they depend in part
upon the specific pulse parameters such as the energy density or
fluence of the laser beam and the number of pulses of the laser
beam applied to the sample. However, various combinations of beam
fluence and shot number may advantageously produce more uniform
materials processing results.
[0075] Example 1 below will describe the effects of beam fluence
and shot number on a gold sample. The results are categorized into
four discrete regions of effect, AB, BC, CD, and DE as shown in
FIG. 1, which shows a graph of the absorptance of a gold (Au)
surface against number of pulses of varying fluence from a
Ti:sapphire laser (central wavelength of 0.8 .mu.m; pulse duration
of 60 fs). The different absorptance values of the laser beam
irradiated gold in various of these regions can be correlated with
differences in the materials alterations achieved; i.e., region AB
is associated with nanoscale roughness (see also FIG. 2); region BC
is associated with nanoscale roughness including nanobranches (see
FIG. 3(a)) and spherical nanoparticles (see FIG. 3(b)) and also
contains microscale structures such as micropores, circular
microgrooves, and central microchannels; and, region CD contains
macroscale structures such as craters, periodic structures, and
other surface deformations (see, e.g., FIG. 7).
[0076] As Example 3 will show, the regions defined in Example 1 and
shown in FIG. 1 are applicable not just to a gold material sample
but are also observed to be very consistent across different
materials. Thus Example 3 is specifically directed to an analysis
of the effects of fluence and shot number of a fs laser beam on
copper, with the SEM results for various experiments shown in FIGS.
16-18 and 20. FIG. 19 is a summary graphic that shows that there
are essentially three regions defined by the data of Example 3: a
region dominated by nanostructures (the X region in the figure); a
region dominated by microstructures with some nanostructures (the
open circle region in the figure); and, a region dominated by
macrostructures with some micro- and nanostructures (the sold
squares in the figure). These three regions correspond to regions
AB, BC, and CD (or possibly CD/DE), respectively, of FIG. 1 and
demonstrate that although there would be no a priori ability to
predict the existence of these regions, once the regions have been
defined, the structures formed for each region are relatively
predictable.
[0077] Further confirmation of the general applicability of the
three regions of FIG. 19 to other materials including, but not
limited to, other metals, semiconductors, and dielectrics is
provided by the data of Example 5, where titanium metal was exposed
to a varying number of fs duration laser pulses of varying fluence.
Specifically, Example 5 will show that nanostructures are present
with low laser fluences (see, e.g., FIGS. 26-29) as expected, and
that for higher fluences of, e.g., 0.16 or 0.35 J/cm.sup.2 and a
sufficient number of laser shots (e.g., 20-200; see FIGS. 30 and
31), microstructuring occurs as predicted by the data of FIG. 19.
Finally, again as predicted by the data of FIG. 19, for 1,500 shots
at a fluence of 0.48 J/cm.sup.2, macrostructures are formed as
predicted (see, e.g., FIG. 32(d)).
Exemplary Applications
[0078] The apparatus and method embodiments described herein may
have utility in a variety of applications including, but not
limited to: aesthetic or marking applications such as the
application of patterning or coloring to the surface of jewelry;
medical applications, e.g., for implantable medical devices, where
the novel properties of the laser altered surface of such a device
may aide in, for example, integration of cells of a subject into
the implant; catalysis, where the properties of the altered
materials and particularly the increased surface area of the
materials resulting from, e.g., nanostructuring, may improve the
ability of the material to catalyze chemical reactions; heat
transfer situations, where alterations resulting in increased
absorptivity may improve, e.g., the efficiency of solar cells and
heat sinks; sensor sensitivity, where the unique alterations to
materials described herein may be used in both a sensor's absorbing
element to increase the amount of electromagnetic radiation
absorbed and also in the shielding around the sensor or sensors to
protect them from various forms of stray electromagnetic radiation,
thereby helping to improve their signal-to-noise ratios; and,
stealth technologies or other technologies where the absorption of
electromagnetic energy such as ultraviolet, visible, infrared,
terahertz radiation, etc., may cloak, conceal, or otherwise obscure
the object coated or shielded with the altered material having the
desired absorptive properties.
Exemplary Aesthetic or Marking Applications
[0079] A non-limiting aspect of the invention is directed to
methods for materials processing that produce altered materials for
aesthetic or marking applications, for example jewelry or other
applications where the nano-, macro-, or micro-structuring of a
material's surface may produces desired effects.
[0080] The data of Example 1 will show that fs-based laser
processing may be used to increase the absorptivity of a material,
which can be observed visually as darkening or blackening of the
surface region of the material so altered. Accordingly, an
embodiment the invention is directed to a method of obtaining the
desired markings. A related embodiment is directed to the materials
obtained by such processing methods.
Exemplary Biomedical Applications
[0081] A non-limiting aspect the invention is directed to a method
for materials processing that produce materials advantageously
suitable for biomedical applications, particularly medical
applications where a metal or metal-clad device is to be implanted
into a subject, and alterations to the metal or metal cladding may
act to improve the biocompatibility of the metal or metal
cladding.
[0082] In this aspect, suitable materials may include metals,
ceramics, composites, and others, that are nanostructured and/or
possibly microstructured and then introduced or implanted into a
biological milieu such as in bone, in tissue, etc., where
biocompatibility is important for successful introduction or
implantation. The materials contemplated include any as are known
for introduction or implantation into the body, and include, but
are not limited to, metals such as titanium, gold, silver, etc.,
alloys of these metals, composites, etc. The "biological milieu"
may include bone, tissue, etc., of a whole organism, or of an
isolated component of an organism, e.g., of an isolated organ,
teeth, bones, etc. Organisms contemplated include animals, and
particularly mammals, including humans.
[0083] Example 5 below will discuss alterations to titanium metal
using a fs laser method described herein to alter the surface
topography of titanium for better biocompatibility, i.e., to
provide a surface containing, e.g., pits, pillars, steps, etc., or
other structural features that serve as anchors or other
attachment, scaffolding, or stimuli for protein and/or cellular
integration.
[0084] "Biocompatibility" as used herein refers generally to
alterations in the surface of a material that increase the ability
of that material to integrate into the body, e.g., increase
structural integration such as by invasion or interpenetration of
the material by cells of the body or proteins or other biological
material. Biocompatibility also refers to alterations that increase
integration by decreasing rejection of the material by the body, as
would occur if the material fails to integrate, i.e., so that the
body recognizes the material as non-integrated and thus acts to
encapsidate or otherwise reject it.
[0085] Biocompatibility may be assayed in a variety of ways. For
implants, for example, biocompatibility may be determined by
assaying the mechanical strength or stability of the integration of
the implant into the body. Thus for example, in osseointegration of
dental implants, biocompatibility may be assayed by determining the
force required to displace or separate out the implant from the
surrounding bone. Biocompatibility may also be determined by
directly observing (e.g., by SEM) the extent to which proteins,
cells, or other biological materials are able to invade or
integrate into the metal or other material altered by the materials
processing methods of the present invention. As another
non-limiting example of an assay for biocompatibility, methods for
measuring cell death or proliferation may be used to determine the
extent to which the altered surface topography of the material
processed by the laser methods of the present invention results in
the activation of cells to proliferate, or the active suppression
of cell death mechanisms that would otherwise occur if the cells
failed to find themselves in a suitable proliferative
environment.
Exemplary Catalysis Applications
[0086] A non-limiting aspect is directed to a method for fs laser
pulse materials processing that can produce materials with
desirable catalytic properties; i.e., materials that contain
nanostructural and macro- and/or micro-structured alterations that
increase catalytic surface area.
[0087] Such alterations may be assayed by SEM or other analyses
that allow for the determination of the porosity or other increased
surface area aspects of the materials altered. Alternatively,
catalytic activity may be measured directly by determining the rate
at which a reaction is catalyzed by an unaltered material (e.g.,
platinum) versus the rate of the reaction using an altered
material.
Exemplary Modifications of the Optical Properties of Materials
[0088] An aspect of the invention is directed to a method for
altering the optical properties of materials, including, but not
limited to, metals such as are provided in Example 6 below. Thus as
shown in Example 6, the materials processing methods of the present
invention may be used to obtain, e.g., metals which appear to the
human observer to have various shades of gray (where "gray" may
alternatively be defined as a material having relatively uniform
reflectance across the entire visible wavelength), including
multiple shades of gray in one metal piece. These materials
processing methods may additionally be used to obtain what appear
to the human observer to be colored materials (where "colored" may
alternatively be defined as a material having preferential
reflectance in some regions of the visible spectrum and not in
others), e.g., colored metals such as are also described in Example
6, and `black` metal. Although these methods are applied to metals
in Example 6, the present invention explicitly contemplates the
application of these methods for certain non-metal materials as
well.
Example 1
[0089] Experiments in support of embodiments of the invention have
demonstrated that a significant amount of residual thermal energy
is deposited in metal samples following multi-shot femtosecond
laser ablation. Traditionally, it was commonly believed that one of
the most important advantages of femtosecond laser ablation is that
the energy deposited by ultrashort laser pulses does not have
enough time to move into the bulk sample; therefore, the residual
thermal energy remaining in the bulk sample should be negligible.
In contrast to this, a significant enhancement in laser light
absorption was observed recently by the inventors following
ablation. To understand the physical mechanisms of laser energy
absorption, the change in absorptance of gold due to structural
modifications following multi-shot femtosecond laser ablation was
directly measured. The measured data indicates that there is a
significant absorption enhancement due to nanostructuring in
addition to the known mechanisms of absorption increase via micro-
and macro-structuring. Moreover, nanostructuring alone may enhance
the absorptance by a factor of about three. The physical mechanism
of the total enhanced absorption is due to a combined effect of
nano-, micro-, and macro-structural surface modifications induced
by femtosecond laser ablation. At a sufficiently high fluence and
with a large number of applied pulses, the absorptance of gold
surface may reach an absorptance value of nearly 100%.
[0090] The absorptance A of a pure metal with a clean surface
consists of two components A=A.sub.INTR+A.sub.SR, where A.sub.INTR
is the intrinsic absorptance and A.sub.SR is the contribution due
to surface roughness. For an optically smooth metal surface,
A.sub.SR is about 1-2% of A.sub.INTR, but the role of A.sub.SR
enhances as the surface roughness increases. For multi-pulse
ablation, only the first femtosecond laser pulse interacts with an
undamaged surface, since the laser-induced surface structural
modification develops long after the ultrashort pulse. In this
case, A is governed by A.sub.INTR, which can be a function of laser
fluence due to laser-induced change in the dielectric constant of
the material. All of the subsequent laser pulses interact with a
structurally modified surface and their absorption is determined by
both A.sub.INTR and A.sub.SR. The absorption of a single
femtosecond laser pulse by an undamaged metal surface is dominated
by A.sub.INTR. However, the coupling of laser energy to a metal in
multi-pulse femtosecond laser ablation has not yet been
investigated, where A.sub.SR may have a significant value due to
surface structural modification.
[0091] The instant non-limiting example discusses the effect of
surface structural modifications on the absorptance of gold in
multi-pulse femtosecond laser ablation when an originally plane and
smooth surface transforms into a blind hole. This effect is
investigated as a function of the number of applied ablation pulses
at various fluences. A reported calorimetry technique allows a
direct measurement of laser energy absorbed by the sample. Our data
indicate that femtosecond laser-induced surface modification
enhances the sample absorptance, which can reach a value close to
100% at a sufficiently high fluence with a large enough number of
applied pulses. Scanning electron microscope (SEM) studies show
that there is absorption enhancement due to nanostructuring, which,
alone was seen to enhance the absorptance by a factor of about
three.
[0092] Experimentally, an amplified Ti:sapphire laser system was
used to generate 60-fs pulses of about 1.5 mJ/pulse at 1 kHz
repetition rate at a central wavelength of 800 nm. The laser beam
was focused onto a mechanically polished sample surface with a
40-cm-focal-length lens at normal incidence. An electromechanical
shutter was used to select the number of pulses, N, applied to the
sample. The absorptance of the ablated spot was studied. After
ablation of the sample with a chosen number of pulses, we reduced
the laser fluence to a level below the ablation threshold.
Subsequently, we irradiated the ablated spot again using a train of
low-fluence laser pulses that would not induce any further surface
modification. A certain amount of energy from this low-fluence
pulse train, E.sub.A, is absorbed in the skin layer of the sample,
dissipates via heat conduction in the sample, and causes a bulk
temperature rise .DELTA.T. We measured this temperature rise with a
thermocouple battery that allows E.sub.A to be determined
calorimetrically as E.sub.A=C.DELTA.T, where C is the known heat
capacity of the sample. To measure energy E.sub.I incident upon the
sample, a certain fraction of incident pulse train energy was split
off by a beam splitter and measured with a joulemeter. Having
measured E.sub.I and E.sub.A, the absorptance of the ablated spot
could be found as A=E.sub.A/E.sub.I. Laser-induced surface
modifications were then studied using a SEM and an optical
microscope.
[0093] The optical properties of surface modifications were studied
following multi-pulse ablation at single-pulse laser fluences of
F=1.1, 0.35, 0.17, and 0.078 J/cm.sup.2 in air. The ablation
threshold F.sub.abl for a pristine surface was found to be
F.sub.abl=0.067 and 0.048 J/cm.sup.2 for single-pulse and 500-pulse
train irradiation, respectively. The numbers of pulses required to
perforate a 1 mm-thick sample at the center of the irradiated spot
were determined to be 16,100, 25,000, and 77,000 pulses at F=1.1,
0.35, and 0.17 J/cm.sup.2, respectively. This corresponds to
average ablation rates of 63, 40, and 13 nm/pulse, indicating that
a single laser pulse produces a nanoscale modification in depth.
Plots of absorptance versus the number of ablation shots, N, at
different F are shown in FIG. 1. For an undamaged surface, the
absorptance remains a constant value of 0.12 when measured at
F=0.0043 J/cm.sup.2, which is an order of magnitude below
F.sub.abl. The absorptance of a structurally modified surface is
significantly greater than that of the undamaged surface and shows
dependence on the number of applied ablation pulses, N.
[0094] The A(N) curves for the ablated surface can be characterized
into distinct regions marked with A, B, C, D, and E on A(N) in the
case of F=0.17 J/cm.sup.2 in FIG. 1. The first of these four
regions is region AB, where the absorptance initially increases
from 0.12 (undamaged surface) to a value in the range of 0.25-0.33.
Typically, this region covers the first 1-10 shots. For example,
this initial enhancement of absorptance can be produced by four
pulses at F=0.17 J/cm.sup.2 or by one pulse at F=0.35 and 1.1
J/cm.sup.2. Optical microscopy showed that the irradiated spot was
entirely covered with surface modification following ablation by
only one pulse when F.gtoreq.0.35 J/cm.sup.2, but four pulses at
F=0.17 J/cm.sup.2. Therefore, the enhancement of A with N at F=0.17
J/cm.sup.2 appears due to both the surface modification and an
increase in size of the modified area from point A to B.
[0095] In the second region, BC, the absorptance undergoes a slight
decrease as N increases. Typically, this region covers
approximately the next 100-300 pulses. Both regions AB and BC
extend to a larger number of pulses when the surface is modified at
F only slightly above F.sub.abl, as seen from the curve at F=0.078
J/cm.sup.2 in FIG. 1.
[0096] The third region, CD, is characterized by a further
enhancement of absorptance with the increase of N. This region
extends to N on the order of 10,000 pulses.
[0097] The fourth region, DE, was where absorptance reached a
maximum value that did not change with further increase of N.
[0098] Reference is now made to the SEM pictures of surface
morphology shown in FIGS. 2-6. In regions AB, BC, and CD, where
absorptance exhibits dependence on N, the following surface
modifications were observed: For region AB, a characteristic
modification is nanoscale roughness (FIG. 2). In region BC, two
major features were observed. First, nanoscale roughness develops
further in the form of nanobranches (FIG. 3 (a)) and spherical
nanoparticles (FIG. 3 (b)). Secondly, microscale structures begin
to develop in the forms of micropores, circular microgrooves, and
central micro-channels. In region CD, features resembling a crater
with a deep central micro-channel, periodic structures with
orientation in the direction perpendicular to the laser light
polarization and with a period roughly equal to the laser
wavelength (FIG. 4(a)), and a visible black halo around the crater
were observed. All these laser-induced surface modifications can
affect the absorptance in various ways. For example, surface
roughness can enhance the absorption of light both by multiple
reflections in micro-cavities and by variation in the angle of
incidence (angular dependence of Fresnel absorption). Nanoscale
structural features can affect absorptance since the optical
properties of a nanostructured material can be quite different from
the bulk. Laser-induced periodic surface structures (LIPSS) may
enhance absorption of laser energy via generation of surface
electromagnetic waves. In accordance with non-limiting aspects of
the instant invention, the observed LIPSS, referred to herein as
nano-structure-covered laser-induced periodic surface structures
(NC-LIPSS) having non-conventional, finer nanoscale structural
features, are shown in FIG. 4(b).
[0099] The absorption of laser energy in femtosecond laser ablation
may also be altered through re-deposition of ablated material.
Examination of the black halo produced around the crater shows that
its elemental composition as determined by energy dispersive X-ray
analysis is identical to that for a pristine surface; i.e., the
black halo is a layer of the ablated and re-deposited gold. SEM
images in FIGS. 5 and 6 demonstrate that the black halo has a
structure of spherical nanoparticle aggregates that is typically
seen in gold-black films and, which, have been known for their high
absorptance in the infrared. Therefore, the gold-black halo can
enhance the absorption of low-intensity wings of the incident
Gaussian beam and contribute to residual heating of the sample.
Since re-deposition of ablated material occurs both outside and
within the ablated spot, the re-deposition of the nanoparticles
produced by ablation can also enhance the absorption of light in
the ablated area. For example, an enhanced absorption of light by a
semiconductor coated with Au nanoparticles has recently been
reported. Therefore, in femtosecond laser ablation, the enhanced
absorption can occur due to surface nano-, micro-, macro-structures
and re-deposition of nanoparticles depending on ablation
conditions. The combined effect of these surface modifications can
lead to virtually 100% absorption of laser light in multi-pulse
ablation with a sufficiently large number of pulses at high fluence
as shown in FIG. 1. Almost all of incident laser energy is retained
in the sample as residual thermal energy. This suggests that the
energy carried away by the ablated material is small in Au, and the
enhanced absorptance appears to be the dominant factor in the
enhanced residual thermal energy deposition in multi-pulse
femtosecond laser ablation at large numbers of applied pulses.
[0100] Since different surface modifications are superimposed on
each other, it is difficult to completely isolate and determine
each individual contribution to the enhanced absorptance.
Therefore, we provide the following estimations on the
contributions of nano-, micro-, and macro-structures induced by
femtosecond laser ablation. Since surface nano-structures are the
dominant feature in region AB and part of region BC for
N<50-100, and the absorptance increases from 0.12 to 0.25-0.33
over these regions (see FIG. 1), nano-structures alone are believed
to account for the additional absorptance increase of about
0.1-0.2. The contribution of two microscale structures, LIPSS and
random roughness, is estimated as follows. To estimate the
contribution of LIPSS, we ablated a sample using p-polarized light
and measured the low-fluence absorptance A(N) of the ablated spot
with both p- and s-polarizations. The curves A(N) of different
polarizations were identical, indicating that the grating effects
of microscale LIPSS on the absorption of laser light by gold is
negligible. To estimate the contribution of microscale random
roughness, we abraded a mechanically polished sample surface with
sandpaper to produce a rms roughness of 3 .mu.m, which is estimated
to be comparable to the laser-induced roughness for
100<N<1000. The absorptance of this abraded surface was then
measured to be about 0.24 as opposed to 0.12 for a mechanically
polished surface. This indirectly shows that the random
micro-roughness accounts for the additional absorptance increase of
about 0.12. Macro-structures come into play in two major forms,
deep central channel and concentric ring grooves, when the number
of pulses is between about 500-1000 and laser fluence is higher
than 0.17 J/cm.sup.2. Two SEM pictures showing typical
macro-structure craters are presented in FIG. 7. The macro-scale
crater formation starts in region CD and, therefore, we believe the
progressive increase of macro-structure size largely accounts for
the absorptance increase from 0.4 to about 1.0. However, nano- and
micro-structures also develop further in regions CD and DE and may
also contribute to absorptance increase to some extent.
[0101] Besides the physical mechanisms of enhanced absorption, we
also make the following observations about femtosecond laser-matter
interactions: First, laser-induced nanostructures alone can enhance
the absorptance of Au by a factor of about three following only 1-3
pulses. This result suggests a new direction for future study of
optical properties of nanostructures imprinted on a metal surface.
Secondly, we produced a new type of microscale periodic structure
with much finer nanoscale structures (NC-LIPSS) following ablation
with a large number of applied pulses. Thirdly, re-deposition of
laser-induced nanoparticles is seen outside of the ablated spot
leading to the formation of a nanostructured material known as gold
black. Finally, we identified potential new applications of
femtosecond laser ablation for modifying optical properties of
metals and producing technologically valuable surface coatings such
as, but not limited to, gold-black films.
Example 2
[0102] In this Example, a comparative study of residual thermal
effects in aluminum following fs laser ablation was performed. At
laser fluences above the ablation threshold where plasmas are
produced and at a sufficiently high ambient gas pressure, an
enhanced coupling of pulsed laser energy into the sample occurs.
Furthermore, in contrast to the conventional understanding that
residual thermal energy is negligible in fs-laser ablation, up to
70% of the incident pulse energy can be retained in the sample
following single-pulse fs-laser ablation in 1-atm air. The major
factors influencing thermal energy coupling to the sample are the
laser fluence and ambient gas pressure. Residual thermal energy
deposition decreases with reducing ambient gas pressure.
[0103] Laser ablation using femtosecond (fs) laser pulses has
numerous applications in the field of materials processing and
machining and, nanotechnology. Comparative studies have
demonstrated that femtosecond laser ablation has advantages over
nanosecond ablation in aspects of higher precision, reduced
heat-affected zone, and smaller amount of debris around the ablated
spot. Following laser ablation, a fraction of absorbed laser energy
is retained in the heat-affected zone, dissipates into the bulk of
the sample and remains inside as residual thermal energy that
induces the bulk temperature of the sample to rise. This is
sometimes referred to as the thermal load and is often undesirable
in laser micro- and nano-machining.
[0104] The coupling of thermal energy into metals has been
previously studied for microsecond and nanosecond laser ablation.
An enhanced residual thermal energy coupling to metals has been
observed when laser fluence is above a certain threshold value. It
has been suggested that, in addition to the direct absorption of
laser light, energy transfer from laser-produced plasma can
contribute to residual heating. However, mechanisms responsible for
thermal coupling are still not fully understood.
[0105] We have observed an enhanced residual heating of metals
following multi-pulse femtosecond laser ablation, where
laser-induced surface modification has been found to play a role in
enhanced residual heating but, where this alone could not fully
account for the observed amount of deposited thermal energy. To
exclude the effect of surface modification on residual thermal
response, we investigated single-pulse fs ablation. We also studied
the residual thermal response of aluminum (Al) following ns-laser
ablation to compare with the results for fs-laser ablation. We used
a calorimetric technique to study effects of laser pulse duration,
ambient gas pressure, and laser wavelength on residual heating of
Al. To characterize the residual thermal response, we defined a
so-called residual energy coefficient (REC) K=E.sub.R/E.sub.I,
where E.sub.R is the residual thermal energy remaining in the
sample following ablation and E.sub.I is the incident laser pulse
energy. By definition, REC is equal to absorptance of the sample
material when laser fluence is below the ablation threshold.
Enhanced residual heating occurred following both single-pulse ns-
and single-pulse fs-laser ablation in ambient gas at a sufficiently
high pressure. The major factors governing the residual heating are
laser fluence and ambient gas pressure. There is a fundamental
difference between multi-pulse versus single-pulse ablation because
multi-pulse ablation may induce absorptance change due to
accumulated surface modifications from multiple laser shots. This
accumulated effect does not occur in single-pulse ablation.
[0106] Both ns and fs duration pulse effects were examined. The
following three laser systems were used: 1) a ruby laser producing
45-ns pulses (FWHM) at wavelength .lamda.=0.69 .mu.m with pulse
energy of 0.6 J; 2) a Nd:YAG laser generating 55-ns pulses at
.lamda.=1.06 .mu.m with pulse energy of 1.4 J; and, 3) a
Ti:sapphire laser producing 60-fs pulses at .lamda.=0.8 .mu.m with
pulse energy of 1.5 mJ. Using each laser system, the laser beam was
focused onto an Al sample at normal incidence. A fraction of the
incident pulse energy E.sub.I was split off using a beamsplitter
and measured with a joulemeter to allow E.sub.I to be
determined.
[0107] The residual energy E.sub.R that remains in the sample
following ablation causes the bulk temperature of the sample to
rise by .DELTA.T. Using a thermocouple attached to the Al sample,
.DELTA.T was measured after thermal equilibrium was reached in the
sample. Knowing the specific heat capacity c.sub.p and the mass m
of the sample, the residual energy can be obtained from
E.sub.R=mc.sub.p.DELTA.T. The thermocouple response time (the time
required for achieving a maximum thermocouple signal in our
calorimeter) was about 2.5 sec. Using measured E.sub.I and E.sub.R,
the residual thermal energy coefficient K=E.sub.R/E.sub.I can be
found as a function of single-pulse laser fluence F=E.sub.I/S,
where S is the laser beam area on the sample. The samples were
mechanically polished. Measurements were performed in various
ambient gases and at different pressures. The sample was translated
with an X-Y stage so each subsequent laser pulse was incident on a
fresh spot. Two parameters, the ablation threshold F.sub.abl and
the plasma ignition threshold F.sub.pl, were determined at the
onset of surface damage visible to eye with subsequent examination
under a scanning electron microscope (SEM). F.sub.pl was determined
by observing the onset of bright violet radiation from the
irradiated spot using either a photomultiplier (PMT) or an
open-shutter camera, both properly filtered to cut off scattered
laser light.
[0108] Although embodiments of the invention are directed to
fs-duration laser pulses, this Example probes some of the effects
of both fs- and ns-duration laser pulses. The dependence of REC on
laser fluence F following single-pulse ns-laser ablation in various
ambient gases under different pressures on Al are plotted in FIG. 8
(for Nd:YAG laser) and 9 (for ruby laser). For the Nd:YAG laser,
ablation and plasma ignition thresholds in 1-atm air are determined
to be F.sub.abl=1.2.+-.0.3 J/cm.sup.2 and F.sub.pl=1.4.+-.0.4
J/cm.sup.2. For the ruby laser, these values are F.sub.abl=1.0+0.2
J/cm.sup.2 and F.sub.pl=1.1.+-.0.3 J/cm.sup.2. Thus
F.sub.abl.apprxeq.F.sub.pl in these experiments. By definition, REC
should be equal to the absorptance of the material when it is
irradiated by low-fluence laser light that does not cause any
surface modification. The measured value of REC (K=0.25) at
F<F.sub.abl in FIG. 2 agrees with the reported value of
absorptance for a mechanically polished Al sample at .lamda.=1.06
.mu.m (D. E. Gray (Ed.): American Institute of Physics Handbook,
3rd edn. (McGraw-Hill, New York, 1972)), and this agreement shows
the accuracy of our measurement technique. Data at 1-atm in FIGS. 8
and 9 show that REC enhances abruptly at a certain fluence
threshold, F.sub.enh, and reaches a maximum value of about 0.5-0.6
indicating that about 50-60% of the laser pulse energy can be
retained in Al following nanosecond laser ablation. Our experiment
also shows that F.sub.enh.apprxeq.F.sub.pl within the experimental
uncertainty for both Nd:YAG and ruby laser ablation.
[0109] Next, we studied the pressure effect on REC; representative
curves are plotted in FIGS. 8 and 9. For Nd:YAG laser ablation, REC
slightly decreases when air pressure, P, decreases from 1 atm to
about 30 torr, but REC abruptly drops when pressure further reduces
from 30 torr to about 0.6 torr. For P<0.6 torr, the onset of
plasma is accompanied with a drop of REC. This drop becomes more
pronounced as the pressure is further reduced to 0.04 torr. At this
pressure, REC eventually reaches a value of about 0.12 that is
smaller than the absorptance of an undamaged surface by a factor of
two. For P<0.04 torr, REC virtually remains independent of the
residual air pressure. This behavior shows that, in contrast to the
observation in air, the onset of plasma in vacuum is accompanied by
a drop of REC. In vacuum, both F.sub.abl and F.sub.pl are higher
than those at 1-atm air pressure by approximately a factor of two.
Dependence of REC on laser fluence is also studied in 1-atm oxygen
and 1-atm helium, and these REC data are shown in FIGS. 8 and 9.
The dependences show virtually the same behavior as those in air,
indicating that REC does not essentially depend on the particular
type of gas. The contribution of possible exothermic chemical
reactions that may occur due to presence of chemically active gases
such as oxygen is negligible.
[0110] In vacuum, the laser plasma mainly consists of ionized
species of ejected material, while in a gas medium, plasma consists
of ionized species of both ablated material and ambient gas. A
characteristic feature of ambient gas plasma produced by ns pulses
is that the plasma expands due to the generation of laser-supported
absorption waves. FIG. 10 shows open-shutter photographs of plasmas
produced by 55-ns Nd:YAG laser pulses for ablation of Al in both
air and vacuum under the same experimental conditions. Distinction
between plasmas can be clearly seen. The size of plasma in air is
larger than that in vacuum. Therefore, the role of plasmas in
residual heating of the sample in air may differ from that in
vacuum.
[0111] The direct absorption of laser energy is a factor that may
influence residual heating. According to the Drude model, when the
temperature increases material absorptivity should also increase
due to an enhanced collision frequency between free electrons and
thermally vibrating lattice atoms. Therefore, one should expect an
increase in REC with laser fluence due to this enhancement of
material absorptivity. However, the fact that REC increases in air
while it decreases in vacuum above a certain laser fluence
indicates that the temperature-enhanced Drude absorption does not
play an essential role in enhanced residual thermal response. This
is also confirmed by our estimation of the laser-induced surface
temperature using the following formula:
T S ( t ) = A a k .pi. .intg. 0 t I ( t - .theta. ) .theta. .theta.
+ T 0 ##EQU00001##
where A is the absorptance, a is the thermal diffusivity, I is the
intensity of incident laser light, k is the thermal conductivity, t
is the time, T.sub.0 is the initial temperature, and .theta. is the
integration variable. FIG. 11(a) shows the computed T.sub.S(t)
induced by the Nd:YAG laser pulse at F.sub.abl=F.sub.pl=1.4
J/cm.sup.2 in 1-atm air and at F.sub.abl.apprxeq.F.sub.pl=2.7
J/cm.sup.2 in vacuum with A=0.25, a=1.0.times.10.sup.-4 m.sup.2/s,
k=240 Js.sup.-1m.sup.-1.degree. C..sup.-1, and T.sub.0=20.degree.
C. One can see that the maximum surface temperature is about
500.degree. C. in air and 1000.degree. C. in vacuum. The estimated
surface temperature in air is below both the melting (660.degree.
C.) and boiling (2495.degree. C.) points of Al. The computed
T.sub.S(t) for ruby laser at F.sub.abl.apprxeq.F.sub.pl=1.1
J/cm.sup.2 in 1-atm air and at F.sub.abl.apprxeq.F.sub.pl=2.1
J/cm.sup.2 in vacuum with A=0.28 are shown in FIG. 11(b). Similar
to the results of Nd:YAG laser in FIG. 5(a), the estimated surface
temperature for ruby laser irradiation in air is also below both
the melting and boiling points of Al. Thus when the enhanced
thermal coupling occurs in 1-atm air, the estimated surface
temperature induced by both Nd:YAG and ruby lasers is too low to
induce a significant increase in absorptance.
[0112] The similar general behavior of REC for Nd:YAG (.lamda.=1.06
.mu.m) and ruby (.lamda.=0.69 .mu.m) lasers shows that laser
wavelength is relatively unimportant in the visible and near
infrared spectral region. Nevertheless, our experiment clearly
demonstrates that REC of the aluminum sample depends mainly on
laser fluence and ambient gas pressure following ns-laser
ablation.
[0113] The dependence of REC on laser fluence for Al following
fs-laser ablation in 1-atm air and in vacuum (P=0.01 torr) are
plotted in FIG. 12. The residual thermal energy coupling is
enhanced in air above a certain threshold value of laser fluence,
while in vacuum it is reduced. The values of F.sub.abl, F.sub.pl,
and F.sub.enh in air are found to be 0.053 J/cm.sup.2, 0.086
J/cm.sup.2, and 0.5 J/cm.sup.2, respectively. These thresholds are
well separated and the enhancement threshold is above the plasma
threshold; i.e. F.sub.enh>F.sub.pl>F.sub.abl, in contrast to
the ns-laser ablation where
F.sub.enh.apprxeq.F.sub.pl.apprxeq.F.sub.abl. We note that our
measured value of F.sub.pl in 1-atm air agrees with reported values
for Al thin film deposited on a silicon substrate. The values of
F.sub.abl and F.sub.pl in vacuum are determined to be 0.058
J/cm.sup.2 and 0.096 J/cm.sup.2, respectively (see FIG. 12).
Contrary to conventional understanding that the residual thermal
energy is negligible in an ablated sample following femtosecond
laser ablation, our data show that REC reaches a value of 0.7
indicating that, at the highest laser fluence achievable in our
experiment (F.apprxeq.4 J/cm.sup.2), about 70% of the incident
laser energy can be retained in the sample following single-pulse
fs-laser ablation in 1-atm air
[0114] FIG. 13(a) shows an SEM image of an undamaged surface that
is mechanically polished. A view of the sample surface after
irradiation in air at F=F.sub.abl is shown in FIG. 13(b). (FIG.
13(b) does not show the same spot on the sample as in FIG. 13(a)).
FIG. 13(b) shows that surface defects are preferential spots for
initial ablation with some sparsely distributed small spherical
nanoparticles in the irradiated area. FIG. 14 shows a typical
laser-induced surface morphology following ablation at F=F.sub.pl
in 1-atm air. It is seen that surface modifications are still
localized around surface defects, but both the number and the size
of nanoparticles are greater than those at F=F.sub.abl. Therefore,
material ejection in fs-laser ablation appears to be initiated at
surface defects. Open-shutter photographs of the femtosecond
laser-induced plumes taken at F=1.16 J/cm.sup.2 (higher than
F.sub.pl) are shown in FIG. 15. The figure shows that the size of
the plume in air is larger than that in vacuum (P=0.01 torr).
[0115] There are three basic distinctions between ns- and fs-laser
ablation. First, fs-laser pulses do not interact with ejected
material because hydrodynamic expansion of ablated material from
the irradiated area occurs on a timescale much longer than
femtosecond pulse duration. Secondly, laser-supported absorption
waves that are commonly generated in ns-laser ablation in a gas
medium do not exist in fs-laser ablation. Thirdly, a material
irradiated with an intense fs-laser pulse can be heated to a
solid-density plasma state.
Example 3
[0116] Unique properties of nanomaterials have been extensively
studied in the past and various nanostructures have found numerous
applications in optics including enhanced x-ray emission and
enhanced absorption in intense light-matter interaction, and
optical biosensing, to name a few. Direct surface nanostructuring
(i.e., not from ablated plume deposition) may be used in a number
of technological applications, for example, manipulation of optical
properties of solids, catalysts, dental implants, etc. We performed
a detailed study of the morphology of surface nanomodifications
produced on bulk metals using a femtosecond laser ablation
technique embodied herein. The effects of laser fluence and number
of applied pulses on the generated surface nanostructures were
studied with a scanning electron microscope (SEM). According to an
aspect, a set of optimal laser irradiation conditions for metal
surface nanostructuring is disclosed.
[0117] In our experiment, we used an amplified Ti:sapphire laser
system that consisted of a mode-locked oscillator and a two-stage
amplifier including a regenerative amplifier and a two-pass power
amplifier. The laser system produces 65-fs pulses with energy
around 1 mJ/pulse at a 1 kHz repetition rate with a central
wavelength .lamda.=800 nm. To produce ablation, the laser beam is
focused normally onto a bulk sample mounted vertically. To measure
the incident pulse energy, a certain fraction of the incident light
is split off by a beam splitter and measured with a pyroelectric
joulemeter. The number of laser shots, N, applied to the sample is
controlled using an electromechanical shutter. All experiments were
performed in air under atmospheric pressure. The morphology of
femtosecond laser-induced surface modifications was studied using a
SEM. The studied samples were mechanically polished copper, gold,
and platinum. The range of laser fluence used in the ablation was
between 0.084 and 9.6 J/cm.sup.2. The number of applied pulses was
varied from 1 to 5.times.10.sup.4 shots. The ablation threshold was
determined as the minimum fluence to generate a surface damage seen
under the SEM.
[0118] A SEM picture of a copper sample surface prior to laser
irradiation is shown in FIG. 16(a). For reference, the ablation
threshold for a copper sample was determined to be F.sub.abl=0.084
J/cm.sup.2 following a total of N=100 shots. The morphology of the
irradiated surface was studied following ablation with laser
fluence of F=0.084, 0.16, 0.35, 1.52, 3.7, and 9.6 J/cm.sup.2 and
the number of applied pulses in the range of 1-5.times.10.sup.4. A
number of representative surface structures produced on the copper
sample are shown in FIGS. 16-18. An analysis of the SEM data shows
that the morphology of femtosecond laser-induced surface
nanostructures depends both on laser fluence and the number of
applied pulses. The effect of the total number of shots on
nanostructuring at F=0.35 and 1.52 J/cm.sup.2 is shown in FIGS. 16
and 17, respectively. FIG. 16(b) shows that nanostructures begin to
occur on some random localized sites after one shot at F=0.35
J/cm.sup.2. A few larger-size structural features are also observed
in the central part of the ablated area, as seen in FIG. 16(b).
These larger structures may be associated with surface defects
and/or laser beam intensity inhomogeneities. FIG. 16(c) shows a
nanoscale surface structure produced by two-shot ablation. The
structure comprises both larger nanocavities and nanoprotrusions
with spherical tips of diameter up to about 75 nm. Therefore, the
one additional shot transforms the sparsely distributed nanoscale
features in FIG. 16(b) to the cellular-like structures in FIG.
16(c). The surface morphology after ablation with 1000 pulses is
shown in FIG. 16(d). One can see that the mean size of
nanoprotrusions becomes larger while at the same time some
nanocavities develop into microcavities. The evolution of the
surface structures following ablation at F=1.52 J/cm.sup.2 and
various N is shown in FIG. 17(a-d). At this middle fluence, pure
nanostructures are only generated by ablation with one or two laser
shots (FIGS. 17(a) and 17(b)). As shown in FIG. 17(c), 10 shot
ablation produces both random nano- and micro-structures. With
further increasing N, the proportion of nanostructures decreases as
can be seen in FIG. 17(d), where microscale structures become
dominant. At the highest fluence used in our experiment,
nanostructures are not present over most of the irradiated area and
a dominant morphological feature is microroughness. However,
nanostructuring can still be observed on the periphery of the
ablated spot where the Gaussian beam intensity is low enough for
nanostructural formation. An example of these surface structural
modifications is shown in FIG. 18 for two-shot ablation at F=9.6
J/cm.sup.2.
[0119] The effect of laser fluence on surface structuring can be
seen from analyzing the surface modifications produced at various F
and fixed N as shown for example in FIG. 16(c) (F=0.35 J/cm.sup.2,
N=2), FIG. 17(b) (F=1.52 J/cm.sup.2, N=2) and FIG. 18 (F=9.6
J/cm.sup.2, N=2). These images show that ablation with high laser
fluence does not actually induce nanostructures and therefore there
exist optimal laser ablation conditions for surface
nanostructuring. In order to determine the optimal conditions for
nanostructuring, we performed an SEM study of laser-induced surface
modifications following ablation with a large variety of F and N.
The obtained data are summarized in FIG. 19. One can see that the
most favorable conditions for pure nanostructuring are ablation at
low and medium values of laser fluence (F<1.5 J/cm.sup.2). FIG.
19 also shows the range of laser irradiation parameters where
femtosecond laser ablation produces different combinations of
surface nano-, micro-, and macro-structures.
[0120] To determine the mechanism of nanostructuring, we performed
a SEM study on the origin of nanoscale modifications. A
representative example of nascent nanostructures following ablation
with F=0.35 J/cm.sup.2 and N=1 is shown in FIG. 20(b), where the
characteristic types of initial nanostructures are labeled. For
comparison, FIG. 20(a) shows an undamaged area of the sample using
the same scale as in FIG. 20(b). It is seen in FIG. 20(b) that
surface structuring is initiated on random, highly-localized
nanoscale sites. The typical structures include circular nanopores
with a diameter in the range of 40-100 nm, randomly-oriented
nanoprotrusions with a diameter in the range of 20-70 nm and a
length of 20-80 nm, nanocavities of arbitrary form, and nanorims
around nanocavities. Under these femtosecond laser processing
conditions, nanoscale features down to a size of 20 nm are
produced. One can see from FIG. 20(b) that a nanopore or nanocavity
is always immediately accompanied by a nanorim or nanoprotrusion,
indicating a nanoscale material relocation to an adjacent site.
These one-to-one nanoscale dips and protrusions occur randomly over
the laser spot, suggesting an initial non-uniform laser energy
deposition. Possible factors responsible for the spatial variation
of the absorbed laser energy include: (1) the spatial inhomogeneity
of the incident beam; (2) the enhancement of absorption by surface
defects; (3) interference of the incident laser light with the
excited surface electromagnetic waves due to structural defects.
When the incident laser fluence is close to the laser ablation
threshold, the spatial variations in deposited laser energy can
produce a melt at localized nanoscale sites within the irradiated
spot. Once the localized nanoscale melts have been formed, a high
radial temperature gradient in a nanomelt can induce a radial
surface tension gradient that expels the liquid to the periphery of
the nanomelt. This will lead to the formation of nanocavities,
nanoprotrusions, and nanorims due to fast freezing of the expelled
liquid on the boundary with the solid state material (see FIG.
20(b)). This mechanism may also be used to explain the formation of
nanobumps on a thin metal film. These initially induced surface
random nanostructures can enhance the absorption of laser light and
facilitate the further growth of surface nanoroughness due to the
increased spatial nonuniform energy absorption. When laser fluence
is sufficiently high to produce ablation, the atoms ejected from
the nanomelts produce a recoil pressure that squirts liquid metal
outside of the nanomelt. For multi-pulse ablation, the repeating
vaporization and re-deposition of nanoparticles back onto the
surface may also affect the surface nanostructuring. SEM morphology
study at high fluence (F>5 J/cm.sup.2; i.e., strong ablation)
shows that melt occurs over a large area of the ablated spot (see
FIG. 18) and the flow dynamics in this large melt pool
predominantly results in microstructuring. We have also studied the
ambient gas pressure effect on nanostructuring by taking SEM images
of platinum following single-pulse ablation in 1-atm air and in a
vacuum at a base pressure of 8.times.10.sup.-3 Torr. Although we
have observed a greater amount of re-deposited nanoparticles in air
than in vacuum, the morphology of nanostructures is still quite
similar under different air pressures. Our study was performed with
samples mounted vertically. It should be noted that the amount of
re-deposited ablated particles back onto the sample surface may be
different when the sample is positioned vertically versus
horizontally, but further studies are required in this aspect of
nanostructuring using fs laser pulses.
Example 4
[0121] Laser-induced periodic surface structures (LIPSS) on solids
have been studied in a number of works in the past. Typically,
LIPSS show regular groove structure with a period on the incident
laser wavelength scale and oriented perpendicularly to the
polarization of the incident light. LIPSS are commonly seen
following long pulse irradiation on a variety of materials,
including semiconductors, metals, and dielectrics.
[0122] In contrast to previous work performed mostly at relatively
high fluence, we studied the formation of LIPSS on platinum and
gold in a special fluence regime, namely, at near damage-threshold
fluence. We found a unique type of LIPS S entirely covered with
nanostructures. A distinctive feature of the nanostructure-covered
LIPSS (NC-LIPSS) is that its period is appreciably less than that
of the regular LIPSS whose period is approximately equal to the
laser wavelength at normal incident laser light. The reduced period
of the nanostructure-covered LIPSS is caused by a significant
increase of the real part of the effective refractive index of the
air-metal interface when nanostructures develop on a metal surface
that affects the propagation of excited surface plasmon polaritons.
Nanostructure-covered LIPSS has a variety of potential
applications, such as modifying optical properties of materials and
chemical catalysts where high surface-to-volume ratio is a crucial
factor.
[0123] In this experiment, we used an amplified Ti:sapphire laser
system that generates 65-fs laser pulses with energy about 1
mJ/pulse at a 1 kHz repetition rate and with a central wavelength
.lamda.=0.8 .mu.m. The horizontally-polarized laser beam is focused
onto a vertically standing metal sample in air at normal incidence.
The number of laser shots, N, applied to the sample is selected
with an electromechanical shutter. We studied the evolution of
NC-LIPSS on metals following irradiation with N=1, 2, 4, 8, 10, 20,
30, 40, 50, 100, 200, 300, 400, 500 pulses at near damage-threshold
fluence. The studied metals were platinum and gold. The laser
fluence of the incident light was varied by changing the distance
between the focusing lens and sample. To measure the laser pulse
energy incident upon the sample, a fraction of the incident laser
beam is split off by a beamsplitter and diverted to a pyroelectric
joulemeter. The morphology of the produced periodic structures is
examined using a scanning electron microscope (SEM). The surface
profile is measured with an atomic force microscope (AFM). All
sample surfaces were mechanically polished using 0.1 .mu.m grade
aluminum oxide powder.
[0124] The evolution of surface structures produced on Pt following
ablation at near damage-threshold laser fluence of F=0.16
J/cm.sup.2 is shown in FIGS. 21(a)-(b). FIG. 21(a) demonstrates
surface random nanoroughness produced after 10 shot ablation. The
inset in FIG. 21(a) shows that this initial surface modification is
characterized by nanocavities and nanoprotrusions of various forms.
At N=20, a microscale periodic pattern starts to form over the
initially produced random nanoroughness. At this stage, only small
patches of periodical structures are observed in various isolated
locations within the irradiated spot, referred to below as
intermediate LIPSS. With increasing N, the intermediate LIPSS grow
and coalesce into a clear extended LIPSS with a period of 0.61
.mu.m at N=30 (FIG. 21(b). For N greater than 70 shots, LIPS S
starts to disappear gradually in the central spot area (FIGS. 22(a)
and 22(b)). However, clear NC-LIPSS continue to form in the
peripheral area (FIGS. 22(c) and 22(d). Using AFM, the initial
undamaged surface rms roughness is found to be about 5.6 nm after
polishing, and a typical AFM surface profile measurement on Pt is
shown in FIG. 23(a). Following ablation with N=1, 2, and 10 shots,
surface rms roughness is found to be about 16.5, 35.2, and 79.8 nm,
respectively. FIG. 23(b) shows typical surface roughness after N=10
shots. The surface profile of LIPSS after 30 laser shots is shown
in FIG. 24. To gain the insight of how initial nanoroughness
affects the formation of NC-LIPSS, we performed a SEM study of the
formation of NC-LIPSS with samples of different initial surface
conditions. We found that the extended LIPSS is produced with a
smaller number of laser shots when the sample has a greater surface
nanoroughness. To understand the material dependency in forming the
nanostructure-covered LIPSS, we also performed a detailed SEM study
of surface structural modifications on Au. Our data show that the
general trend is similar for Au and Pt in forming the initial
nanoroughness. The period of nanostructure-covered LIPSS on Au is
observed to be 0.58 .mu.m and is also markedly less than the laser
wavelength (FIG. 25). However, the periodic patterns on Au are much
less clear compared to Pt. Recently, we performed a comparison
study on regular LIPSS on various metals following femtosecond
laser radiation where LIPSS shows distinctly different level of
morphological clearness among various metals even under identical
experimental conditions. The electron-phonon energy coupling
coefficient, g, is shown to directly correlate to the morphological
clearness of LIPSS. A larger g coefficient usually leads to more
pronounced LIPSS. In this study, g coefficient for Pt and Au are
25.times.10.sup.16 and 2.1.times.10.sup.16 W/m.sup.3K,
respectively, and the much larger g coefficient explains why LIPSS
is much more clear on Pt than Au.
[0125] The periodic patterns induced by femtosecond laser
processing are distinctly different from those produced by longer
pulses in two aspects. First, femtosecond laser-induced periodic
structures are covered by random nanostructures. Secondly, the
LIPSS period induced by femtosecond pulses at normal incidence is
appreciably less than the laser wavelength while the period is
roughly equal to the wavelength for longer pulses. To account for
our observation, we carefully examined the evolution of surface
structural modifications on both Pt and Au, and we propose the
following mechanism for the formation of NC-LIPSS. The first few
laser shots usually produce sparsely and randomly distributed
nanostructures. It is known that surface plasmons, both localized
and propagating along a surface, can be excited by coupling laser
energy into nanostructures. With further increase of the number of
laser shots, more nanostructures appear allowing excitation of more
localized and propagating surface plasmons. The produced
nanoroughness includes nanorods, nanocones, and nanospheres, and
these nanostructures will excite propagating cylindrical surface
plasmons that subsequently interfere with the incident light. This
interference causes the formation of intermediate periodic surface
microstructures. As the number of laser shots increases, the
intermediate microstructures will grow as well as the area occupied
by these structures. The developed intermediate periodic surface
microstructures will further excite propagating plane surface
plasmons that interfere with the plane incident laser light wave,
and this interference will finally result in the permanent extended
periodic microstructures.
[0126] For normally incident linearly polarized light, the period d
of the surface grating formed due to the interference between the
incident laser light and the excited surface plasmon wave is given
by equation 1 as:
d=.lamda./.eta. (1)
with g|E, where .lamda. is the incident light wavelength,
.eta.=Re[.di-elect cons./(.di-elect cons.+1)].sup.1/2 is the real
part of the effective refractive index of the air-metal interface
for surface plasmons, .di-elect cons. is the dielectric constant of
the metal, g is the grating vector, and E is the electrical field
vector of the incident wave. For a plane vacuum-metal interface,
.eta. is calculated to be 1.0096 at .lamda.=800 nm for Pt
(.di-elect cons..sub.1=-15.5 and .di-elect cons..sub.2=23.5) and
1.022 for Au (.di-elect cons..sub.1=-23.4, .di-elect
cons..sub.2=1.55). Using Eq. (1), the grating period is found to be
0.79 .mu.m for Pt and 0.78 .mu.m for Au. However, the observed
period is 0.61 .mu.m for Pt (FIG. 1 (b)) and 0.58 .mu.m for Au
(FIG. 5). If we substitute these values of the observed period into
Eq. (1), we will have .eta.=1.31 for Pt and .eta.=1.38 for Au. To
explain this discrepancy, we note that the table values of
.di-elect cons..sub.1 and .di-elect cons..sub.2 for Pt and Au are
obtained from smooth surface and at room temperature, and therefore
these values may not be suitable when the metals are heated by
high-intensity femtosecond pulses and covered with nano- and
micro-structures. To better understand the high-intensity effects
on NC-LIPSS period, we performed a detailed study of LIPSS in
various locations within the damaged spots on metals. From these
data summarized in Table 1, we can see that the NC-LIPSS period
remains the same in the central and peripheral areas of an
irradiated spot despite the fact that the two locations have
different intensities due to the Gaussian beam profile. On the
other hand, the period of our NC-LIPSS decreases with increasing N
when the surface roughness grows while the light intensity remains
constant. Furthermore, the NC-LIPSS produced using a higher fluence
of 0.16 J/cm.sup.2 exhibit a similar period as that produced at
F=0.084 J/cm.sup.2. Our observations indicate that the
high-intensity effect on dielectric constant is not essential,
whereas the effects of surface morphology (nano- and
micro-roughness) are more dominant. It is known that surface
roughness causes an increase in the modulus of the surface plasmon
wave vector, and this will correspond to an increase in the real
part of the refractive index. According to Eq. (1), an increased
real part of the refractive index for propagating surface plasmons
will cause a reduced NC-LIPSS period, which agrees with our
experimental observation.
TABLE-US-00001 TABLE 1 Nanostructure-covered LIPSS period in
different areas of the irradiated spot on platinum at F = 0.084
J/cm.sup.2. LIPSS period (.mu.m) Number of shots Central area
Peripheral area 30 0.62 0.61 50 0.58 0.61 100 0.57 0.57 200 0.55
0.54 500 0.55 0.53
[0127] Under certain conditions we also produced a large number of
nanoprotrusions and nanocavities on a metal surface (see FIG. 22).
The nanostructures produced can greatly increase the effective
surface area, which may be of importance in many technological
applications, such as but not limited to producing better chemical
catalysts where a high surface-to-volume ratio is a crucial
factor.
Example 5
[0128] In this study we performed a femtosecond laser surface
treatment of titanium to help determine the potential of this
technology for surface structuring of titanium implants. We found
find that femtosecond laser processing produces a large variety of
nanostructures (nanopores, nanoprotrusions) with a size down to 20
nm, multiple parallel grooved surface patterns with a period on the
sub-micron level, microroughness in the range of 1-15 .mu.m with
various configurations, smooth surface with smooth
micro-inhomogeneities, and smooth surface with sphere-like
nanostructures down to 10 nm. Also, we have determined the optimal
conditions for producing these surface structural modifications.
Femtosecond laser treatment may produce a richer variety of surface
structures on titanium for implants and other biomedical
applications than long-pulse laser treatments.
[0129] Due to good biostability, biocompatibility, mechanical
performance, and long-term durability, titanium has been widely
used in a variety of biomedical applications such as dental and
orthopedic implants, and implantable electronic devices. In
numerous in vitro and in vivo studies, surface topography of
titanium implants has been shown to be important in enhancing
implant performance. It has been shown that both microstructures
and nanostructures influence biological processes at implant
interfaces. Various methods of implant surface structuring have
been studied in the past such as grit-blasting, chemical etching,
laser treatment, and the combinations of the various methods.
Recent studies have shown that laser processing of implant surfaces
provides both suitable surface topography and less surface
contamination as compared with other methods. Another advantage of
laser processing is that the technique is also suitable for
texturing of implants of more complicated shapes. In the past,
surface structures have been produced using long-pulse lasers,
including nanosecond Nd:YAG laser, copper vapor laser, nanosecond
excimer lasers, picosecond Nd:YAG laser, and sub-picosecond excimer
laser. Femtosecond lasers have advantages over nanosecond lasers in
aspects of higher precision, reduced heat-affected zone, and
smaller amount of debris around the ablated spot.
[0130] The effects of laser fluence and the number of applied
pulses on laser-induced surface topography in titanium are reported
herein. We found that a femtosecond laser produces a large variety
of nanostructures (nanopores, nanoprotrusions) with a size down to
20 nm, multiple parallel grooved surface patterns with a period on
the sub-micron level, microroughness in the range of 1-15 .mu.m
with various configurations, smooth surface with smooth
micro-inhomogeneities, and smooth surface with sphere-like
nanostructures down to 10 nm. Our results suggest that femtosecond
laser treatment can produce a richer variety of surface structures
on titanium for implants and other biomedical applications than
long-pulse laser treatments.
[0131] Commercially pure titanium flat plates with a dimension of
15.times.17.times.1.5 mm were used in our experiment. The plates
were mechanically polished using 0.1-.mu.m-grade aluminum oxide
powder and further cleaned with acetone. For surface texturing, we
used an amplified Ti:sapphire laser system that generates 65-fs
laser pulses with the pulse energy over 1 mJ at a 1 kHz repetition
rate with a central wavelength of 0.8 .mu.m. The laser beam is
horizontally polarized and was focused at normal incidence onto a
vertically standing titanium sample in air at a pressure of 1 atm.
For laser beam focusing, we used an achromatic lens with a focal
length of 20 cm. The laser fluence of the incident light was varied
by changing the distance between the focusing lens and the sample.
The diameter of laser-irradiated spots on the titanium sample was
varied from 100 to 1200 .mu.m. The number of laser shots, N,
applied to the sample was selected with an electromechanical
shutter. The surface structuring of titanium was studied following
the treatment with laser fluence of F=0.067, 0.084, 0.16, 0.35,
0.48, and 2.9 J/cm.sup.2 and the number of applied pulses, N, in
the range of 1-30,000. Following femtosecond laser treatment, the
topography of surface modifications was studied using a SEM.
[0132] As a reference, FIG. 26(a) shows a SEM image of the titanium
surface prior to laser irradiation. FIGS. 26(b) through 26(d)
demonstrate surface topography produced by femtosecond laser
processing at near-damage-threshold fluence of F=0.067 J/cm.sup.2
for different numbers of laser shots, where the characteristic
features are random nanopores and sphere-like nanoprotrusions with
the size down to about 15-20 nm. Laser-induced surface
nano-topography depends on both the number of applied pulses and
laser fluence. At higher fluence of F=0.084 J/cm.sup.2, the
nanoroughness produced is shown in FIGS. 27(a) through 27(d), where
the average size of the nanostructures at this higher fluence is
larger than those at lower fluence in FIG. 1. For N>10-15,
periodic ordering of surface nanoroughness begins to occur. FIGS.
28 and 29 show some typical periodic patterns for laser fluences
F=0.067 and 0.084 J/cm.sup.2, respectively. The period of the
grooves is about 0.53 .mu.m. These periodic patterns with
sub-micron periods are covered with nanoroughness, as shown in
detail in FIGS. 28(d) and 29(d). With increasing laser fluence, the
periodic patterns are less likely produced and microroughness
becomes a more dominant surface structure. FIG. 30 shows surface
topography produced following treatment at F=0.16 J/cm.sup.2 at
various N. At this middle-level laser fluence, pure nanoroughness
is observed only after one-shot laser processing (FIG. 30(a)). A
clear microscale roughness covered with nanoroughness develops
after 20-shot treatment (FIG. 30(b)). With further increasing N,
microroughness continues to develop with deepening of cavities
(FIG. 30(c)). At a large enough N, columnar surface
micro-structures covered with nanoroughness are seen in FIG. 30(d).
At higher laser fluence of F=0.35 J/cm.sup.2, a combination of
nano- and micro-structures is produced after only one laser shot,
as shown in FIG. 31(a). With increasing N, columnar microstructures
rapidly develop as the dominating structures (see FIGS.
31(b)-31(d)). When the laser fluence is increased to the level of
F=0.48 J/cm.sup.2, a different type of surface microstructures is
observed, as shown in FIGS. 32(a)-32(c). At this laser fluence and
for N>1000, a pore of the size of the focused laser beam can be
created. An example of such a pore with the diameter of 350 .mu.m
is shown in FIG. 32(d), where microstructures are also seen at
bottom of the pore. At the highest fluence used in our experiment,
F=2.9 J/cm.sup.2, one laser shot can produce surface melting over
the entire irradiated surface area, and resolidification of this
surface melt results in a smooth surface covered with some
micro-inhomogeneities as shown in FIG. 33(a). Following two-pulse
irradiation, an even smoother surface is seen in FIG. 33(b). A
magnified picture showing nanoscale features of such smooth
surfaces is shown in FIG. 33(c). A detail picture of the titanium
surface after four laser shots is shown in FIG. 33(d), where one
can see nanoscale structures as small as down to 10 nm. The smooth
surface is produced with a low number of laser shots (N<10). At
a larger N (N>10), micro-inhomogeneities develop rapidly and
eventually a crater of the size of the focused laser beam will be
formed.
[0133] It has been shown that implant surface topography is an
important factor affecting the behaviors of both proteins and cells
on implant surfaces. It is generally accepted that proteins
typically respond to surface structural features (pits, pillars,
steps) about 1-10 nm, while cells can be sensitive to structural
features on the scale of 15 nm-100 .mu.m. It was also found that
structured implants have a better mechanical interlocking of the
bone-implant interfaces than smooth implant surfaces due to an
increased surface area. Also, it has been reported that extended
parallel groove structures may cause cells to align and migrate
along the grooves, a contact guidance phenomenon. Our SEM study
shows that all of these types of surface textures can be produced
by femtosecond laser treatment.
[0134] Little work has been done on laser fabrication of surface
nanostructures on titanium. Our study shows that femtosecond laser
technique can produce a large variety of both pure nanostructures
(FIGS. 26(b)-26(d), 27(a)-27(d), and 33(c)) and various
combinations of micro- and nanostructures (FIGS. 28(d), 29(d), 30,
31). There are two types of pure nanostructures observed in our
experiment. The first type (FIGS. 26(b)-26(d), 27(a)-27(d)) is
produced at low laser fluence (near the damage threshold) and a low
number of laser shots; the size of these nanostructures is down to
20 nm. The second type (FIGS. 33(c) and 33(d)) is produced at high
fluence and low N when laser irradiation causes the surface to melt
uniformly over the entire irradiated area; the size of these
nanostructures is down to 10 nm.
[0135] Examination of shot-to-shot SEM images of surface topography
suggests the following mechanism for the formation of
nanostructures of the first type. It is seen from FIG. 26(d) that a
nanopore is always accompanied by a nearby nanoprotrusion,
indicating a nanoscale material relocation to an adjacent site.
This one-to-one nanoscale pores/protrusions relationship occurs
randomly over the laser spot, suggesting an initial non-uniform
laser energy deposition. When the incident laser fluence is close
to the laser damage threshold, spatial nonuniformity in the
deposited laser energy can produce a melt at localized nanoscale
sites within the irradiated spot. Once the localized nanoscale melt
has been formed, a high radial temperature gradient in a nanomelt
can induce a radial surface tension gradient that expels the liquid
to the periphery of the nanomelt. This can lead to the formation of
nanocavities and nanoprotrusions due to fast freezing of the
expelled liquid on the boundary with the solid state material.
These initially induced surface random nanostructures enhance the
absorption of laser light and facilitate further growth of surface
nanoroughness with increasing number of laser shots due to the
increased spatial non-uniform energy absorption. When laser fluence
is sufficiently high to produce ablation, particles will be ejected
from the nanomelts and produce a recoil pressure that squirts the
liquid metal outside of the nanomelt. It should be noted that for
multi-pulse ablation, the repeating vaporization and re-deposition
of nanoparticles back onto the surface can also promote surface
nanostructuring. As seen from FIGS. 26 and 27, the average size and
density of femtosecond laser-induced nanostructural features can be
controlled by varying both the laser fluence and number of laser
shots.
[0136] Mechanisms for the formation of nanostructures of the second
type cannot be straightforwardly derived from our SEM study. The
formation of these nanostructural features may be due to
redeposition of ablated nanoparticles back onto the irradiated
surface.
[0137] Multiple parallel grooved surface patterns for biomedical
applications are commonly produced using lithographic or laser
holographic techniques. However, fabrication of these type of
patterns on biomaterials using a single laser beam has not been
reported. Below we discuss the optimal conditions for producing
these structures and explain the physical mechanisms of their
formation.
[0138] Our study shows that optimal conditions for producing
periodic groove patterns on titanium are at near-damage-threshold
fluence and with the laser shot number in the range between 20 and
800. In the past, multiple parallel grooved surface patterns have
been produced by long-pulse lasers and are known as laser-induced
periodic surface structures (LIPSS). The formation of LIPSS on
metals is believed to result from the interference of the incident
laser light with the excited surface plasmon polaritons that result
in spatial periodic energy distribution on the surface. Usually,
LIPSS shows a regular groove structure with a period on the
incident laser wavelength scale and is oriented perpendicularly to
the polarization of the incident light. Our results of the
evolution of surface structural modifications on titanium suggest
the following mechanism for the formation of the observed LIPSS. In
our experiment, the first few laser shots produce sparsely and
randomly distributed nanostructures. It is known that propagating
cylindrical surface plasmons can be excited by coupling laser
energy into nanoroughness, and this can give rise to their
interference with the incident light. This interference will,
first, cause the formation of intermediate periodic surface
structures in localized areas of the irradiated spot. An example of
such intermediate periodic surface structure can be seen in FIG.
26(c). With further increasing number of laser shots, the number of
intermediate periodic structures will grow as well as the area
occupied by these structures. The developed intermediate periodic
surface structures will further excite propagating plane surface
plasmons and their interference with the plane incident laser light
wave will, finally, result in the permanent extended periodic
grating.
[0139] For linearly polarized incident laser light, the period d of
the surface grating formed due to the interference between the
incident laser light wave and the excited surface plasmon wave is
given by d=.lamda./(.eta..+-.sin .theta.) with g|E, where in this
equation .lamda. is the incident light wavelength,
.eta.=Re[.di-elect cons./(.di-elect cons.+1).sup.1/2 is the real
part of the effective refractive index of the air-metal interface
for surface plasmons, E is the dielectric constant of the metal,
.theta. is the laser light incidence angle, g is the grating
vector, and E is the electrical field vector of the incident wave.
The above equation shows that the period of laser-fabricated
grating can be varied by changing the laser wavelength, the
incidence angle, or the real part of the effective refractive
index. An important parameter affecting the cell behavior is known
to be groove depth, and this parameter in fabricating LIPSS can be
controlled by the number of applied laser shots.
[0140] A unique feature of the periodic groove structures produced
according to the embodiments herein is that both ridges and grooves
are covered with nanoroughness following femtosecond laser
treatment, in contrast to rectangular surface grooves fabricated
using lithography techniques that usually have smooth ridges and
rough floors.
[0141] Laser microtexturing of titanium has been studied in the
past using long-pulse lasers. It has been shown that laser
processing of implant surfaces provides both suitable surface
microstructures and the least surface contamination as compared
with other methods. As shown in FIGS. 30(b)-30(d), 31(a)-31(d),
32(a)-32(c), and 33(a), a rich variety of microstructures can be
produced by femtosecond laser treatment, and these structures can
be characterized as the following two types. The first type (see
FIGS. 30(b)-30(d), 31(a)-31(d), 32(a)-32(c)) is produced at the
middle levels of the laser fluence (F=0.16 and 0.35 J/cm.sup.2).
The characteristic size of this type of microroughness is in the
range of 1-15 .mu.m. Both the characteristic size and configuration
of the surface microroughness can be controlled by both laser
fluence and the number of applied shots. This type of
microroughness seems to be only produced by femtosecond laser
treatment. The second type of microroughness (see FIG. 33(a)) is
characterized by a smooth surface with smooth
micro-inhomogeneities. This type of microroughness is produced at
the highest laser fluence in our experiment (F=2.9 J/cm.sup.2) when
melting occurs over the entire irradiated area. If the melted
surface has some structural inhomogeneities, fast resolidification
of this melted surface may result in smooth micro-scale
roughness.
[0142] Some parts of implant surfaces may be required to be smooth.
Previously, nanosecond excimer lasers have been used for polishing
machined titanium implants, and effects of both polishing and
cleaning of the surfaces have been reported). Our study shows that
smoothed surface can be also obtained with femtosecond laser
treatment, as shown in FIG. 33(b).
[0143] It is known that open pores with a diameter in the range of
100-400 .mu.m can improve the strength of bone-implant interfaces.
Recently, long-pulse lasers have been used for fabricating 100-300
.mu.m pores on Ti6Al4V implants. Our study shows that pores of this
size can be produced with femtosecond laser treatment, as shown in
FIG. 32(d). We note that we can further produce various surface
structures on the pore bottom through femtosecond laser
treatment.
Example 6
[0144] Another application of femtosecond laser surface structuring
to produce the materials processing of the presently embodied
invention is to provide the controllable modification of the
optical properties of metals, where these optical properties range
from the UV to THz spectral range, and where the modifications may
be used to create various black, grayed, and colored metals.
[0145] As an example, FIG. 34 shows the % reflectance from 0.25 to
2.5 .mu.m of "black" aluminum obtained by the materials processing
methods of the present invention. In the visible this aluminum
appears pitch black as illustrated in FIG. 35(a).
[0146] By varying the materials processing parameters, we have also
produced aluminum that appears to be various shades of gray. Thus
in the case of the grayed aluminum as shown in FIG. 35(b), the
materials processing was performed at laser fluence F=7.9
J/cm.sup.2, a scanning speed of the laser beam across the surface
of the Al of v=1 mm/s, and translation between scanning lines S=100
.mu.m. The two gray shades of aluminum shown in FIG. 35(b) are
obtained by varying the laser pulse repetition rate (f=100 Hz for
the darker shade and 93 Hz for the lighter one). The spectral
reflectance of this darker gray aluminum sample is shown in FIG.
34.
[0147] In addition to producing various shades of gray as discussed
above, the materials processing methods of the present invention
can also produce colored metals; i.e., metals that appear to have a
particular color or that appear to have multiple colors.
[0148] To produce colored metals, two types of femtosecond laser
processing techniques were performed. The first technique involved
tailoring laser-induced surface random structures, while the second
technique produced femtosecond laser-induced periodic surface
structures (FLIPSS). The colored metals produced by the first
technique exhibit the same apparent color at various viewing
angles, while the colored metals produced by the second technique
exhibit different colors at different viewing angles due to a
grating effect.
[0149] FIG. 35(c) shows a picture of a colored aluminum sample
produced by the controlled tailoring of random surface roughness.
The aluminum appears golden in color because the tailored surface
structures preferentially enhance the absorption at blue and green
wavelengths. The spectral reflectance of the golden aluminum is
shown in FIG. 34.
[0150] Colored metals produced by the second technique, FLIPSS,
exhibit different colors at different viewing angles. FIG. 36 shows
various colors of an aluminum sample structured with FLIPSS under
experimental conditions of F=0.05 J/cm.sup.2, f=83 Hz, v=1 mm/s,
and S=100 .mu.m. The spectral reflectance of the color aluminum
structured with FLIPSS is shown in FIG. 34. Structuring with FLIPSS
can cause a polarization effect on the absorption of light that
provides an additional way for controlling the optical properties.
The size of the optically modified metal surface area can be as
small as a tightly focused laser spot; i.e. down to about 10 .mu.m,
or as large as desired by using a scanning laser beam (for example,
FIGS. 35 and 36 show samples with structurally modified area of
about 24 mm in diameter).
[0151] Given the additional advantages of laser processing such as
low contamination and capability to process complicated shapes, the
black, grayed, and colored metals created by femtosecond laser
surface structuring have numerous potential applications in such
areas including, but not limited to, photonics, plasmonics,
optoelectronics, stealth technology, thermal radiation sources,
solar cell absorbers, radiative heat transfer devices, infrared
sensing, biooptical devices, thermophotovoltaics, and
airborne/space borne devices.
[0152] While specific embodiments of the present invention have
been described herein, it will be appreciated by those skilled in
the art that many equivalents, modifications, substitutions, and
variations may be made thereto without departing from the spirit
and scope of the invention as defined in the appended claims.
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