U.S. patent application number 11/265876 was filed with the patent office on 2006-06-15 for structures for polarization and beam control.
Invention is credited to Xuegong Deng, Feng Liu, Jian Wang.
Application Number | 20060127830 11/265876 |
Document ID | / |
Family ID | 36584385 |
Filed Date | 2006-06-15 |
United States Patent
Application |
20060127830 |
Kind Code |
A1 |
Deng; Xuegong ; et
al. |
June 15, 2006 |
Structures for polarization and beam control
Abstract
In certain aspects, the invention features articles that include
a layer including a plurality of rows of a first material extending
along a first direction, the rows being spaced apart from each
other and a center of each adjacent row being separated by a
distance less than a wavelength .lamda. so that for radiation of
wavelength .lamda. propagating along a path through the layer, the
layer has a first effective index of refraction, n.sub.1, for the
radiation having a first polarization state and the layer has
second effective index of refraction, n.sub.2, for the radiation
having a second polarization state orthogonal to the first
polarization state, where n.sub.1 and n.sub.2 are different, where
a surface of the layer includes a plurality of trenches, the
trenches extending along a second direction and being spaced apart
from each other, where a center of each adjacent trench is
separated by a distance more than wavelength .lamda., and the
trenches are filled with a second material having a refractive
index, n.sub.3, different from n.sub.2.
Inventors: |
Deng; Xuegong; (Piscataway,
NJ) ; Wang; Jian; (Orefield, PA) ; Liu;
Feng; (Allentown, PA) |
Correspondence
Address: |
FISH & RICHARDSON PC
P.O. BOX 1022
MINNEAPOLIS
MN
55440-1022
US
|
Family ID: |
36584385 |
Appl. No.: |
11/265876 |
Filed: |
November 3, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60636303 |
Dec 15, 2004 |
|
|
|
Current U.S.
Class: |
431/188 |
Current CPC
Class: |
G02B 5/3083
20130101 |
Class at
Publication: |
431/188 |
International
Class: |
F23M 9/00 20060101
F23M009/00 |
Claims
1. An article, comprising: a layer including a plurality of rows of
a first material extending along a first direction, the rows being
spaced apart from each other and a center of each adjacent row
being separated by a distance less than a wavelength .lamda. so
that for radiation of wavelength .lamda. propagating along a path
through the layer, the layer has a first effective index of
refraction, n.sub.1, for the radiation having a first polarization
state and the layer has second effective index of refraction,
n.sub.2, for the radiation having a second polarization state
orthogonal to the first polarization state, where n.sub.1 and
n.sub.2 are different, where a surface of the layer includes a
plurality of trenches, the trenches extending along a second
direction and being spaced apart from each other, where a center of
each adjacent trench is separated by a distance more than
wavelength .lamda., and the trenches are filled with a second
material having a refractive index, n.sub.3, different from
n.sub.2.
2. The article of claim 1, wherein the article is configured so
that for radiation having wavelength .lamda. incident on the layer
along the path, the article transmits about 50% or more of the
incident radiation having the first polarization state along a
first direction and transmits about 50% or more of the incident
radiation having the second polarization state along one or more
directions non-parallel to the first direction.
3. The article of claim 2, wherein the article is configured to
diffract about 50% or more of radiation having wavelength .lamda.
and the second polarization state incident on the article along the
path into one or more non-zero diffraction orders.
4. The article of claim 3, wherein the article is configured to
diffract about 10% or less of radiation having wavelength .lamda.
and the first polarization state incident on the article along the
path into one or more non-zero diffraction orders.
5. The article of claim 3, wherein the article is configured to
transmit substantially all of the radiation having wavelength
.lamda. and the first polarization state incident on the article
along the path along the zero order diffraction direction.
6. The article of claim 1, wherein the center of each adjacent
trench is separated by a distance of about 2.lamda. or more.
7. The article of claim 1, wherein the center of each adjacent
trench is separated by a distance of about 20.lamda. or less.
8. The article of claim 1, wherein the center of each adjacent
trench is separated by a distance of about 1 micrometer or
more.
9. The article of claim 1, wherein the center of each adjacent
trench is separated by a distance of about 20 micrometers or
less.
10. The article of claim 1, wherein the rows of the first material
are periodically spaced in a direction orthogonal to the first
direction.
11. The article of claim 1, wherein the center of each adjacent row
of the first material is separated by a distance of about 400 nm or
less.
12. The article of claim 1, wherein the center of each adjacent row
of the first material is separated by a distance of about 200 nm or
less.
13. The article of claim 1, wherein the center of each adjacent row
of the first material is separated by a distance in a range from
about 70 nm to about 300 nm.
14. The article of claim 1, wherein the row of the first material
have a rectangular, trapezoidal, oval, or convex hull profile.
15. The article of claim 1, wherein the first and second directions
are non-parallel.
16. The article of claim 15, wherein the first and second
directions are substantially orthogonal to each other.
17. The article of claim 1, wherein the layer is form-birefringent
for radiation at wavelength .lamda. and n.sub.1 corresponds to
either the ordinary or extraordinary refractive index of the
layer.
18. The article of claim 1, wherein n.sub.3 is approximately equal
to n.sub.1.
19. The article of claim 1, wherein the first material is a
dielectric material.
20. The article of claim 1, wherein the first material comprises at
least one material selected from a group consisting of SiO.sub.2,
SiN.sub.x, Si, Al.sub.2O.sub.3, ZrO.sub.2, Ta.sub.2O.sub.5,
TiO.sub.2, HfO.sub.2, Nb.sub.2O.sub.5, and MgF.sub.2.
21. The article of claim 1, wherein the first material is a
nanolaminate material.
22. The article of claim 1, wherein the second material is a
dielectric material.
23. The article of claim 1, wherein the first material comprises at
least one material selected from a group consisting of SiO.sub.2,
SiN.sub.x, Si, Al.sub.2O.sub.3, ZrO.sub.2, Ta.sub.2O.sub.5,
TiO.sub.2, HfO.sub.2, Nb.sub.2O.sub.5, and MgF.sub.2.
24. The article of claim 1, wherein the second material is a
nanolaminate material.
25. The article of claim 1, wherein the surface including the
trenches has a rectangular, trapezoidal, oval, or convex hull
profile.
26. The article of claim 1, wherein .lamda. is in a range from
about 150 nm to about 5,000 nm.
27. The article of claim 1, wherein .lamda. is in a range from
about 400 nm to about 700 nm.
28. The article of claim 1, wherein .lamda. is in a range from
about 1,200 nm to about 1,700 nm.
29. The article of claim 1, wherein the layer includes a plurality
of rows of a third material extending along the first direction,
the rows of the third material alternating with the rows of the
first material and the first and third materials being
different.
30. The article of claim 1, wherein the third material has a
refractive index at .lamda. that is different from n.sub.1.
31. The article of claim 30, herein the third material is a
dielectric material.
32. The article of claim 1, wherein the first and second
polarization states are linear polarization states.
33. The article of claim 1, wherein the layer has a thickness, t,
that is about 1 micrometer or less.
34. The article of claim 1, wherein the trenches have a depth, d,
less than a thickness, t, of the layer.
35. The article of claim 1, further comprising a substrate that
supports the layer.
36. The article of claim 35, wherein the substrate is a planar
substrate.
37. The article of claim 35, wherein the substrate is comprises an
inorganic glass material.
38. The article of claim 35, wherein the substrate is substantially
transparent for radiation having wavelength .lamda..
39. The article of claim 35, further comprising an anti-reflection
film supported by the substrate.
40. An apparatus, comprising: a first element comprising the
article of claim 1; and a second element comprising the article of
claim 1, wherein the elements are configured so that the apparatus
splits an incident beam at wavelength .lamda. into a pair of beams
that emerge from the apparatus spatially separated from one another
and propagating along substantially parallel paths.
41. The article of claim 40, wherein the pair of beams are
substantially polarized orthogonal to each other.
42. An article, comprising: a layer comprising a plurality of rows
of a composite material alternating with rows of a second material,
the rows of the composite material and the rows of the second
material being arranged to form a diffraction grating, where the
diffraction grating has a period greater than a wavelength .lamda.
and the composite material is form-birefringent for radiation at
wavelength .lamda..
43. The article of claim 42, wherein the second material has a
refractive index at .lamda. approximately equal to either the
ordinary or extraordinary refractive index of the composite
material at .lamda..
44. An article, comprising: a polarizing beam splitter comprising a
layer of a material that is form birefringent for radiation having
a wavelength .lamda., wherein the polarizing beam splitter is
configured so that for radiation having wavelength .lamda. incident
on the polarizing beam splitter along a path, the polarizing beam
splitter transmits about 50% or more of the incident radiation
having a first polarization state along a first direction and
transmits about 50% or more of the incident radiation having a
second polarization state along one or more directions non-parallel
to the first direction, where the first and second polarization
states are orthogonal.
45. The article of claim 44, wherein the polarizing beam splitter
transmits about 80% or more of the incident radiation having the
first polarization state along the first direction.
46. The article of claim 44, wherein the polarizing beam splitter
transmits about 80% or more of the incident radiation having the
second polarization state along the one or more directions
non-parallel to the first direction.
47. The article of claim 44, wherein the polarizing beam splitter
transmits about 80% or more of the incident radiation having the
second polarization state along a single of the directions
non-parallel to the first direction.
48. The article of claim 44, wherein the layer of the material is
in the form of a diffraction grating for radiation having
wavelength .lamda..
49. The article of claim 48, wherein the first direction
corresponds to zeroth order diffraction of the diffraction
grating.
50. The article of claim 48, wherein the one or more directions
non-parallel to the first direction correspond to non-zero order
diffraction of the diffraction grating.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority under 35 U.S.C. .sctn.
119(e)(1) to U.S. Provisional Application No. 60/636,303, entitled
"MULTILAYER STRUCTURES FOR POLARIZATION AND BEAM CONTROL," and
filed on Dec. 15, 2004, the entire contents of which are
incorporated herein by reference.
TECHNICAL FIELD
[0002] This disclosure relates to optical devices and systems that
use optical devices.
BACKGROUND
[0003] Optical devices and systems are commonly used where
manipulation of light is desired. Examples of optical devices
include lenses, polarizers, optical filters, antireflection films,
optical retarders (e.g., waveplates), and beam splitters (e.g.,
polarizing and non-polarizing beam splitters). Sub-wavelength
structures may be used to control properties of optical beams such
as polarization. Articles that are sensitive to the polarization of
a beam may include sub-wavelength structure.
SUMMARY
[0004] In general, in one aspect, the invention features articles
that include a first layer including a plurality of rows of a first
material extending along a first direction, the rows being spaced
apart from each other and a center of each adjacent row being
separated by a distance less than a wavelength .lamda., and a
second layer supported by the first layer, the second layer
comprising a second material, wherein the first layer is configured
to transmit about 50% or more of radiation of wavelength .lamda.
having a first polarization state incident on the first layer along
a path and to specularly reflect about 80% or more of radiation of
wavelength .lamda. having a second polarization state incident on
the first layer along the path, the first and second polarization
states being orthogonal, and the second layer is configured so that
the article specularly reflects about 10% or less of the radiation
of wavelength .lamda. having the second polarization incident on
the article along the path, where the path intersects the first and
second layers.
[0005] Embodiments of the articles may include one or more of the
following features and/or features of other aspects.
[0006] The second layer can include a plurality of portions
including the second material, the portions being spaced apart from
each other and a center of each adjacent portion being separated by
a distance more than about .lamda. (e.g., more than about 2.lamda.,
more than about 5.lamda., more than about 10.lamda.). The second
layer can include a plurality of portions including the second
material, the portions being spaced apart from each other and a
center of each adjacent portion being separated by a distance less
than about 50.lamda.. (e.g., less than about 40.lamda., less than
about 30.lamda., less than about 20.lamda.). The plurality of
portions of the second material can extend along a second
direction. Portions that include the second material can be rows
that extend along the second direction. The rows that include the
second material can have a rectangular or trapezoidal profile. The
portions including the second material can be periodically spaced
in a direction perpendicular to the second direction.
[0007] The rows including the second material have a width of about
1 .mu.m or more (e.g., about 2 .mu.m or more, about 5 .mu.m or
more). In some embodiments, the rows including the second material
have a width of about 10 .mu.m or less (e.g., about 8 .mu.m or
less).
[0008] The center of adjacent portions can be separated by a
distance of about 1 .mu.m or more (e.g., about 2 .mu.m or more,
about 3 .mu.m or more, about 4 .mu.m or more, about 5 .mu.m or
more). In some embodiments, the center of adjacent portions are
separated by a distance of about 50 .mu.m or less (e.g., about 40
.mu.m or less, about 30 .mu.m or less, about 20 .mu.m or less).
[0009] The plurality of portions in the second layer can each
include a plurality of rows of the second material extending along
the first direction, the rows of the second material being spaced
apart from each other and a center of each adjacent row being
separated by a distance less than .lamda.. The center of each
adjacent row of the second material can be separated by a distance
of about 400 nm or less (e.g., about 300 nm or less, about 250 nm
or less, about 200 nm or less, about 180 nm or less, about 160 nm
or less). In some embodiments, the center of each adjacent row of
the second material is separated by a distance in a range from
about 70 nm to about 300 nm. The rows of the first material in the
first layer can be continuous with the rows of the second material
in the second layer.
[0010] The second layer can have a thickness of about 1,000 nm or
less (e.g., about 800 nm or less, about 600 nm or less, about 500
nm or less, about 400 nm or less, about 300 nm or less, about 200
nm or less). In certain embodiments, the second layer has a
thickness of about 10 nm or more (e.g., about 20 nm or more, about
30 nm or more, about 40 nm or more, about 50 nm or more, about 60
nm or more, about 70 nm or more, about 80 nm or more).
[0011] The first and second materials can be the same or different.
The second material can be a dielectric material. In some
embodiments, the second material includes at least one material
selected from a group including SiO.sub.2, SiN.sub.x, Si,
Al.sub.2O.sub.3, ZrO.sub.2, Ta.sub.2O.sub.5, TiO.sub.2, HfO.sub.2,
Nb.sub.2O.sub.5, and MgF.sub.2. In certain embodiments, the second
material is a metallic material. The second material can include at
least one metal selected from the group including Al, Au, Ag, Cr
and Cu.
[0012] Portions of the second layer between the portions including
the second material can include a third material different from the
second material. The third material can be a dielectric material.
The third material can be a nanolaminate material. In some
embodiments, the second material is a metallic material. The third
material can have a refractive index at .lamda. that is
approximately equal to an effective refractive index of the rows
comprising the second material for radiation of wavelength .lamda.
having the second polarization state propagating along the axis. In
certain embodiments, the second layer is a continuous layer.
[0013] The second layer can be configured so that the article
specularly reflects about 8% or less (e.g., about 5% or less, about
4% or less, about 3% or less, about 2% or less) of the radiation of
wavelength .lamda. having the second polarization incident on the
article along the path.
[0014] The rows of the first material can be periodically spaced in
a direction orthogonal to the first direction.
[0015] The first material can be a dielectric material. In some
embodiments, the first material includes at least one material
selected from a group including SiO.sub.2, SiN.sub.x, Si,
Al.sub.2O.sub.3, ZrO.sub.2, Ta.sub.2O.sub.5, TiO.sub.2, HfO.sub.2,
Nb.sub.2O.sub.5, and MgF.sub.2. The first material can be a
nanolaminate material.
[0016] In certain embodiments, the first material is a metallic
material. The first layer can include a plurality of rows of a
dielectric material extending along the first direction, the rows
of the dielectric material alternating with the rows of the first
material. The first material can include at least one metal
selected from the group including Al, Au, Ag, Cr and Cu.
[0017] The rows of the first material can have a width of about 200
nm or less (e.g., about 150 nm or less, about 100 nm or less, about
80 nm or less, about 60 nm or less, about 50 nm or less). In some
embodiments, the rows of the first material have width in a range
from about 50 nm to about 200 nm. Each adjacent rows of the first
material are separated by a trench having a width in a range from
about 50 nm to about 300 nm.
[0018] The first layer can have a thickness of about 10 nm or more
(e.g., about 20 nm or more, about 50 nm or more, about 70 nm or
more, about 100 nm or more). In certain embodiments, the first
layer has a thickness of about 1,000 nm or less (e.g., about 800 nm
or less, about 600 nm or less, about 500 nm or less, about 400 nm
or less). The first and second layers can have a combined thickness
of about 5 .mu.m or less (e.g., about 3 .mu.m or less, about 2
.mu.m or less, about 1 .mu.m or less, about 800 nm or less, about
500 nm or less).
[0019] The article can include a third layer between the first
layer and the second layer. The third layer can include the first
material. The rows of the first material in the first layer can be
continuous with the first material in the third layer. The first
and second layers can be separated by a distance of about 5 .mu.m
or less (e.g., about 3 .mu.m or less, about 2 .mu.m or less, about
1 .mu.m or less, about 800 nm or less, about 500 nm or less).
[0020] The first layer can be a continuous layer.
[0021] The rows of the first material can have a rectangular,
trapezoidal, oval, or convex hull profile.
[0022] The first layer can transmit about 80% or more (e.g., about
90% or more, about 95% or more) of radiation of wavelength .lamda.
having the first polarization state incident on the layer along the
path. The first the layer can block about 90% or more (e.g., about
95% or more, about 98% or more, about 99% or more) of radiation of
wavelength .lamda. having the second polarization state incident on
the layer along the path.
[0023] The first and second polarization states can be linear
polarization states. The first and second polarization states can
be TM and TE polarization states, respectively. .lamda. can be
between about 150 nm and about 5,000 nm (e.g., between about 400 nm
and about 700 nm, between about 1,200 nm and about 1,700 nm).
[0024] The first layer can transmit about 50% or more of radiation
of wavelength .lamda. having a first polarization state incident on
the layer along a path and the layer blocks about 80% or more of
radiation of wavelength .lamda.' having a second polarization state
incident on the layer along the path, wherein |.lamda.-.lamda.'| is
about 50 nm or more (e.g., about 100 nm or more, about 150 nm or
more, about 200 nm or more, about 250 nm or more, about 300 nm or
more).
[0025] The article can further include a substrate, wherein the
first layer is supported by the substrate. The substrate can be a
planar substrate. The substrate can include a layer of an inorganic
glass material. The article can further include an anti-reflection
film supported by the substrate.
[0026] In a further aspect, the invention features an apparatus
including a Faraday rotator and the article positioned relative to
the Faraday rotator so that the path intersects the Faraday
rotator. The Faraday rotator can rotate incident radiation having
the first polarization state propagating along the path by an
amount between about 30.degree. and about 60.degree. (e.g., by
about 45.degree.). The apparatus can include a polarizer, wherein
the Faraday rotator is positioned between the article and the
polarizer. In certain embodiments, the apparatus includes an
optical isolator including the Faraday rotator and the article. The
apparatus can also include a source (e.g., a laser source) of
radiation at .lamda..
[0027] In general, in another aspect, the invention features
articles that include a first layer supported by the substrate, the
first layer including a plurality of rows of a first material
extending along a first direction, the rows being spaced apart from
each other and a center of each adjacent row being separated by a
distance less than a wavelength .lamda., wherein the first layer is
configured to transmit about 50% or more of radiation of wavelength
.lamda. having a first polarization state incident on the first
layer along a path and to specularly reflect about 80% or more of
radiation of wavelength .lamda. having a second polarization state
incident on the first layer along the path, the first and second
polarization states being orthogonal, and a second layer supported
by the first layer, the second layer including a plurality of rows
that include a second material extending along a second direction,
the rows being spaced apart from each other and a center of each
adjacent row being separated by a distance more than .lamda.,
wherein .lamda. is in a range from about 150 nm to about 5,000
nm.
[0028] Embodiments of the articles can include one or more of the
following features and/or features of other aspects.
[0029] For example, the portions that include the second material
can extend along a second direction. The second direction can be
parallel to the first direction. The portions can be rows extending
along the second direction. The plurality of portions in the second
layer each can include a plurality of rows of the first material
extending along the first direction, the rows of first material
being spaced apart from each other and a center of each adjacent
row being separated by a distance less than .lamda..
[0030] In general, in another aspect, the invention features
articles that include a first layer including a plurality of rows
of a first material extending along a first direction, the rows
being spaced apart from each other and a center of each adjacent
row being separated by a distance less than a wavelength .lamda.,
and a second layer supported by the first layer, the second layer
including a plurality of portions that include a second material
extending along a second direction, the portions being spaced apart
from each other and a center of each adjacent portion being
separated by a distance more than .lamda., wherein the plurality of
portions in the second layer each include a plurality of rows of
the second material extending along the first direction, the rows
of the second material being spaced apart from each other and a
center of each adjacent row being separated by a distance less than
.lamda., and wherein .lamda. is in a range from about 150 nm to
about 5,000 nm. Embodiments of the articles can include one or more
of the features of other aspects.
[0031] In general, in a further aspect, the invention features
articles that include a first layer including a plurality of rows
of a first material extending along a first direction, the rows
being spaced apart from each other and a center of each adjacent
row being separated by a distance less than a wavelength .lamda.,
wherein the first layer is configured to transmit about 50% or more
of radiation of wavelength .lamda. having a first polarization
state incident on the first layer along a path and to specularly
reflect about 80% or more of radiation of wavelength .lamda. having
a second polarization state incident on the first layer along the
path, the first and second polarization states being orthogonal,
and a second layer supported by the first layer, the second layer
including a first plurality of rows that include a second material
and a second plurality of rows of a third material different from
the second material, where rows of the first plurality alternate
with rows of the second plurality and a center of each adjacent row
of the first plurality is separated by a distance more than
.lamda., wherein .lamda. is in a range from about 150 nm to about
5,000 nm. Embodiments of the articles can include one or more of
the features of other aspects.
[0032] In general, in another aspect, the invention features
articles that include a layer including a plurality of portions
that include a metallic material, the plurality of portions being
spaced apart from each other so each portion is spaced from an
adjacent portion by a distance in a range from about 1 .mu.m to
about 50 .mu.m, wherein the metallic material in each portion is
arranged in a plurality of rows extending along a first direction,
the metallic rows being spaced apart from each other and a center
of each row being spaced about 500 nm or less from an adjacent
row.
[0033] Embodiments of the articles can include one or more of the
following features and/or features of other aspects.
[0034] For example, spaces between adjacent portions can be filled
with a first dielectric material. Spaces between adjacent metallic
rows can be filled with a second dielectric material. The first and
second dielectric materials can be the same. The first dielectric
material can have a refractive index at a wavelength .lamda. that
is approximately equal to an effective refractive index of the
portions of metallic material for radiation of wavelength .lamda.
having being polarized parallel to the first direction, where
.lamda. is in a range from about 150 nm to about 5,000 nm. The
first dielectric material can be a nanolaminate material.
[0035] In general, in a further aspect, the invention features
methods that include forming a layer including a plurality of rows
of a first material extending along a first direction, the rows
being spaced apart from each other and a center of each adjacent
row being separated by a distance less than a wavelength .lamda.,
and removing portions of the rows of the first material to form a
plurality of trenches in the layer, the trenches having a width of
about .lamda. or more, where .lamda. is in a range from about 150
nm to about 5,000 nm.
[0036] Implementations of the methods can include one or more of
the following features and/or features of other aspects.
[0037] The method can further include depositing a second material
in the plurality of trenches. The second material can be deposited
using atomic layer deposition. The first and second materials can
be different. The first material can be a metallic material and the
second material can be a dielectric material.
[0038] The portions of the first layer can be removed by etching
(e.g., reactive ion etching) the first layer. The first material
can be a metallic material.
[0039] The trenches can have a depth less than a thickness of the
layer. The trenches can be periodically spaced along at least one
direction. The trenches can be periodically spaced along a
direction perpendicular to the first direction.
[0040] In general, in a further aspect, the invention features
articles that include a layer including a plurality of rows of a
first material extending along a first direction, the rows being
spaced apart from each other and a center of each adjacent row
being separated by a distance less than a wavelength .lamda. so
that for radiation of wavelength .lamda. propagating along a path
through the layer, the layer has a first effective index of
refraction, n.sub.1, for the radiation having a first polarization
state and the layer has second effective index of refraction,
n.sub.2, for the radiation having a second polarization state
orthogonal to the first polarization state, where n.sub.1 and
n.sub.2 are different, and a plurality of trenches formed in a
surface of the layer, the trenches extending along a second
direction and being spaced apart from each other, where a center of
each adjacent trench is separated by a distance more than
wavelength .lamda., and the trenches are filled with a second
material having a refractive index, n.sub.3, different from
n.sub.2.
[0041] Embodiments of the articles can include one or more of the
following features and/or features of other aspects.
[0042] For example, the article can be configured so that for
radiation having wavelength .lamda. incident on the layer along the
path, the article transmits about 50% or more of the incident
radiation having the first polarization state along a first
direction and transmits about 50% or more of the incident radiation
having the second polarization state along one or more directions
non-parallel to the first direction. The articles can be configured
to diffract about 50% or more of radiation having wavelength
.lamda. and the second polarization state incident on the article
along the path into one or more non-zero diffraction orders. The
article can be configured to diffract about 10% or less (e.g.,
about 8% or less, about 5% or less) of radiation having wavelength
.lamda. and the first polarization state incident on the article
along the path into one or more non-zero diffraction orders. The
article can be configured to transmit substantially all of the
radiation having wavelength .lamda. and the first polarization
state incident on the article along the path along the zero order
diffraction direction.
[0043] The center of each adjacent trench can be separated by a
distance of about 2.lamda. or more (e.g., about 3% or more, about
4.lamda. or more, about 5% or more). The center of each adjacent
trench can be separated by a distance of about 20.lamda. or less
(e.g., about 15.lamda. or less, about 10.lamda. or less).
[0044] The center of each adjacent trench can be separated by a
distance of about 0.5 .mu.m or more (e.g., about 1 .mu.m or more,
about 2 .mu.m or more, about 3 .mu.m or more, about 5 .mu.m or
more). In some embodiments, the center of each adjacent trench is
separated by a distance of about 20 .mu.m or less (e.g., about 15
.mu.m or less, about 10 .mu.m or less, about 8 .mu.m or less).
[0045] The rows of the first material can be periodically spaced in
a direction orthogonal to the first direction. The center of each
adjacent row of the first material can be separated by a distance
of about 400 nm or less (e.g., about 300 nm or less, about 200 nm
or less, about 150 nm or less). In some embodiments, the center of
each adjacent row of the first material is separated by a distance
in a range from about 70 nm to about 300 nm.
[0046] The row of the first material can have a rectangular,
trapezoidal, oval, or convex hull profile.
[0047] The first and second directions can be non-parallel. For
example, the first and second directions can be substantially
orthogonal to each other.
[0048] The layer can be form-birefringent for radiation at
wavelength .lamda. and n.sub.1 can correspond to either the
ordinary or extraordinary refractive index of the layer. n.sub.3
can be approximately equal to n.sub.1.
[0049] The first material can be a dielectric material. In some
embodiments, the first material can include at least one material
selected from a group that includes SiO.sub.2, SiN.sub.x, Si,
Al.sub.2O.sub.3, ZrO.sub.2, Ta.sub.2O.sub.5, TiO.sub.2, HfO.sub.2,
Nb.sub.2O.sub.5, and MgF.sub.2. The first material can be a
nanolaminate material.
[0050] The second material can be a dielectric material. In some
embodiments, the second material includes at least one material
selected from a group consisting of SiO.sub.2, SiN.sub.x, Si,
Al.sub.2O.sub.3, ZrO.sub.2, Ta.sub.2O.sub.5, TiO.sub.2, HfO.sub.2,
Nb.sub.2O.sub.5, and MgF.sub.2. The second material can be a
nanolaminate material.
[0051] The surface including the trenches can have a rectangular,
trapezoidal, oval, or convex hull profile.
[0052] .lamda. can be in a range from about 150 nm to about 5,000
nm (e.g., from about 400 nm to about 700 nm, from about 1,200 nm to
about 1,700 nm).
[0053] The layer can include a plurality of rows of a third
material extending along the first direction, the rows of the third
material alternating with the rows of the first material and the
first and third materials being different. The third material can
have a refractive index at .lamda. that is different from n.sub.1.
The third material can be a dielectric material.
[0054] The first and second polarization states can be linear
polarization states.
[0055] The layer can have a thickness, t, that is about 1 .mu.m or
less (e.g., e.g., about 800 nm or less, about 600 nm or less, about
500 nm or less, about 400 nm or less). The trenches can have a
depth, d, less than a thickness, t, of the layer.
[0056] The article can include a substrate that supports the layer.
The substrate can be a planar substrate. The substrate can include
an inorganic glass material. In some embodiments, the substrate is
substantially transparent for radiation having wavelength
.lamda..
[0057] The article can include an anti-reflection film supported by
the substrate.
[0058] In another aspect, the invention features apparatus that
include a first element comprising an article and a second element
comprising an article wherein the elements are configured so that
the apparatus splits an incident beam at wavelength .lamda. into a
pair of beams that emerge from the apparatus spatially separated
from one another and propagating along substantially parallel
paths. The pair of beams can be substantially polarized orthogonal
to each other.
[0059] In general, in a further aspect, the invention features
articles that include a layer including a plurality of rows of a
composite material alternating with rows of a second material, the
rows of the composite material and the rows of the second material
being arranged to form a diffraction grating, where the diffraction
grating has a period greater than a wavelength .lamda. and the
composite material is form-birefringent for radiation at wavelength
.lamda..
[0060] Embodiments of the articles can include one or more of the
following features and/or features of other aspects. For example,
the second material can have a refractive index at .lamda.
approximately equal to either the ordinary or extraordinary
refractive index of the composite material at .lamda..
[0061] In general, in another aspect, the invention features
articles that include a polarizing beam splitter including a layer
of a material that is form birefringent for radiation having a
wavelength .lamda., wherein the polarizing beam splitter is
configured so that for radiation having wavelength .lamda. incident
on the polarizing beam splitter along a path, the polarizing beam
splitter transmits about 50% or more of the incident radiation
having a first polarization state along a first direction and
transmits about 50% or more of the incident radiation having a
second polarization state along one or more directions non-parallel
to the first direction, where the first and second polarization
states are orthogonal.
[0062] Embodiments of the articles can include one or more of the
following features and/or features of other aspects.
[0063] The polarizing beam splitter can transmit about 80% or more
of the incident radiation having the first polarization state along
the first direction. The polarizing beam splitter can transmit
about 80% or more of the incident radiation having the second
polarization state along the one or more directions non-parallel to
the first direction. The polarizing beam splitter can transmit
about 80% or more of the incident radiation having the second
polarization state along a single of the directions non-parallel to
the first direction.
[0064] The layer of the material can be in the form of a
diffraction grating for radiation having wavelength .lamda.. The
first direction can correspond to zeroth order diffraction of the
diffraction grating. The one or more directions non-parallel to the
first direction can correspond to non-zero order diffraction of the
diffraction grating.
[0065] Among other advantages, embodiments can provide wire-grid
polarizers that have reduced specular reflectivity of block state
radiation. The reduction in specular reflectivity of block state
radiation can be achieved without substantially affecting the
polarizer's transmission of the pass state radiation. In some
embodiments, the wire-grid polarizers can operate in the visible
portion of the electromagnetic spectrum. The wire-grid polarizers
can be broadband polarizers.
[0066] Embodiments also include transmissive polarizing beam
splitters formed using a form-birefringent medium. The polarizing
beam splitters operate by transmitting orthogonal polarization
components of an incident beam along different paths.
[0067] Embodiments such as wire-grid polarizers and/or transmissive
polarizing beam splitters can have a relatively compact
form-factor, being formed as thin films on a substrate. Embodiments
include monolithic layers having diffractive and/or sub-wavelength
structure, providing mechanically robust devices. Furthermore,
additional layers can be formed over the monolithic layers,
providing compound devices. Large numbers of small components may
be efficiently manufactured by, e.g., forming a large area device
and dicing it into many smaller devices.
[0068] Unless otherwise defined, all technical and scientific terms
used herein have the same meaning as commonly understood by one of
ordinary skill in the art to which this invention belongs. Other
features and advantages of the invention will be apparent from the
description and drawings, and from the claims.
DESCRIPTION OF DRAWINGS
[0069] FIG. 1A is a cross-sectional view of an embodiment of a
reflective polarizer that includes a sub-wavelength grating layer
and a diffractive layer.
[0070] FIG. 1B is a cross-sectional view of a portion of the
sub-wavelength grating layer and the diffractive layer of the
reflective polarizer shown in FIG. 1A.
[0071] FIG. 2A is a cross-sectional view of an embodiment of a
transmissive polarizer that includes a sub-wavelength grating layer
and a diffractive layer.
[0072] FIG. 2B is a cross-sectional view of another embodiment of a
transmissive polarizer that includes a sub-wavelength grating layer
and a diffractive layer.
[0073] FIG. 3 is a perspective view of an embodiment of a polarizer
that includes a sub-wavelength grating layer and a diffractive
layer.
[0074] FIG. 4 is a plan view of another embodiment of a diffractive
layer.
[0075] FIG. 5 is a plan view of another embodiment of a diffractive
layer.
[0076] FIG. 6 is a plan view of yet another embodiment of a
diffractive layer.
[0077] FIG. 7 is a cross-sectional view of an embodiment of a
polarizer that includes a sub-wavelength grating layer and a
diffractive layer.
[0078] FIG. 8 is a cross-sectional view of an embodiment of a
polarizer that includes a sub-wavelength grating layer, a
diffractive layer, and another layer disposed between the
sub-wavelength grating layer and diffractive layer.
[0079] FIG. 9A is a perspective view of an embodiment of a
polarizer that includes a sub-wavelength grating layer and a
diffractive layer. FIGS. 9B and 9C are plan views of the
diffractive and sub-wavelength grating layers, respectively, of the
polarizer shown in FIG. 9A.
[0080] FIG. 10 is a schematic diagram of an embodiment of a
polarizer that includes two diffractive structures offset from one
another.
[0081] FIGS. 11A-I show steps in the manufacture of a
sub-wavelength structured layer.
[0082] FIG. 12 is a schematic diagram of an embodiment of a
walk-off polarizing beamsplitter.
[0083] FIG. 13 is a schematic diagram of an embodiment of a
polarizer device that includes a reflective polarizer.
[0084] FIG. 14 is a schematic diagram of an embodiment of a liquid
crystal display that includes a reflective polarizer.
[0085] FIG. 15 is a schematic diagram of an embodiment of an
optical isolator.
[0086] FIG. 16 is a schematic diagram of an embodiment of an
optical system including an optical isolator.
[0087] FIG. 17 is a plot showing a modeled transmission spectrum
for an example embodiment of a polarizer.
[0088] FIG. 18 is a plot showing a modeled reflection spectrum for
the example embodiment of a polarizer.
[0089] Like reference symbols in the various drawings indicate like
elements.
DETAILED DESCRIPTION
[0090] The disclosure generally relates to articles that are
sensitive to and can be used to control properties of
electromagnetic (EM) radiation, such as the polarization and/or
direction of beams incident on the articles. Examples of EM
radiation include the visible region of the EM spectrum, the
infrared region, the microwave region, the radiowave region, and/or
other regions. In some embodiments, the articles can be sensitive
to and/or can be used to control the properties of incident
radiation in more than one region of the EM spectrum. In general,
the articles can be used to control properties of incident
radiation at one or more wavelengths (including a wavelength
.lamda.), and typically include structural features that diffract
light at .lamda., as well as sub-wavelength structural features.
Generally, the structural features that diffract light at .lamda.
are of similar size or larger than .lamda.. In embodiments,
surfaces with diffractive structural features may be covered with a
material that reduces the optical effects of the diffractive
structural features for at least one polarization state of incident
radiation at .lamda..
[0091] Referring to FIGS. 1A and 1B, a reflective polarizer 10
includes a sub-wavelength grating layer 14 and a diffractive layer
16. Sub-wavelength grating layer 14 has alternating portions 20 and
22 which form a sub-wavelength structure for radiation at .lamda..
Portions 20 are formed from a material that is substantially
transmissive at .lamda. while portions 22 are formed from a
material that is substantially not transmissive at .lamda.. For
example, embodiments designed for operation in the visible or IR
regions, portions 20 may be formed from a dielectric material and
portions 22 can be formed from a metal. The sub-wavelength
structure of layer 14 gives rise to an optical anisotropy for
incident radiation at % (e.g., the layer has different optical
properties for radiation polarized along the x-axis compared to
radiation polarized along the y-axis). As a result, sub-wavelength
grating layer 14 linearly polarizes incident radiation 102 of
wavelength .lamda. propagating parallel to an axis 101,
corresponding to the z-axis of the Cartesian coordinate system
shown in FIGS. 1A and 1B. In other words, for radiation of
wavelength .lamda. incident on reflective polarizer 10 propagating
parallel to the z-axis, reflective polarizer 10 transmits a
relatively large amount of the component of incident radiation
plane-polarized in the x-z plane (referred to as pass state
polarization), shown as transmitted radiation 103, while blocking a
relatively large amount of the component plane-polarized in the y-z
plane (referred to as block state polarization), depicted at least
in part as reflected radiation 104. A layer transmits a relatively
large amount of a component of incident radiation if it transmits
about 60% or more (e.g., about 80% or more, about 90% or more,
about 95% or more, about 98% or more, about 99% or more) of the
incident component. A layer blocks a relatively large amount of a
component of incident radiation if it blocks about 60% or more
(e.g., about 80% or more, about 90% or more, about 95% or more,
about 98% or more, about 99% or more) of the incident
component.
[0092] Diffractive layer 16 includes alternating portions 15 and 18
which form a diffractive structure for radiation at .lamda..
Portions 20 and 22 extend from layer 14 into portions 15 of layer
16. Thus, portions 15 also have sub-wavelength structure and are
anisotropic for incident radiation at .lamda.. Portions 18, on the
other hand, are formed from a material that is isotropic at .lamda.
and also transmissive at .lamda.. As a result, optical properties
(e.g., refractive index) of portions 18 are the same for pass and
block state radiation.
[0093] The composition of portions 15 and 18 are selected so that
the refractive index of portions 18, n.sub.18, is substantially the
same as the refractive index of portions 15 for pass state
radiation at .lamda. (referred to as n.sub.P), but differs from the
refractive index of portions 15 for block state radiation at
.lamda. (referred to as n.sub.B). As a result, the super-wavelength
structure of layer 16 does not substantially affect incident pass
state radiation, but does affect incident block state radiation at
.lamda..
[0094] Generally, reflective polarizer 10 blocks a relatively large
amount of incident radiation at .lamda. having the block state
polarization by reflecting and/or absorbing block state radiation.
For example, reflective polarizer 10 can reflect about 80% or more
(e.g., about 90% or more, about 95% or more, about 98% or more,
about 99% or more) of incident radiation at .lamda. having the
block polarization state. When reflective polarizer 10 reflects a
relatively large amount block state radiation, absorption of the
block state radiation is relatively low. For example, block state
absorption by polarizer 10 can be about 10% or less (e.g., about 5%
or less, about 4% or less, about 3% or less, about 2% or less,
about 1% or less).
[0095] Due to the diffractive structure, layer 16 substantially
diffracts and/or scatters reflected block state radiation. This can
result in a substantially diffuse reflection of block state
radiation with reflective polarizer 10 specularly reflecting a
relatively small amount of block state radiation at .lamda.. For
example, reflective polarizer can specularly reflect about 20% or
less (e.g., about 15% or less, about 10% or less, about 8% or less,
about 5% or less, about 3% or less, about 2% or less, about 1% or
less) of the block state radiation.
[0096] In certain embodiments, reflective polarizer 10 can absorb a
relatively large amount of the incident radiation at .lamda. having
the block polarization state. For example, reflective polarizer 10
can absorb about 30% or more (e.g., about 40% or more, about 50% or
more) of the block state polarization. High absorption of block
state radiation can occur, for example, where the material forming
portions 22 has relatively high absorption at .lamda. (e.g.,
absorbs about 40% or more incident radiation at .lamda., about 50%
or more incident radiation at .lamda.).
[0097] In some embodiments, reflective polarizer 10 polarizes
radiation at more than one wavelength, such as for a continuous
band of wavelengths. For example, reflective polarizer 10 can
polarize radiation for a band of wavelengths about 50 nm or more in
width (e.g., about 100 nm wide or more, about 200 nm wide or more,
about 300 nm wide or more). In certain embodiments, reflective
polarizer 10 polarizes incident radiation across substantially the
entire visible portion of the electromagnetic spectrum (e.g., for
.lamda. from about 400 nm to about 700 nm). Alternatively,
reflective polarizer 10 can polarize radiation for substantially
the entire near infrared portion of the electromagnetic spectrum
(e.g., from about 700 nm to 2,000 nm). In certain embodiments,
reflective polarizer 100 polarizes radiation for substantially the
entire visible and near infrared portions of the electromagnetic
spectrum (e.g., from about 400 nm to about 2,000 nm).
[0098] Furthermore, while reflective polarizer 10 has been
described as polarizing incident radiation propagating parallel to
the z-axis (e.g., normally incident on the surface of polarizer
10), polarizer 10 can polarize radiation at .lamda. for radiation
at non-normal angles of incidence (i.e., for radiation incident on
reflective polarizer 10 propagating at an angle .theta. with
respect to the z-axis, where .theta. is non-zero). In certain
embodiments, reflective polarizer 10 can polarize radiation
incident at more than one angle of incidence, such as for a range
of incident angles. For example, in some embodiments, reflective
polarizer 10 polarizes radiation incident within a cone of incident
angles for .theta. of about 10.degree. or more (e.g., about
15.degree. or more, about 20.degree. or more). Note that for
non-normal incidence, the pass state corresponds to radiation
polarized parallel to the x-z plane, while the block state
corresponds to radiation polarized orthogonal to the x-z plane.
[0099] Reflective polarizer 10 can have a relatively high
extinction ratio, E.sub.T, for transmitted radiation at .lamda..
For transmitted radiation, the extinction ratio refers to the ratio
of pass state intensity at .lamda. to the block state intensity
transmitted by reflective polarizer 10. E.sub.T can be, for
example, about 30:1 or more (e.g., about 50:1 or more, about 100:1
or more, about 150:1 or more) at .lamda.. In certain embodiments
where block state transmission is relatively low, E.sub.T can be
very high, such as about 1000:1 or more.
[0100] Turning now to the structure of reflective polarizer 10,
portions 20 and 22 extend along the y-direction, forming a periodic
structure consisting of a series of alternating rows of materials
that are substantially transmissive at .lamda. and materials that
are substantially not transmissive at .lamda.. The rows
corresponding to portions 20 have a width .LAMBDA..sub.20 in the
x-direction, while the rows corresponding to portions 22 have a
width .LAMBDA..sub.22 in the x-direction. The sub-wavelength
grating period, .LAMBDA., equal to .LAMBDA..sub.20+.LAMBDA..sub.22,
is smaller than .lamda. and as a result radiation of wavelength
.lamda. interacts with sub-wavelength grating layer 14 without
encountering significant high-order diffraction that can occur when
radiation interacts with periodic structures. For reflective
polarizers that operate in the visible portion of the EM spectrum,
grating layer 14 is an example of a nanostructured layer.
[0101] In general, .LAMBDA..sub.20 can be about 0.2 .lamda. or less
(e.g., about 0.1 .lamda. or less, about 0.05 .lamda. or less, about
0.04 .lamda. or less, about 0.03 .lamda. or less, about 0.02
.lamda. or less, 0.01 .lamda. or less). For example, in some
embodiments, .LAMBDA..sub.20 is about 200 nm or less (e.g., about
150 nm or less, about 100 nm or less, about 80 nm or less, about 70
nm or less, about 60 nm or less, about 50 nm or less, about 40 nm
or less, about 30 nm or less). Similarly, .LAMBDA..sub.22 can be
about 0.2 .lamda. or less (e.g., about 0.1 .lamda. or less, about
0.05 .lamda. or less, about 0.04 .lamda. or less, about 0.03
.lamda. or less, about 0.02 .lamda. or less, 0.01 .lamda. or less).
For example, in some embodiments, .LAMBDA..sub.22 is about 200 nm
or less (e.g., about 150 nm or less, about 100 nm or less, about 80
nm or less, about 70 nm or less, about 60 nm or less, about 50 nm
or less, about 40 nm or less, about 30 nm or less). .LAMBDA..sub.20
and .LAMBDA..sub.22 can be the same as each other or different.
[0102] In general, .LAMBDA. is less than .lamda., such as about
0.5% or less (e.g., about 0.3 .lamda. or less, about 0.2 .lamda. or
less, about 0.1 .lamda. or less, about 0.08 .lamda. or less, about
0.05 .lamda. or less, about 0.04 .lamda. or less, about 0.03
.lamda. or less, about 0.02 .lamda. or less, 0.01 .lamda. or less).
In some embodiments, A is about 500 nm or less (e.g., about 300 nm
or less, about 200 nm or less, about 150 nm or less, about 130 nm
or less, about 100 nm or less, about 80 nm or less, about 60 nm or
less, about 50 nm or less, about 40 nm or less).
[0103] The duty cycle of sub-wavelength grating layer 14, given by
the ratio .LAMBDA..sub.22/.LAMBDA., can vary as desired. In some
embodiments, the duty cycle is about 50% or less (e.g., about 40%
or less, about 30% or less, about 20% or less). Alternatively, in
certain embodiments, the duty cycle is more than about 50% (e.g.,
about 60% or more, about 70% or more, about 80% or more).
[0104] The thickness, h.sub.14, of sub-wavelength grating layer 14
measured along the z-axis can vary as desired. In general, the
thickness of sub-wavelength layer 14 is selected based on the
desired optical properties of sub-wavelength grating layer 14 at
.lamda., and the effect of layer thickness on various optical
properties is discussed below. In some embodiments, h.sub.14 can be
about 50 nm or more (e.g., about 75 nm or more, about 100 nm or
more, about 125 nm or more, about 150 nm or more, about 200 nm or
more, about 250 nm or more, about 300 nm or more, about 400 nm or
more, about 500 nm or more, about 1,000 or more, such as about
2,000 nm).
[0105] The aspect ratio of grating layer thickness, h.sub.14, to
.LAMBDA..sub.20 and/or d to .LAMBDA..sub.22 can be relatively high.
For example h.sub.14:.LAMBDA..sub.111 and/or
h.sub.14:.LAMBDA..sub.112 can be about 2:1 or more (e.g., about 3:1
or more, about 4:1 or more, about 5:1 or more, about 8:1 or more,
about 10:1 or more).
[0106] Regarding the specific structure of diffractive layer 16,
portions 15 and 18 also extend along the y-direction, forming a
periodic structure consisting of a series of alternating rows,
where adjacent rows have different optical properties. As discussed
above, portions 18 are formed from a material that has a refractive
index, n.sub.18, substantially equal to the refractive index of
portions 15 for the pass state radiation at .lamda..
|n.sub.18-n.sub.P| can be, for example, about 0.001 or less (e.g.,
about 0.0001 or less, about 0.0002 or less, about 0.0001 or less,
about 0.00005 or less, about 0.00001 or less).
[0107] More generally, in embodiments, |n.sub.18-n.sub.P| can be
about 0.03 or less (e.g., about 0.2 or less, about 0.1 or less,
about 0.05 or less, about 0.01 or less).
[0108] The rows corresponding to portions 15 have a width W.sub.15
in the x-direction, while the rows corresponding to portions 18
have a width W.sub.18 in the x-direction. The diffractive grating
period, W, equal to W.sub.15+W.sub.18, is on the order of or larger
than about .lamda. and as a result layer 16 diffracts incident
block state radiation of wavelength .lamda..
[0109] In general, W.sub.15 is about 0.2 .lamda. or more (e.g.,
about 0.5 .lamda. or more, about .lamda. or more, about 2 .lamda.
or more, about 5 .lamda. or more). In some embodiments, W.sub.15 is
about 200 nm or more (e.g., about 400 nm or more, about 500 m or
more, about 750 nm or more, about 1,000 nm or more, about 1,500 nm
or more, about 2,000 nm or more). Similarly, W.sub.18 can be about
0.2 .lamda. or more (e.g., about 0.5 .lamda. or more, about .lamda.
or more, about 2 .lamda. or more, about 5 .lamda. or more). In some
embodiments, W.sub.18 is about 200 nm or more (e.g., about 400 nm
or more, about 500 nm or more, about 750 nm or more, about 1,000 mm
or more, about 1,500 nm or more, about 2,000 nm or more). W.sub.15
and W.sub.18 can be the same as each other or different.
[0110] In general, W is on the order of or larger than about
.lamda., such as about 1.5 .lamda. or more (e.g., about 2 .lamda.
or more, about 3 .lamda. or more, about 4 .lamda. or more, about 5
.lamda. or more, about 10 .lamda. or more, about 20 .lamda. or
more, about 30 .lamda.). In embodiments, .LAMBDA. is about 400 nm
or more (e.g., about 500 nm or more, about 600 nm or more, about
750 nm or more, about 1,000 nm or more, about 1,500 nm or more,
about 2,000 nm or more, about 3,000 nm or more, about 4,000 nm or
more, about 5,000 m or more, about 6,000 nm or more, about 8,000 nm
or more, about 10,000 nm or more, about 15,000 or more).
[0111] The duty cycle of diffractive layer, given by the ratio
W.sub.15/W, can vary. In some embodiments, the duty cycle is less
than about 50% (e.g., about 40% or less, about 30% or less, about
20% or less). Alternatively, in certain embodiments, the duty cycle
is more than about 50% (e.g., about 60% or more, about 70% or more,
about 80% or more).
[0112] The thickness, h.sub.16, of diffractive layer 16 measured
along the z-axis can vary as desired. Generally, the thickness of
diffractive layer 16 is selected based on the desired optical
properties of diffractive layer 16 at .lamda.. In some embodiments,
h.sub.16 can be about 50 nm or more (e.g., about 75 nm or more,
about 100 nm or more, about 125 nm or more, about 150 nm or more,
about 200 nm or more, about 250 nm or more, about 300 nm or more,
about 400 nm or more, about 500 nm or more, about 1,000 or more,
such as about 2,000 nm).
[0113] The aspect ratio of diffractive layer thickness, h.sub.16,
to W.sub.15 can be relatively high. For example h.sub.16/W.sub.15
can be about 2:1 or more (e.g., about 3:1 or more, about 4:1 or
more, about 5:1 or more, about 8:1 or more, about 10:1 or
more).
[0114] Together, sub-wavelength grating layer 14 and diffractive
layer 16 have a combined thickness that is relatively thin. For
example, the combined thickness of layers 14 and 16 may be about 5
.mu.m or less (e.g., about 2 .mu.m or less, about 1 .mu.m or less,
about 500 nm or less).
[0115] The composition of portions 20 and 22 are selected so that
polarizer 10 has desired polarizing properties, while the
composition of portions 18 are selected so that the portions have a
desired refractive index at .lamda. (e.g., so that n.sub.18 is
substantially the same as n.sub.P).
[0116] The compositions of portions 18, 20, and 22 are also
selected based compatibility with the processes used to manufacture
polarizer 10 and their compatibility with the materials used to
form other portions of polarizer 10.
[0117] Portions 18, 20, and/or 22 can include inorganic and/or
organic materials. Examples of inorganic materials include metals,
semiconductors, and inorganic dielectric materials (e.g., glass).
Examples of organic materials include organic polymers. As
discussed previously, in some embodiments, portions 18 and 20 are
formed from materials that are substantially transmissive at
.lamda. and can include one or more dielectric materials, such as
dielectric oxides (e.g., metal oxides), fluorides (e.g., metal
fluorides), sulphides, and/or nitrides (e.g., metal nitrides).
Examples of oxides include SiO.sub.2, Al.sub.2O.sub.3,
Nb.sub.2O.sub.5, TiO.sub.2, ZrO.sub.2, HfO.sub.2, SnO.sub.2, ZnO,
ErO.sub.2, Sc.sub.2O.sub.3, and Ta.sub.2O.sub.5. Examples of
fluorides include MgF.sub.2. Other examples include ZnS, SiN.sub.x,
SiO.sub.yN.sub.x, AlN, TiN, and HfN.
[0118] Portions 22 are formed from a material that is substantially
non-transmissive at .lamda.. In certain embodiments, portions 22
include a metal, such as Au, Ag, Al, Cr, and Cu. Portions 22 can be
formed from more than one metal (e.g., portions 22 can be formed
from a metal alloy).
[0119] A one millimeter thick sample of a substantially
transmissive material transmits about 80% or more (e.g., about 90%
or more, about 95% or more, about 98% or more, about 99% or more)
of radiation at .lamda. normally incident thereon. Examples of
substantially transmissive materials for visible and infrared
wavelengths include various dielectric materials, such as
SiO.sub.2.
[0120] A one millimeter thick sample of a substantially
non-transmissive material transmits about 1% or less (e.g., about
0.5% or less, about 0.1% or less, about 0.01% or less, about 0.001%
or less) of radiation at .lamda. normally incident thereon.
Substantially non-transmissive materials include materials that
reflect and/or absorb a relatively large amount of radiation at
.lamda.. Examples of non-transmissive materials for visible and
infrared wavelengths include various metals, such as Al, Au, Ag,
Cr, and Cu. Al and Ag are examples of materials that have high
reflectance across the visible portion of the electromagnetic
spectrum, while Au and Cu have high reflectance for the yellow and
red portions of the spectrum, while absorbing relatively more of
the shorter visible wavelengths (e.g., the green and blue
wavelengths).
[0121] In some embodiments, the compositions of portions 18, 20
and/or 22 have a relatively low absorption at .lamda., so that
sub-wavelength grating layer 14 has a relatively low absorption at
.lamda.. For example, sub-wavelength grating layer 14 can absorb
about 10% or less (e.g., about 5% or less, about 3% or less, about
2% or less, about 1% or less) of radiation at .lamda. incident on
reflective polarizer 10 propagating along axis 101.
[0122] Portions 18, 20 and/or 22 can be formed from a single
material or from multiple different materials. In some embodiments,
one or more of portions 18, 20, and 22 are formed from a
nanolaminate material, which refers to materials that are composed
of layers of at least two different materials and the layers of at
least one of the materials are extremely thin (e.g., between one
and about 10 monolayers thick). Optically, nanolaminate materials
have a locally homogeneous index of refraction that depends on the
refractive index of its constituent materials. Varying the amount
of each constituent material can vary the refractive index of a
nanolaminate. Examples of nanolaminate portions include portions
composed of SiO.sub.2 monolayers and TiO.sub.2 monolayers,
SiO.sub.2 monolayers and Ta.sub.2O.sub.5 monolayers, or
Al.sub.2O.sub.3 monolayers and TiO.sub.2 monolayers
[0123] Generally, portions 18, 20. and/or 22 can include
crystalline, semi-crystalline, and/or amorphous materials.
Typically, an amorphous material is optically isotropic and may
transmit radiation better than materials that are partially or
mostly crystalline. As an example, in some embodiments, both
portions 18 and/or 20 are formed from amorphous materials, such as
amorphous dielectric materials (e.g., amorphous TiO.sub.2 or
SiO.sub.2), while portions 22 are formed from a crystalline or
semi-crystalline material (e.g., crystalline or semi-crystalline
Si).
[0124] In general, the structure and composition of sub-wavelength
grating layer 14 and diffractive layer 16 are selected based on the
desired optical performance of reflective polarizer 10. Structural
parameters that affect the optical performance of linear polarizer
10 include, for example, h.sub.14, h.sub.16, .LAMBDA.,
.LAMBDA..sub.111, and .LAMBDA..sub.112, W, W.sub.15, W.sub.18.
Typically, varying a single parameter affects multiple different
performance parameters. For example, the overall transmission of
the polarizer at .lamda. can be varied by changing the relative
thickness of portions formed from a transmissive material, e.g.,
.LAMBDA..sub.20, to the thickness or portions formed from a
non-transmissive material, e.g., .LAMBDA..sub.22. However, while a
higher ratio .LAMBDA..sub.20/.LAMBDA..sub.22 may provide relatively
higher transmission of the pass state polarization, it also results
in higher transmission of the block state polarization, which
decreases E.sub.T. As a result, optimizing the polarizer's
performance involves trade offs between different performance
parameters and the polarizer's structure and composition is varied
depending on the desired performance for the polarizer's end use
application.
[0125] In general, to effectively polarize radiation at wavelength
.lamda., the period .LAMBDA. of the sub-wavelength grating layer
should be shorter than .lamda., such as about .lamda./4 or less
(e.g., about .lamda./6 or less, about .lamda./10 or less).
Moreover, for effective broadband performance, .LAMBDA. should be
shorter than the shortest wavelength in the wavelength band. For a
broadband polarizer in the visible spectrum, for example, .LAMBDA.
should be less than about 300 nm, such as about 200 nm or less
(e.g., about 150 nm or less, about 130 nm or less, about 110 nm or
less, about 100 nm or less, about 90 nm or less, about 80 nm or
less). Furthermore, to effectively reduce specular reflection of
the block state, the period W of the diffractive layer should be in
a range from about .lamda. to about 20.lamda. (e.g., from about
.lamda. to about 10.lamda.).
[0126] Typically, the reflectance of sub-wavelength grating layer
14 for block state radiation can be increased by forming at least
some of the portions from a material having a relatively high
reflectance at .lamda.. The reflectance of the polarize can also be
increased by increasing the relative size of the portions of
reflective material relative to the portions of transmissive
material. In other words, a larger duty cycle can provide increased
reflectance at .lamda.. However, this can also reduce pass state
transmission. Conversely, the transmission of sub-wavelength
grating layer 14 can be increased by reducing the duty cycle.
Typically, the duty cycle is in the range of about 20% to about
80%.
[0127] In some embodiments, E.sub.T can be increased by increasing
the depth of sub-wavelength grating layer 14, h.sub.14. Increasing
h.sub.14 can provide increased E.sub.T without substantially
reducing the amount of pass state transmission.
[0128] Furthermore, the extent to which block state radiation is
specularly or diffusely reflected can be varied by modifying the
thickness and/or duty cycle of diffractive layer 16.
[0129] Theoretical models can be used to assess the performance of
the sub-wavelength and/or diffractive layers and to determine
structure and composition that will provide desired optical
performance. For example, the performance of sub-wavelength grating
polarizers can be modeled using coupled-wave analysis as described
by J. J. Kuta et al. in the article entitled "Coupled-wave analysis
of lamellar metal transmission gratings for the visible and the
infrared," J. Opt. Soc. Am. A, Vol. 12, No. 5, pp. 1118-1127
(1995). Theory relating to the performance of diffractive gratings
is discussed, for example, in Diffraction Gratings and
Applications, by E. G Loewen and Evgeny Popov, Marcel Dekker, Inc.,
New York (1997).
[0130] Referring now to substrate 12, in general, the substrate
provides mechanical support to polarizer 10. Substrate 12 can be
formed from any material compatible with the manufacturing
processes used to produce retarder 12 that can support the other
layers. In certain embodiments, substrate 12 is transparent at
wavelength .lamda., transmitting substantially all radiation
impinging thereon at wavelength .lamda. (e.g., transmitting about
90% or more, about 95% or more, about 97% or more, about 99% or
more, about 99.5% or more). In certain embodiments, substrate 12 is
formed from a glass, such as BK7 (available from Abrisa
Corporation), borosilicate glass (e.g., pyrex available from
Corning), aluminosilicate glass (e.g., C1737 available from
Corning), or quartz/fused silica. In some embodiments, substrate 12
can be formed from a crystalline material, such as a non-linear
optical crystal (e.g., LiNbO.sub.3 or a magneto-optical rotator,
such as garnett) or a crystalline (or semicrystalline)
semiconductor (e.g., Si, InP, or GaAs). Substrate 12 can also be
formed from an organic material, such as a polymer (e.g., a
plastic).
[0131] While reflective polarizer 10 substantially blocks one
polarization state of incident light, certain articles that include
sub-wavelength and diffractive structure substantially transmit
both polarization states. Referring to FIG. 2A, an example of such
an article is transmissive polarizer 200. Structurally,
transmissive polarizer 200 is the same as reflective polarizer 10.
However, in this case, portions 22 are formed from a material that
is substantially transmissive at .lamda.. Thus, polarizer 210
substantially transmits all radiation 102 at .lamda. incident
thereon, but transmits orthogonal polarization states along
different paths.
[0132] The substantially transmissive material forming portions 22
has a different refractive index at .lamda. from the material
forming portions 20. For example, |n.sub.20-n.sub.22| can be about
0.01 or more (e.g., about 0.02 or more, about 0.05 or more, about
0.1 or more, about 0.15 or more, about 0.2 or more, about 0.25 or
more, about 0.3 or more), wherein n.sub.20 and n.sub.22 are the
refractive indices of the materials forming portions 20 and 22,
respectively.
[0133] As a result of the sub-wavelength structure, layer 14 is
form-birefringent for radiation at .lamda. with the radiation
polarized in the x-z plane having a different refractive index from
radiation polarized perpendicular to the x-z plane. As for
reflective polarizer 10, portions 18 are formed from a material
having a refractive index approximately equal to the refractive
index of portions 15 for radiation polarized perpendicular to the
x-z plane. Thus, the diffractive structure of layer 16 diffracts
radiation polarized perpendicular to the x-z plane into non-zero
order diffracted states (depicted as 204 in FIG. 2 and referred to
as "diffracted state" polarization), while it transmits radiation
polarized in the x-z plane in the zero-order diffracted state
(depicted as 203 in FIG. 2 and referred to as "pass state"
polarization).
[0134] In general, the diffracted state radiation can be diffracted
into one or more non-zero order diffracted states (e.g., into the
.+-.1, .+-.2, .+-.3 . . . diffracted states). The angular
dispersion of the non-zero order diffracted states, corresponding
to an angle .omega..sub.i for each state (where i corresponds to
the diffraction order and=.+-.1, .+-.2, .+-.3 . . . ), can vary
depending on the structure and composition of the diffractive
layer. In some embodiments, diffracted state radiation can be
dispersed into relatively high angles. For example, .omega..sub.i
can be about 20.degree. or more (e.g., about 30.degree. or more,
about 45.degree. or more). Alternatively, in certain embodiments,
.omega..sub.i can be relatively small (e.g., about 10.degree. or
less, about 5.degree. or less, such as about 3.degree.).
[0135] In some embodiments, transmissive polarizer 200 directs a
relatively large amount of the incident diffracted state
polarization in a single direction (e.g., into the +1 or -1
diffracted order). For example, transmissive polarizer 200 can
direct about 50% or more (e.g., about 60% or more, about 70% or
more, about 80% or more, about 90% or more, about 95% or more,
about 98% or more, about 99% or more) of the diffracted state
polarization in a single direction. In some embodiments, the
structure of transmissive polarizer 200 is arranged so that the
polarizer is at blazing conditions at the wavelength .lamda..
Blazing is discussed, for example, by D. A. Buralli and G. M.
Morris in the article, "Effects of diffraction efficiency on the
modulation transfer function of diffractive lenses," Appl. Opt. 31,
4389-4396, (1992).
[0136] While n.sub.P is substantially the same as n.sub.18 in the
reflective polarizer and transmissive polarizer described above, in
certain embodiments n.sub.P and n.sub.18 need not be the same. For
example, in some embodiments diffraction of pass state radiation
may be desired, in which case portions 18 can be formed from a
material that has a different refractive index than n.sub.P. In
some embodiments of reflective polarizers, specular reflection of
block state radiation in addition to diffraction of pass state
radiation may be desired. Accordingly, portions 18 can be formed
from a material that has a refractive index substantially equal to
n.sub.B at .lamda..
[0137] Furthermore, while the material used to form portions 18 is
optically isotropic for reflective polarizer 10 described above,
anisotropic materials can also be used. For example, in some
embodiments, portions 18 can be formed from materials that are
birefringent at .lamda.. Examples include using form birefringent
materials (e.g., materials with sub-wavelength structure), liquid
crystalline materials, and/or anisotropic crystalline
materials).
[0138] In polarizers 10 and 200, elongated rows 20 and 22 have a
rectangular profile. More generally, these rows can have profiles
with different shapes. For example, in some embodiments, elongated
rows 20 and 22 can have a trapezoidal, triangular (also referred to
as "saw-tooth"), or curved (e.g., oval, convex hull or sinusoidal)
profile.
[0139] Similarly, the diffractive layer can also have a
non-rectangular profile. For example, in some embodiments,
diffractive layer 16 can have a trapezoidal, triangular, or curved
(e.g., oval, convex hull or sinusoidal) profile. An example of a
transmission-only polarizer having a diffractive layer with a
triangular profile is shown in FIG. 2B. Polarizer 210 includes a
diffractive layer 216 that includes isotropic portions 218 and
form-birefringent portions 215, that both extend along the
y-direction. Portions 218 and 215 form a grating with a triangular
profile, with portions 18 corresponding to a series of filled,
V-shaped, trenches.
[0140] In the polarizers described above, the rows forming the
diffractive and sub-wavelength structure extend parallel to each
other. In general, however, the relative orientation of the
sub-wavelength and diffractive structures can vary as desired. In
some embodiments, for example, the rows forming the diffractive and
sub-wavelength structure can extend along orthogonal directions.
For example, referring to FIG. 3, a polarizer 310 can include a
sub-wavelength grating 314 and a layer 316 having a diffractive
structure that includes portions 324 and 326 extending along one
direction (the x-direction), where portions 324 include a
sub-wavelength structure consisting of rows extending along a
different direction (the y-direction).
[0141] Furthermore, sub-wavelength grating layers can include more
than two alternating portions. For example, layer 314 includes a
periodic structure that includes three repeating portions 320, 321,
and 322. In general, sub-wavelength grating layers can include
periodic structures that have more than three portions (e.g., four
repeating portions, five repeating portions, six repeating
portions). For example, one or more portions may be included for
enhancing adhesion between the other portions. For example, Cr
portions may be used to enhance adhesion of a structure that
includes Au portions and SiO.sub.2 portions. As another example,
one or more portions may be used to adjust the dispersion
properties of the grating. For example, portions of dielectric
materials with different dispersion properties can be used to
modify the dispersion properties of the grating.
[0142] In some embodiments, the diffractive layer may be periodic
in two dimensions. For example, FIG. 4 shows an embodiment of a
diffractive layer 416 that includes portions 424 surrounded by
portions 426. Portions 424 include rows of different materials
forming sub-wavelength structured regions, while portions 426 are
isotropic. Layer 416 is periodic in two non-orthogonal directions
(e.g., the y-direction and a direction non-parallel to the x- and
y-directions). Alternatively, diffractive structured layer 416 may
be periodic in two orthogonal directions, such as along the x- and
y-directions. FIG. 5 shows another example of a layer 516 having
diffractive structure that is periodic along two directions. Layer
516 includes portions 526 that have closed boundaries surrounded by
portions 524 which are sub-wavelength structured regions.
[0143] In some embodiments, the boundaries between different
portions in the diffractive structured layer may be curved. For
example, referring to FIG. 6, a diffractive structured layer 616
includes portions 624 that include sub-wavelength structured
regions periodic in the x-direction, and portions 626. The
boundaries between portions 624 and 626 extend generally along the
y-direction, but follow curved paths. The shapes of the boundaries
can repeat, such that diffractive structured layer 616 is periodic
in both the x- and y-directions.
[0144] While sub-wavelength grating layer 14 in polarizers 10 and
200 is periodic in the x-direction with period .LAMBDA., more
generally, other structures can also be used. For example, the
period of the sub-wavelength grating layer may vary in different
portions of the layer (e.g., the grating can be a chirped grating).
Alternatively, the spacing between adjacent portions in layer 14
can vary randomly.
[0145] Similarly, diffractive layer 16 can have a varying period
(e.g., a chirped grating structure). In some embodiments, portions
18 can be randomly distributed through the layer. For example,
portions 18 can be distributed so that layer 16 substantially
scatters, rather than diffracts, incident block/diffracted state
radiation.
[0146] While the polarizers described above include a
sub-wavelength grating layer in addition to a layer that has both
sub-wavelength and diffractive structures, in some embodiments,
polarizers can be formed without the sub-wavelength layer (e.g.,
without layer 14 in polarizers 10 and 200). Moreover, in general,
embodiments of polarizers can include one or more layers in
addition to the sub-wavelength and diffractive layers. For example,
referring to FIG. 7, a polarizer 700 includes an etch stop layer
720, a cap layer 722, and antireflection films 724 and 726 in
addition to a substrate 712, a sub-wavelength grating layer 714 and
diffractive layer 716. Diffractive layer 716 includes portions
718.
[0147] Etch stop layer 720 is formed from a material resistant to
etching processes used to etch a material(s) from which the
sub-wavelength layer is formed (see discussion below). The
material(s) forming etch stop layer 720 should also be compatible
with substrate 712 and with the materials forming sub-wavelength
grating layer 714. Examples of materials that can form etch stop
layer 720 include HfO.sub.2, SiO.sub.2, Ta.sub.2O.sub.5, TiO.sub.2,
SiN.sub.x, or metals (e.g., Cr, Ti, Ni).
[0148] The thickness of etch stop layer 720 can be varied as
desired. Typically, etch stop layer 720 is sufficiently thick to
prevent significant etching of substrate 712, but should not be so
thick as to adversely impact the optical performance of polarizer
700 significantly. In some embodiments, etch stop layer is about
500 nm or less (e.g., about 250 nm or less, about 100 nm or less,
about 75 nm or less, about 50 nm or less, about 40 nm or less,
about 30 nm or less, about 20 nm or less).
[0149] Cap layer 722 is typically formed from the same material(s)
as portions 718 and provides a surface 721 onto which additional
layers, such as the layers forming antireflection film 724, can be
deposited. Surface 721 can be substantially planar.
[0150] Antireflection films 724 and 726 can reduce the reflectance
of pass state radiation of wavelength .lamda. impinging on and
exiting polarizer 700. Antireflection film 724 and 726 generally
include one or more layers of different refractive index. As an
example, one or both of antireflection films 724 and 726 can be
formed from four alternating high and low index layers. The high
index layers can be formed from TiO.sub.2 or Ta.sub.2O.sub.5 and
the low index layers can be formed from SiO.sub.2 or MgF.sub.2. The
antireflection films can be broadband antireflection films or
narrowband antireflection films.
[0151] In some embodiments, polarizer 700 has a reflectance of
about 20% or less (e.g., about 10% or less, about 5% or less, about
2% or less, about 1% or less, about 0.5% or less, about 0.2% or
less, about 0.1% or less, about 0.02% or less) of radiation
impinging thereon at wavelength .lamda. for pass state
polarization.
[0152] Referring to FIG. 8, in some embodiments, a layer can be
disposed between the sub-wavelength grating layer and the
diffractive layer. A polarizer 800 includes a substrate 812, a
sub-wavelength grating layer 814, and a diffractive layer 816.
Diffractive layer 816 includes portions 18 formed from an
homogenous material and portions 815 that have a sub-wavelength
structure. Another layer 818 is disposed between sub-wavelength
grating layer and diffractive layer 816.
[0153] While the rows forming the sub-wavelength structure in the
sub-wavelength layer in polarizers 10 and 200 are continuous with
the sub-wavelength structure in the diffractive layer, other
structures are also possible. In some embodiments, such as in
polarizer 800, the sub-wavelength structure in sub-wavelength layer
is discontinuous with the sub-wavelength structure in the
diffractive layer. Moreover, the sub-wavelength structure in the
diffractive layer can have a different orientation, period, duty
cycle, and/or composition than the sub-wavelength structure in the
sub-wavelength grating layer.
[0154] For example, referring to FIGS. 9A-9C, a polarizer 900 has a
sub-wavelength grating layer 914 and a diffractive layer 916. Layer
914 is periodic in the x-direction, while layer 916 includes
portions 915 that have a sub-wavelength structure that is periodic
along another direction in the x-y plane. Layer 916 also includes
portions 918 that are formed from a homogeneous material. Referring
to FIG. 9B, in diffractive layer 916 portions 918 and 915, and the
alternating rows of material forming the sub-wavelength structure
of portions 915 extend along a direction .theta..sub.g with respect
to the x-axis. FIG. 9C shows the structure of sub-wavelength layer
914 which includes rows extending parallel to the y-axis.
[0155] In some embodiments, polarizers can include more than one
diffractive layer. For example, referring to FIG. 10, a polarizer
1000 has a sub-wavelength grating layer 1014 and diffractive layers
1016 and 1026. Diffractive layer 1016 includes alternating portions
1015 and 1018. Similarly, diffractive layer 1026 includes
alternating portions 1025 and 1028. Portions 1018 and 1028 are
formed from a material having a sub-wavelength structure. A cap
layer 1020 separates layer 1026 from layer 1014.
[0156] The relative spatial arrangement of the portions forming
diffractive layers 1016 and 1026 may vary as desired. For example,
as shown in FIG. 10, layers 1016 and 1026 are periodic in the
x-direction with the same period, but are spatially offset from one
another in the x-direction by one-half period. More generally,
layers 1026 and 1026, having the same period, may be offset from
one another by a desired amount in order to control performance
properties of polarizer 1000, such as the amount of radiation
having the pass state polarization that is transmitted by polarizer
1000.
[0157] Turning now to the fabrication of polarizers, in general, a
variety of techniques can be used to form such structures. In some
embodiments, a sub-wavelength grating layer can be prepared
initially, and subsequently a diffractive grating structure can be
formed in the sub-wavelength grating layer by etching trenches into
the surface of the sub-wavelength grating layer. Material can be
deposited onto the diffractive grating structure to fill in the
trenches corresponding to portions 18. FIGS. 11A-I show different
phases of an example of a preparation process for a sub-wavelength
structured layer. The procedures may be repeated, albeit using a
mask corresponding to a diffractive grating structure, to form the
diffractive grating structure in the sub-wavelength grating
layer.
[0158] Referring to FIG. 11A, initially, substrate 1140 is
provided. Surface 1141 of substrate 1140 can be polished and/or
cleaned (e.g., by exposing the substrate to one or more solvents,
acids, and/or baking the substrate).
[0159] Referring to FIG. 11B, etch stop layer 1130 is deposited on
surface 1141 of substrate 1140. The material forming etch stop
layer 1130 can be formed using one of a variety of techniques,
including sputtering (e.g., radio frequency sputtering),
evaporating (e.g., electron beam evaporation, ion assisted
deposition (LAD), electron beam evaporation), or chemical vapor
deposition (CVD) such as plasma enhanced CVD (PECVD), atomic layer
deposition (ALD), or by oxidization. As an example, a layer of
HfO.sub.2 can be deposited on substrate 1140 by IAD electron beam
evaporation.
[0160] Referring to FIG. 11C, an intermediate layer 1210 is then
deposited on surface 1131 of etch stop layer 1130. Portions 1112
are etched from intermediate layer 1210, so intermediate layer 1210
is formed from the material used for portions 1112. The material
forming intermediate layer 1210 can be deposited using one of a
variety of techniques, including sputtering (e.g., radio frequency
sputtering), evaporating (e.g., election beam evaporation), or
chemical vapor deposition (CVD) (e.g., plasma enhanced CVD).
[0161] In certain embodiments intermediate layer 1210 is formed
from a metal, such as aluminum. Metal layers can be formed by
evaporation (e.g., thermal evaporation), for example. In
embodiments, metal layers are formed by evaporating the metal onto
surface 1131 at relatively fast rates, such as about 5 Angstroms
per second or more (e.g., about 10 Angstroms per second or more,
about 12 Angstroms per second or more, about 15 Angstroms per
second or more), for example. Fast deposition rates can improve the
purity of the metal layer by reducing the amount of impurities
(such as oxygen) that can incorporate into the film as it is
deposited.
[0162] In some embodiments, the substrate can be cooled prior to
and/or during metal deposition. For example, the substrate can be
cooled to about 0.degree. C. or less (e.g., about -20.degree. C. or
less, about -50.degree. C. or less). Cooling the substrate can
increase the size of metal grains formed on the substrate during
deposition. Without wishing to be bound by theory, it is believed
that lower substrate temperature can reduce the kinetic energy of
the metal clusters that tend to prevent the clusters from forming
larger grains. Larger metal grain size may be beneficial by
providing improved optical characteristics, such as higher
reflectance compared to metal layers composed of smaller grains.
Moreover, grating layers having short periods can be more easily
formed from metal layers having larger grain sizes.
[0163] Evaporation can also be performed under relatively high
vacuums, such as vacuums of about 10.sup.-6 Torr or less (e.g.,
about 5.times.10.sup.-7 Torr or less, about 2.times.10.sup.-7 Torr
or less). High vacuum deposition can also improve the purity of the
metal layer by reducing the amount of impurities (such as oxygen)
present in the vicinity of the deposited layer as it is formed,
thereby reducing the amount of impurities that are incorporated in
the film.
[0164] In some embodiments, substrate 1140 is positioned relatively
far from the source of the deposited metal in the deposition
chamber (e.g., about 12 inches or more, about 15 inches or more,
about 20 inches or more, about 24 inches or more). This can
increase the uniformity of the deposited material across surface
1131 relative to systems in which the source is positioned closer
to the substrate.
[0165] In general, the thickness of intermediate layer 1210 is
selected based on the desired thickness of sub-wavelength
structured layer 1110.
[0166] Intermediate layer 1210 is processed to provide portions
1112 of grating layer 1110 using lithographic techniques. For
example, portions 1112 can be formed from intermediate layer 1210
using electron beam lithography or photolithography (e.g., using a
photomask or using holographic techniques).
[0167] In some embodiments, portions 1112 are formed using
nano-imprint lithography. Referring to FIG. 11D, nano-imprint
lithography includes forming a layer 1220 of a resist on surface
1211 of intermediate layer 1210. The resist can be
polymethylmethacrylate (PMMA) or polystyrene (PS), for example.
Referring to FIG. 11E, a pattern is impressed into resist layer
1220 using a mold. The patterned resist layer 1220 includes thin
portions 1221 and thick portions 1222. Patterned resist layer 1220
is then etched (e.g., by oxygen reactive ion etching (RIE)),
removing thin portions 1221 to expose portions 1224 of surface 1211
of intermediate layer 1210, as shown in FIG. 11F. Thick portions
1222 are also etched, but are not completely removed. Accordingly,
portions 1223 of resist remain on surface 1211 after etching.
[0168] Referring to FIG. 11G, the exposed portions of intermediate
layer 1210 are subsequently etched, forming trenches 1212 in
intermediate layer 1210. The unetched portions of intermediate
layer 1210 correspond to portions 1112 of sub-wavelength structure
1110. Intermediate layer 1210 can be etched using, for example,
reactive ion etching, ion beam etching, sputtering etching,
chemical assisted ion beam etching (CAIBE), or wet etching. The
exposed portions of intermediate layer 1210 are etched down to etch
stop layer 1130, which is formed from a material resistant to the
etching method. Accordingly, the depth of trenches 1212 formed by
etching is the same as the thickness of portions 1112. After
etching trenches 1212, residual resist 1223 is removed from
portions 1112. Resist can be removed by rinsing the article in a
solvent (e.g., an organic solvent, such as acetone or alcohol), by
O.sub.2 plasma ashing, O.sub.2 RIE, or ozone cleaning.
[0169] In some embodiments, an etch mask is formed on the surface
of intermediate layer 1210 prior to depositing resist layer 1220.
Etch masks are provided to prevent etching of layer 1210 by the
etchant used to remove portions of the resist layer. Certain oxide
materials (e.g., SiO.sub.2) are examples of materials suitable for
masking intermediate layer 1210 from certain etchants (e.g.,
reactive ion etchants). For example, a layer of SiO.sub.2 can be
used to mask a metal layer from a chlorine-based reactive ion
etchant. Etch mask layers can be relatively thin (e.g., about 100
nm or less, 50 nm or less, such as in a range from about 20 nm to
about 25 nm).
[0170] Etching can be performed using commercially-available
equipment, such as a TCP.RTM. 9600DFM (available from Lam Research,
Fremont, Calif.).
[0171] More than one etch step can be used. For example, in some
embodiments, a two-step etch is used. An example of a two step
etching process for Al is as follows. The first etch is performed
using a gas mixture composed of BCl.sub.3 (e.g., at about 90 sccm),
Cl.sub.2 (e.g., at about 30 sccm), N.sub.2 (e.g., at about 10
sccm), He (e.g., at about 10 Torr) for backside cooling. The radio
frequency (RF) power is about 500 W and the chamber pressure about
5 mtorr. The second etch is performed using Cl.sub.2 (e.g., at
about 56 sccm), HCl (e.g., at about 14 sccm), N.sub.2 (e.g., at
about 35 sccm), H.sub.2 (e.g., at about 10 Torr) for back side
cooling. The RF power is about 300 W and the chamber pressure is
about 7 mtorr. For a typical 150 nm deep aluminum etching, the
first etching time can be about 4 seconds and the second etching
time can be about 15 seconds.
[0172] In certain embodiments, a post-etching passivation step can
be employed to provide a passivation layer on the surface of the
etched layer. Post-etching passivation can be done, for example, by
exposing the etched layer to an oxidant to produce an oxide layer
at the surface of the etched layer. Post-etch passivation of an
etched Al layer, for example, can be performed by exposing the
etched layer to water vapor at an elevated temperature (e.g., at
about 200.degree. C. or more, about 250.degree. C. or more, about
300.degree. C. or more).
[0173] Referring to FIG. 111, after removing residual resist,
material is deposited onto the article, filling trenches 1212 and
forming cap layer 1120. The filled trenches correspond to portions
1111 of sub-wavelength structured layer 1110. Material can be
deposited onto the article in a variety of ways, including
sputtering, electron beam evaporation, CVD (e.g., high density CVD)
or atomic layer deposition (ALD). Note that where cap layer 1120 is
formed and trenches 1212 are filled during the same deposition
step, portions 1111 and cap layer 1120 are formed from a continuous
portion of material.
[0174] In some embodiments, cap layer 1120 can be removed prior to
forming the diffractive layer. Cap layer can be planarized or
removed by polishing (e.g., chemical mechanical polishing), ion
milling, or etching the surface of the cap layer. Once the cap
layer is removed, the diffractive grating layer can be formed by
etching a diffractive structure into sub-wavelength structured
layer 1110, and filling the trenches of diffractive structure with
an isotropic material. Alternatively, a second sub-wavelength
structured layer can be formed the exposed surface of layer 1110 or
on the surface of cap layer 1120. The diffractive layer can be
formed in the second sub-wavelength structured layer as
described.
[0175] Methods can include other processes in addition to those
described. For example, in some embodiments, directional deposition
process can be used to deposited one or more materials onto a
trench wall in an etched material. Directional deposition can be
achieved by evaporation (e.g., electron beam or thermal
evaporation) or sputtering while orienting the substrate at a
non-normal angle with respect to the deposition material source.
Material deposited on top of trench walls can be removed in a
subsequent step (e.g., using etching or chemical mechanical
polishing).
[0176] In some embodiments, multiple polarizers can be prepared
simultaneously by forming a relatively a large structure comprising
sub-wavelength structures and diffractive structures on a single
substrate, which is then diced into individual units. For example,
a structure can be formed on a substrate that has a single-side
surface area about 10 square inches or more (e.g., a circular
substrate with a four inch, six inch, or eight inch diameter
substrate). After forming the structure, the substrate can be diced
into multiple units of smaller size (e.g., having a single-side
surface area of about one square inch or less).
[0177] In embodiments, polarizers may be combined with one or more
optical components and/or incorporated into a variety of different
devices and systems. As an example, a polarizer may be fabricated
with a cap layer having a surface adjacent to the diffractive
grating layer that is highly reflective of radiation at a
wavelength .lamda.. The polarizer may be configured to transmit
radiation polarized in one direction, such as TM-polarized
radiation, and to block radiation polarized in an orthogonal
direction, such as TE-polarized radiation. By fabricating the
polarizer with a reflective cap layer, TM-polarized radiation is
transmitted by the polarizer, reflects from the surface of the cap
layer, and then passes back through the polarizer in the opposite
direction and emerges from the same interface that radiation
originally entered the polarizer. The addition of a reflective cap
layer converts the polarizer from a transmission-only polarizer to
a reflection-only polarizer.
[0178] In certain applications, a polarizer may be constructed atop
an optically non-reciprocal substrate such as, for example, a
rotation garnet. Polarizers fabricated in this way may be used in
beam isolation applications to replace standard polarizers in, for
example, Faraday rotators.
[0179] Polarizers may also be used to spatially separate orthogonal
polarization components of an incident radiation beam. Polarizers
performing this function may be referred to as walk-off polarizers.
FIG. 12 shows an example of a walk-off polarizer 1200 that includes
two diffractive layers, 1266 and 1268, having portions 1222 and
1224 that include sub-wavelength structure, and portions 1223 and
1226 that are homogeneous. Diffractive layers 1266 and 1268 are
disposed on opposing sides of a substrate 1210.
[0180] A radiation beam 1258 incident along the z-direction and
including TM- and TE-polarized components is directed to a surface
1270 of polarizer 1200. Layers 1266 and 1268 are configured to
transmit TM-polarized radiation in a direction that is
substantially unchanged, i.e., parallel the z-direction.
TE-polarized radiation is diffracted by layer 1266 at an angle
.theta. to the z-axis. Layer 1268, having a structure and
composition similar to layer 1266 but positioned in the opposite
sense with respect to the TE-polarized radiation, diffracts the
TE-polarized beam so that the beam emerges from surface 1272 of
polarizer 1260 propagating nominally along the z-direction, but
spatially displaced from the TM-polarized beam. The separation of
the two beams, s, exiting the device can vary depending on the
thickness of the substrate, T, in addition to diffraction angle
.theta.. In embodiments, walk-off polarizer 1200 can include
antireflection films 1230 and 1240 to reduce reflection of incident
radiation from the polarizer.
[0181] Referring to FIG. 13, in some embodiments, a reflective
polarizer 1310 can be combined with another polarizer 1320, such as
an absorbing polarizer, to provide a polarizing device 1300 with
enhanced pass state extinction ratio compared to reflective
polarizer 1310. Polarizer 1320 increases pass state extinction by
absorbing substantially all of the block state radiation
transmitted by reflective polarizer 1310.
[0182] Referring to FIG. 14, in some applications, a reflective
polarizer 1410 can be used as the top polarizer of a reflective
liquid crystal display (LCD) 1400. In addition to reflective
polarizer 1410, LCD 1400 includes a reflective substrate electrode
1420, and a liquid crystal (LC) layer 1401. The reflective
polarizer transmits substantially only the pass state component
1431 of incident ambient light 1430. This pass state light is
specularly reflected by substrate electrode 1420. LC layer retards
the transmitted light by a variable amount, depending upon the
desired brightness of each particular element (e.g., pixel) of the
display. If a dark element is desired, the LC retards the
transmitted light so that when the light reflected by the substrate
electrode reaches reflective polarizer 1410, it is in the block
state and diffusely reflected back towards the substrate electrode,
as denoted by rays 1432. However, where a bright element is
desired, the light reflected by the substrate electrode is in the
pass state at reflective polarizer 1410. This light 1433 is
transmitted by polarizer 1410 and propagates to an observer 1450.
Note that block state ambient light 1434 is diffusely reflected by
reflective polarizer 1410, reducing glare to observer 1450.
[0183] As mentioned previously, in certain applications, wire-grid
polarizers can be used as components in optical isolators. Optical
isolators are passive, non-reciprocal devices that typically
utilize magneto-optic polarization rotation to isolate a radiation
source from reflections in an optical system. In conventional
optical isolators, polarization rotation is performed using a
Faraday rotator using, for example, Yttrium Iron Garnet (YIG) or
Terbium Gallium Garnet (TGG) single crystals. These, among other
Faraday media have the ability, when in an appropriate magnetic
field, to rotate the plane of linearly polarized light by an amount
proportional to the crystals length. YIG crystal is used for
wavelengths from about 1,100 to about 2,100 nm. TGG is typically
used in free space optical isolators for wavelengths between about
500 nm to about 1,100 nm.
[0184] Referring to FIG. 15, an optical isolator 1500 includes a
first wire-grid polarizer 1510 and a second wire-grid polarizer
1520. Both the first and second polarizers include diffractive
structure to reduce specular reflection of block state radiation.
First polarizer 1510 has its pass axis oriented parallel to the
y-axis, while second polarizer 1520 has its pass axis oriented in
the x-y plane at about 45.degree. with respect to the y-axis. A
Faraday rotator 1530, such as a YIG crystal in a magnetic field, is
positioned between first polarizer 1510 and second polarizer 1520.
Faraday rotator 1530 is configured to rotate by 45.degree. the
polarization state of linearly polarized radiation at .lamda.
incident radiation propagating parallel to the z-axis.
[0185] Optical isolator 1500 is configured to reduce specular
reflection of radiation polarized parallel to the x-axis for
radiation at .lamda. incident on the optical isolator propagating
along path 1512. The amount of reflected radiation polarized
parallel to the x-axis is reduced as follows. First, radiation
polarized parallel to the x-axis is in the block state of polarizer
1510, so specular reflection of this polarization state from
polarizer 1510 is relatively small (e.g., about 2% or less, about
1% or less). Block state polarization transmitted by polarizer 1510
propagates along path 1514 and is rotated by 45.degree. during its
passage through Faraday rotator 1530. Thus, this radiation emerges
from Faraday rotator 1530 along path 1516 polarized in the block
state of second polarizer 1520. Specular reflection of block state
radiation from polarizer 1520 is relatively small (e.g., about 2%
or less, about 1% or less), so only a small amount of block state
radiation is reflected by polarizer 1520 back towards Faraday
rotator 1530. Faraday rotator 1530 rotates block state radiation
reflected by polarizer 1520 so that it emerges from Faraday rotator
1530 polarized parallel to the y-axis, and is passed by polarizer
1510.
[0186] Radiation polarized parallel to the y-axis propagating along
path 1512 is substantially transmitted by polarizer 1510, rotated
by 45.degree. by Faraday rotator 1530, and substantially
transmitted by polarizer 1530 along path 1518. Thus, optical
isolator 1500 substantially transmits radiation at .lamda.
polarized parallel to the y-axis incident on the isolator along
path 1512, while reflecting substantially no radiation polarized
parallel to the x-axis.
[0187] While polarizers 1510 and 1520 are shown as physically
separated from Faraday rotator 1530 (e.g., as a free space optical
isolator), other constructions are also possible. For example, in
some embodiments, one or both of the polarizers can be physically
attached to a surface of the Faraday rotator. For example, one or
both of the polarizers can be bonded to a corresponding surface of
the Faraday rotator. In some embodiments, a polarizer can be formed
on one or more surfaces of the Faraday rotator. For example, a YIG
crystal can be used as a substrate for a polarizer.
[0188] Furthermore, while both polarizers 1510 and 1520 are
wire-grid polarizers with both sub-wavelength and diffractive
structure, in general, one of the polarizers can be a different
type of polarizer. For example, one of the polarizers can be an
absorptive polarizer or a reflective polarizer (e.g., a wire-grid
polarizer without diffractive structure).
[0189] Isolators, such as isolator 1500, can be used in a variety
of different devices. For example, referring to FIG. 16, in some
embodiments, an isolator 1610 is used in a laser system 1600. Laser
system 1600 includes a laser source 1620 (e.g., a laser diode) and
isolator 1610 positioned at the output of the laser source.
Isolator 1610 can reduce the amount of radiation of one
polarization state reflected back into the laser source. In some
embodiments, isolator 1610 is optically coupled to laser source
1620. In certain embodiments, isolator 1610 is physically attached
to laser source 1610.
[0190] Various structures and devices that utilize these structures
can be used for systems involving high power laser beam delivery
systems. For example, devices and structures can be used in
short-pulse and/or high repetition rate laser beam isolation at UV
or visible spectrum.
EXAMPLE
[0191] A polarizer is prepared by first depositing an approximately
50 nm thick etch-stop layer of HfO.sub.2 and approximately 450 nm
thick SiO.sub.2 onto a surface of a BK7 substrate by ion-assisted
e-beam evaporation (IAD). Next, the SiO.sub.2 layer is patterned
into a 200 nm-period grating by imprint lithography and reactive
ion etching (RIE), using CHF.sub.3 and O.sub.2. The SiO.sub.2 layer
is etched through its entire thickness, down to the HfO.sub.2
interface. In a subsequent process step, a 3 nm wide chromium (Cr)
layer is deposited onto one side of the SiO.sub.2 trench walls. A
22 nm wide gold layer is the deposited onto the chromium layers.
The chromium and gold layers are deposited using e-beam evaporation
in which the substrate is oriented at approximately 22.degree. with
respect to the metal source. The depth of the chromium/gold
portions extend about 450 nm in thickness along and over the
SiO.sub.2 grating trench walls.
[0192] After gold deposition, a SiO.sub.2 layer is deposited over
the grating structure using atomic layer deposition (ALD),
providing an approximately 105 nm thick planarization layer over
the grating layer. The ALD process corresponds to the process
described by J. Wang et al., in the article, "High-performance
optical retarders based on all-dielectric immersion gratings,"
Optics Lett., 30, 1864-1866, (2005). The SiO.sub.2 planarization
layer is etched back to the top surface of the grating layer using
RIEr. Ion milling is used to remove gold and chromium that deposits
on top of the trench walls. Subsequently, the grating layer is a
monolithic layer having a thickness of approximately 370 nm and
having sub-wavelength structure for wavelengths in the 1,200 nm to
1,700 nm range.
[0193] By using the same process as described above, a second layer
of the above sub-wavelength structure, approximately 260 nm in
thickness, is formed on top of the first layer. Periodic parallel
trenches are etched into the monolithic sub-wavelength structured
layer using a photolithographic exposure and reactive-ion etch.
These trenches have a depth of approximately 260 nm and a width of
approximately 2.4 .mu.m wide. The trenches are separated by rows of
material having the sub-wavelength grating structure that are
approximately 2.4 .mu.m wide. The trenches extend in a direction
substantially orthogonal to the gold rows in the sub-wavelength
structure. The trenches are filled with AlSiO.sub.x having a
refractive index of approximately 1.56 in the 1,200 nm to 1,700 nm
wavelength range, providing a monolithic grating layer having a
diffractive structure in that wavelength range. The AlSiO.sub.x is
deposited as a nanolaminate material in the trenches using ALD. The
structure is then planarized using chemical mechanical polishing
resulting in a layer of AlSiO.sub.x having a thickness of
approximately 105 nm with a substantially planar surface on top of
the second layer of sub-wavelength structure, providing, in
addition, a monolithic grating layer having diffractive structure
for light in the 1,200 nm to 1,700 nm range. A layer of HfO.sub.2,
approximately 105 nm thick, is deposited onto the AlSiO.sub.x layer
using IAD e-beam evaporation. Finally, a layer of SiO.sub.2,
approximately 330 nm thick, is deposited onto the layer of
HfO.sub.2.
[0194] The surface of the BK7 substrate opposite the grating layer
is coated with an antireflection film. The antireflection film is
composed of a first Ta.sub.2O.sub.5 layer, 70 nm thick, adjacent
the substrate surface, a first SiO.sub.2 layer 70 nm thick adjacent
the first Ta.sub.2O.sub.5 layer, a second Ta.sub.2O.sub.5 layer 209
nm thick adjacent the first SiO.sub.2 layer, and a second SiO.sub.2
layer 262 nm thick. The coatings are formed using the same method
as for the etch stop layer.
[0195] The performance of the polarizer is illustrated in FIGS. 17
and 18. FIG. 17 shows modeled transmission of TM (pass state)
radiation and extinction in transmission of TE (block state)
radiation at different wavelengths for radiation normally incident
on the grating side of the polarizer. Pass state transmission,
labeled Ts-WGP(p)(%) in FIG. 17, is relatively high for radiation
in about the 1,200 nm to 2,000 nm range (e.g., about 90% or more),
with particularly high transmission (e.g., about 98% or more) for
radiation in about the 1,400 nm to 2,000 nm range. Correspondingly,
extinction of block state radiation, labeled Et(dB), is relatively
high at these wavelengths, e.g., about 45 dB or more for radiation
in about the 1,200 nm to 2,000 nm range.
[0196] FIG. 18 shows modeled reflection of TE (block state)
radiation at different wavelengths for radiation normally incident
on the grating side of the polarizer. The polarizer has relatively
low reflection of block state radiation. For example, the polarizer
has a reflectance of about 5% or less for radiation in about the
1,200 nm to 1,600 nm range.
[0197] The data shown in FIGS. 17 and 18 was modeled using a hybrid
model. In this model, the effective indices of the sub-wavelength
grating are calculated by using effective medium theory. Effective
medium theory is discussed, for example, by H. Kikuta et al., in
"Achromatic quarter-wave plates using the dispersion of form
birefringence," Applied Optics, Vol. 36, No. 7, pp. 1566-1572
(1997), by C. W. Haggans et al., in "Effective-medium theory of
zeroth order lamellar gratings in conical mountings," J. Opt. Soc.
Am. A, Vol. 10, pp 2217-2225 (1993), and by H. Kikuta et al., in
"Ability and limitations of effective medium theory for
subwavelength gratings," Opt. Rev., Vol. 2, pp. 92-99 (1995). After
calculating the effective indices of the sub-wavelength grating,
the performance of the diffractive optical structures are
calculated by rigorous coupled wave analysis, as discussed by X.
Deng, et al. in the article, "Multiscale structures for
polarization control by using imprint and UV lithography," Proc.
SPIE, Vol. 6003, (2005), Boston, Mass.
[0198] Other embodiments are in the claims.
* * * * *