U.S. patent application number 13/755801 was filed with the patent office on 2014-07-31 for plasmon resonance based strain gauge.
This patent application is currently assigned to HEWLETT-PACKARD DEVELOPMENT COMPANY, L.P.. The applicant listed for this patent is HEWLETT-PACKARD DEVELOPMENT COMPANY, L.P.. Invention is credited to STEVEN J BARCELO, GARY GIBSON, ANSOON KIM, ZHIYONG LI, MINEO YAMAKAWA.
Application Number | 20140211195 13/755801 |
Document ID | / |
Family ID | 51222588 |
Filed Date | 2014-07-31 |
United States Patent
Application |
20140211195 |
Kind Code |
A1 |
BARCELO; STEVEN J ; et
al. |
July 31, 2014 |
PLASMON RESONANCE BASED STRAIN GAUGE
Abstract
A strain gauge or other device may include a deformable medium
and discrete plasmon supporting structures arranged to create one
or more plasmon resonances that change with deformation of the
medium and provide the device with an optical characteristic that
indicates the deformation of the medium.
Inventors: |
BARCELO; STEVEN J; (Palo
Alto, CA) ; LI; ZHIYONG; (Redwood City, CA) ;
GIBSON; GARY; (Palo Alto, CA) ; YAMAKAWA; MINEO;
(Campbell, CA) ; KIM; ANSOON; (Palo Alto,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
HEWLETT-PACKARD DEVELOPMENT COMPANY, L.P. |
Houston |
TX |
US |
|
|
Assignee: |
HEWLETT-PACKARD DEVELOPMENT
COMPANY, L.P.
Houston
TX
|
Family ID: |
51222588 |
Appl. No.: |
13/755801 |
Filed: |
January 31, 2013 |
Current U.S.
Class: |
356/33 ;
356/32 |
Current CPC
Class: |
G01L 1/241 20130101;
G01B 11/16 20130101 |
Class at
Publication: |
356/33 ;
356/32 |
International
Class: |
G01L 1/24 20060101
G01L001/24 |
Claims
1. A strain gauge comprising: a medium that is deformable; and a
plurality of discrete plasmon supporting structures arranged with
the medium to create one or more plasmon resonances that change
with deformation of the medium so that a color of light scattered
from the strain gauge visibly changes with the deformation of the
medium.
2. The strain gauge of claim 1, wherein the medium is a surface of
or is attached to a surface of an object that is subject to strain
to be observed.
3. (canceled)
4. The strain gauge of claim 6, wherein each of the first optical
characteristic and the second optical characteristic is selected
from a group consisting of a frequency, an amplitude, and a width
of a feature that the one or more plasmon resonances create in a
spectral distribution of light scattered from the strain gauge.
5. A strain gauge comprising: a medium that is deformable; and a
plurality of discrete plasmon supporting structures arranged with
the medium to create one or more plasmon resonances that change
with deformation of the medium and provide the strain gauge with an
optical characteristic that depends on the deformation of the
medium, wherein the discrete plasmon supporting structures are
arranged to form a hierarchical structure that includes: a
plurality of first assemblies of the discrete plasmon supporting
structures, wherein the plasmon supporting structures have first
separations within the first assemblies; and one or more second
assemblies containing the first assemblies, wherein the first
assemblies have second separations within the one or more second
assemblies.
6. The strain gauge of claim 5, wherein: a first of the one or more
plasmon resonance that has a first optical characteristic that
depends on the first separations; and a second of the one or more
plasmon resonances has a second optical characteristic that depends
on the second separations.
7. The strain gauge of claim 5, wherein the one or more second
assemblies comprises a plurality of the second assembles arranged
in an array with third separations, wherein: the first separations
are less than about 50 nm; the second separations are more than 100
nm; and the third separations are larger than the second
separations and cause a diffraction pattern that depends on
deformation of the medium.
8. The strain gauge of claim 1, wherein the discrete plasmon
supporting structures include: a plurality of discrete plasmon
supporting structures that have a first size; and a plurality of
discrete plasmon supporting structures that have a second size that
is larger than the first size.
9. A strain gauge comprising: a medium that is deformable; and a
plurality of discrete plasmon supporting structures arranged with
the medium to create one or more plasmon resonances that change
with deformation of the medium and provide the strain gauge with an
optical characteristic that depends on the deformation of the
medium, wherein the plasmon supporting structures comprise a
ferromagnetic material and provide the strain gauge with a
measureable magnetic characteristic that depends on deformation of
the medium.
10. The strain gauge of claim 1, wherein the plasmon supporting
structures comprise a conductive material and provide the strain
gauge with a measureable electrical resistance that depends on
deformation of the medium.
11. The strain gauge of claim 5, further comprising a lighting
system that is controllable to switch between illuminating the
discrete plasmon supporting structures with light having a first
polarization corresponding to a first symmetry axis of an
arrangement of the discrete plasmon supporting structures and
illuminating the discrete plasmon supporting structures with light
having a second polarization corresponding to a second symmetry
axis of the arrangement of the discrete plasmon supporting
structures.
12. The strain gauge of claim 5, further comprising: an optical
sensor; and a controller coupled to the optical sensor and
configured to derive a measurement result from measurements that
the optical sensor provides of output light from the discrete
plasmon supporting structures.
13. The strain gauge of claim 12, wherein the optical sensor is
operable to separately measure light having first and second
polarizations, wherein the first polarization corresponds to a
first symmetry axis of an arrangement of the discrete plasmon
supporting structures and the second polarization corresponds to a
second symmetry axis of the arrangement of the discrete plasmon
supporting structures.
14. The strain gauge of claim 1, wherein the discrete plasmon
supporting structures are arranged in a hierarchical array that
provides multiple spacing length scales including a spacing length
scale less than 50 nm.
15. A measuring method comprising: providing a plurality of
discrete plasmon supporting structures on an object and arranged to
create a plasmon resonance that depends on deformation of the
object; and observing, with a naked eye, a change in an optical
characteristic of the object to detect a deformation of the object,
wherein the optical characteristic observed is caused by the
plasmon resonance.
16. The method of claim 15, wherein observing the change in the
optical characteristic comprises visually observing a change in
color of light scattered from the object.
17. The method of claim 18, wherein observing the change comprises
measuring and analyzing a spectral content of light reflected from
the plasmon supporting structures.
18. A measuring method comprising: providing a plurality of
discrete plasmon supporting structures on an object and arranged to
create a plasmon resonance that depends on deformation of the
object; observing a change in optical characteristics of the object
to detect a deformation of the object, wherein the optical
characteristic observed is caused by the plasmon resonance;
measuring a secondary characteristic that the discrete plasmon
supporting structures provide across an area of the object, wherein
the secondary characteristic is one of an electrical resistance and
a magnetic characteristic; and using the secondary characteristic
measured in determination of the deformation of the object.
19. (canceled)
20. (canceled)
21. The strain gauge of claim 9, further comprising: an optical
sensor; a magnetic sensor; and a controller to derive a measurement
result using a measurement of the optical characteristic from the
optical sensor and a measurement of the magnetic characteristic
from the magnetic sensor.
Description
BACKGROUND
[0001] A strain gauge can measure the strain or deformation of an
object. Typical strain gauges are based on the change in resistance
of a metal undergoing strain. For example, one type of strain gauge
includes a metallic foil pattern that may be attached to an object,
so that deformation of the object deforms and changes the
electrical resistance of the foil. A measurement of the change in
the electrical resistance can thus provide a measure of the strain
or deformation of the object. Strain gauges can measure other
physical quantities such as force, acceleration, pressure, torque,
or even temperature by placing the strain gauge on an object that
has known deformations in response to the physical quantity being
measured. In particular, a measurement of the strain may indicate
an ambient or applied physical quantity.
BRIEF DESCRIPTION OF THE DRAWINGS
[0002] FIGS. 1A and 1B illustrate general principles of one
implementation of a strain measurement.
[0003] FIGS. 2A and 2B illustrate alternative implementations of
measurement systems.
[0004] FIGS. 3A and 3B illustrate respective implementations of
measurement processes using arrays of discrete plasmon supporting
structures.
[0005] FIGS. 4A and 4B illustrate interactions of polarized light
in an implementation of a strain measurement.
[0006] FIGS. 5A and 5B show hierarchical arrangements of plasmon
supporting structures in respective implementations of strain
gauges.
[0007] FIGS. 6A, 6B, and 6C respectively illustrate three different
assemblies of plasmon supporting structures capable of indicating
strain in at least two independent directions.
[0008] FIG. 7 illustrates an implementation of a strain gauge
including a three-dimensional array of plasmon supporting
structures.
[0009] FIGS. 8A, 8B, 8C, and 8D illustrate a fabrication process
for a strain gauge in accordance with one implementation.
[0010] FIG. 9 is a block diagram of an implementation of a strain
gauge that combines optical, electrical, and magnetic measurements
when measuring strain.
[0011] The drawings illustrate examples for the purpose of
explanation and are not of the invention itself. Use of the same
reference symbols in different figures indicates similar or
identical items.
DETAILED DESCRIPTION
[0012] A strain gauge can employ a deterministic arrangement or
array of discrete plasmon supporting structures such as plasmonic
nanoparticles. The deterministic arrangement of the plasmon
supporting structures can create relationships between deformation
of the object and resulting changes in optical characteristics such
as the number, frequencies, widths, and relative magnitudes
associated with plasmon resonances in the strain gauge. For
example, one or more frequencies, widths, or relative magnitudes of
plasmon resonances may change in response to changes in separations
of the plasmon supporting structures that the deformation of an
object or a medium may cause. Similarly, one or more plasmon
resonances may appear or disappear in response to changes in
separations of the plasmon supporting structures. Optical
measurement of light interactions with the plasmon resonances in a
strain gauge can measure changes such as changes in the frequencies
of the plasmon resonances, and the measured changes can then be
converted into a quantitative measurement of strain, torsion, or
other physical quantity that may depend on the strain or torsion of
an array. Alternatively, a strain gauge including an array of
discrete plasmon supporting structures may exhibit color changes in
visible wavelengths that can be detected with the human eye or with
a relatively simple interface.
[0013] The arrays within a strain gauge may be hierarchical, for
example, by arraying assemblies of plasmon supporting structures. A
lowest level assembly may include an array of discrete plasmon
supporting structures with separations arranged so that plasmon
resonances that the structures collectively support within an
assembly have frequencies that depend on the separations of the
particles within the assembly. The separations of assemblies from
other assemblies may be larger and create other plasmon resonances
that are collectively supported in multiple assemblies. The plasmon
resonances associated with longer separations often have lower
frequencies than plasmon resonances associated with shorter
separations. However, changing the separation between plasmonic
structures can result in hybrid resonances that are not apparent in
structures with other separations, so that optical characteristics
resulting from different separations are not always simply a
shifting of the same resonances. Still, an array with multiple
hierarchical levels can thus create plasmon resonances with
frequencies at multiple scales respectively associated with the
multiple hierarchical levels. A strain sensor with such assemblies
may thus display plasmon resonances with a wide dynamic range of
plasmon frequencies.
[0014] Arrays of discrete plasmon supporting structures can be
created with a wide variety of specific geometries or patterns to
create sensors with different sensitivities to strain and different
color or spectral responses to strain. In particular, arrays or
assemblies may have multiple symmetry axes, and a single sensor can
measure multiple dimensions or components of a physical quantity
such as strain or torsion, where the measured components are
related to the symmetry axes. These strain gauges can also be
configured to yield measurements of many physical quantities
including but not limited to pressure, force, acceleration, torque,
and any other physical quantities that may be related to the
measured strain, torsion, or other deformation of the sensor.
[0015] FIGS. 1A and 1B illustrate general principles of one
implementation of a strain measurement. As shown in FIG. 1A, an
object 110 may have a surface with an attached deterministic array
120 of discrete plasmon supporting structures that collectively
have one or more plasmon resonance. FIG. 1A is intended to
illustrate an example where array 120 has a single plasmon
resonance with a frequency f.sub.R0 when array 120 has no
deformation. Input light 130 that is incident on array 120 can have
any desired spectral content or polarization characteristics, but
as one example, input light 130 may be unpolarized light and
include electromagnetic radiation with a range of wavelengths,
e.g., white light. Input light 130 can interact with array 120 or
object 110, so that output light 140, which may be reflected,
scattered, diffracted, or otherwise radiated from object 110, may
have a spectral content, an angular distribution, or polarization
characteristics that differ from input light 130 in a manner that
depends on the deformation of array 120. For example, if object 110
or a medium of array 120 is a strong absorber of the wavelengths in
input light 130, array 120 may absorb or scatter most of input
light 130. FIG. 1A shows an example in which most frequencies of
input light 130 may be strongly absorbed, but a plasmon resonance
of array 120 makes object 110 highly efficient at scattering light
having frequency f.sub.R0. Alternatively, a resonance of array 120
may primarily absorb light with frequency f.sub.R0, and object 110
reflects the remainder of input light 130 so that output light 140
is similar to input light 130 but has a `spectral hole` at
resonance frequency f.sub.R0. In both examples, input light 130,
which in different implementations may have many different spectral
distributions, can consistently produce output light 140 having
spectral distribution with a prominent feature at frequency
f.sub.R0. This spectral feature depends on the pattern of array 120
and may give object 110 a color characteristic of frequency
f.sub.R0 if frequency f.sub.R0 corresponds to a visible frequency
of light.
[0016] FIG. 1B illustrates an effect that may result from strain on
object 110. For example, strain may change the shape, e.g., stretch
or compress one or more dimensions, of object 110, and the
separations of the discrete plasmon supporting structures in array
120 may correspondingly change. Such changes are known to change
the collective plasmon resonance frequencies, and FIG. 1B
illustrates an example where stretching of object 110 and array 120
shifts the plasmon resonance from the unstressed frequency f.sub.R0
to a frequency f.sub.R1. As a result, even with the same input
light 130 as used in the example of FIG. 1A, output light 142 when
object 110 is under strain has a different spectral content, even a
different color if frequency f.sub.R0 or f.sub.R1 is in the visible
range. As shown in FIG. 1B, a feature in the spectral distribution
of output light 140 may have a frequency shift .DELTA.f
corresponding to the difference between plasmon resonance
frequencies f.sub.R0 and f.sub.R1. Frequency shift .DELTA.f is
related to a strain .DELTA.X in object 110. As a result, strain
.DELTA.X in object 110 can be observed or measured by observing or
measuring the spectral content of output light 140 and 142.
[0017] The differences in output light 140 and 142 of FIGS. 1A and
1B illustrate relatively simple techniques for observing
deformation of object 110 by observing optical response of array
120 and particularly by observing a frequency shift in a spectral
feature corresponding to a plasmon resonance. Observing the optical
response of an array of plasmons supporting structures in some
other implementations may include observing other optical effects
caused by plasmon resonances. For example, deformations can be
detected by observing frequency shifts in features (e.g., peaks or
holes) of the output light, observing changes in the relative
amplitude of features in the output light, observing changes in the
widths of one or more features in the output light, or observing
the emergence or disappearance of a spectral peak or hole in the
output light, or observing any combination of these.
[0018] FIG. 2A illustrates how strain in object 110 can be simply
observed using the naked eye in any available lighting 210. For
example, object 110 may be a mechanical member such as a beam,
joint, or cantilever in a structure such as a bridge, building, or
other edifice that is coated with an array of discrete plasmon
supporting structures. A simple visible inspection of member/object
110 can indicate an amount of strain in the object from the color
of the object 110. In a working environment, an inspector can
simply look at object 110 and determine the level of strain in
object 110. Light 210 may be any available lighting in the
environment of object 110 or may or may not be a source of light
with known spectral or polarization characteristics.
[0019] The principles or techniques illustrated in FIG. 2A can be
employed to detect strain on any object 110. For example, object
110 may be a mechanical member or part, a container, a pipe, a
valve, a tool, a panel, a plate, a casing, or a handle that may be
coated with arrays of discrete plasmon supporting structures that
change color when strain is present, for example, when object 110
may be close to a mechanical failure. Alternatively, the principles
or techniques illustrated in FIG. 2A can be employed to effect a
cosmetic change in object 110, e.g., when object 110 is a toy or
decorative item, having a color that changes to a desired color in
response to the application of a strain.
[0020] FIG. 2B illustrates a system 200 for obtaining a
quantitative measurement of strain on an object 110 with an
attached array 120 of discrete plasmon supporting structures.
System 230 uses a lighting system 230 that produces input light 130
for illumination of array 120. Lighting system 230 may include any
type of light source such as ambient light, an incandescent or
fluorescent lighting fixture, a light emitting diode (LED), or a
laser used with or without additional optical elements such as
color or polarization filters. In one implementation, lighting
system 230 has known spectral and polarization characteristics,
which may be characteristic of the light source that lighting
system 230 uses or characteristic of filters or other optical
components that light source 230 applies to produce input light
130, and a controller 250 may operate lighting system 230 to alter
input light 130 in a controlled manner. In one implementation
described below, controller 250 may alter the operation of a light
source or other optical components within lighting system 230 to
change the polarization of input light 130.
[0021] An optical sensor 240 in system 200 measures output light
140 and may particularly measure the spectral content of output
light 140 as a whole, within a specific or selectable angular
range, or having a specific or selectable polarization. Optical
sensor 240 may, for example, include a spectrometer,
spectrophotometer, an image detector, or one or more photodiodes
with or without polarization filters. (A photodiode used with color
filters could act as a crude spectrometer, or if the wavelength
shift is large enough a color filter may not be necessary).
Polarization filters may be employed when selective measurement,
e.g., spectral measurement, of one or more particular light
polarizations is desired. As noted above, strain in object 110 can
change the frequencies of plasmon resonances in array 120 and
correspondingly change the optical response of array 120 and the
optical characteristics of output light 140. Optical sensor 240, in
general, can measure any optical characteristics of output light
140 that deformation of array 120 changes.
[0022] Controller 250 may include one or more processors capable of
executing instructions or may be entirely a hardware controller.
Controller 250 generally controls light source 230 and optical
sensor 240 during measurement of output light 140 and can further
analyze the measured characteristics of output light 140 and
determine one or more measurements of a physical quantity such as
the strain in object 110.
[0023] FIG. 3A illustrates one implementation of a measurement
process 300 that can be performed with system 200. Process 300
basically includes measuring 320 of output light 140 during
illumination of array 120 and based on the measured optical
characteristics of output light 140, deriving or inferring 330 the
strain or a physical quantity that depends on the strain.
[0024] In one specific implementation of system 200 and process
300, lighting system 230, optical sensor 240, and controller 250 of
system 200 may be in a handheld measuring device, while object 110
is relatively immobile. For example, if object 110 is a large
object such as components in a factory, building, or bridge, an
inspector may employ the handheld device to illuminate a portion,
e.g., a joint, of object 110 and measure light from the array 120
on object 110 as in step 320. The measuring device can then process
the optical measurements to produce a measurement result.
[0025] FIG. 3B shows a more specific implementation of measurement
process 300, which further includes characterization 310 of the
optical response of array 120 as a function of deformation in array
120. Characterization 310 may, for example, provide a mapping from
a set of frequencies corresponding to peaks (maxima) or valleys
(minima) in the spectral content of output light 140 to a
combination of one or more components of deformation of object 110
or array 120 that would produce the measured frequencies. In a
simple case, only one component of strain is of interest, e.g.,
stretching or compression along a linear direction or torsion about
an axis, and characterization 310 may provide a simple function or
lookup table that maps a frequency of a spectral peak to a value of
the measurement result of interest. Characterization 310 of the
output light from an array 120 may involve more complicated
formulations that may require multiple functions or look-up tables
for derivation of a measurement result from a spectral
distribution. Characterization 310 of array 120 may possibly be
derived from first principles based on the geometry and composition
of array 120 or may be determined empirically from a calibration of
array 120 or an archetype of array 120. Characterization 310 may be
performed before or after fabrication of array 120.
[0026] The characteristics of output light 140 from array 120 may
depend on the characteristics of input light 130. Characterization
310 may be for specific input light characteristics, and
illumination may be controlled during optical measurement 330. In
particular, variations in input lighting 130 can be used during
optical measurement 320 to provide more information for derivation
of a measurement result. Additionally, the characteristics of
output light 140 may differ for different polarization components
of output light 140, and optical measurement 320 may select and
measure a particular polarization component. FIG. 3B illustrates an
implementation of process 300 in which optical measurement 320
includes one or more illuminations 322 of array with input light
130 having different characteristics for each illumination 322 and
with respective optical measurements 324 of output light 140.
[0027] An even more specific implementation of optical measurement
320 uses different polarizations for input light 130. Light with
different polarizations may interact differently with different
plasmon resonant modes in array 120. FIG. 4A, for example, shows an
example in which linearly polarized input light 410 interacts with
an array 420 of plasmon supporting structures 421-424. In array
420, plasmon supporting structures 421 and 422 are separated by a
distance .DELTA.X along a horizontal or X direction, and plasmon
supporting structures 423 and 424 are separated by a distance
.DELTA.Y along a vertical or Y direction. In FIG. 4A, linearly
polarized input light 410 has the electric field directed in an X
direction and may interact most strongly with horizontal plasmon
oscillations and therefore with a plasmon resonant mode associated
with structures 421 and 422, collectively. Deformations of array
420 that change separation .DELTA.X may thus significantly change
the frequency of the plasmon resonant mode that input light 410
tends to excite. In contrast, plasmon resonant modes that
structures 423 and 424 support may more weakly interact with
horizontally polarized input light 410, or deformations of array
420 that change separation .DELTA.X may not significantly change
the frequency of the plasmon resonant modes that primarily reside
in structures 423 and 424. In either case, horizontally polarized
input light 410 may provide output light 140 that includes a strong
peak associated with the resonances primarily associated with
structures 421 and 422 or that changes most with deformations of
array 420 in the X direction.
[0028] FIG. 4B shows the example of linearly polarized input light
430 having an electric field directed in the vertical or Y
direction, so that input light 430 may interact most efficiently
with a plasmon resonant mode associated with plasmon oscillations
in the Y direction such as a plasmon resonance supported
collectively in structures 423 and 424. Deformations of array 420
that change separation .DELTA.Y may thus significantly change the
frequency of the plasmon resonant modes that input light 420 tends
to excite, and vertically polarized input light 420 may provide
output light that includes strong peaks associated with the plasmon
resonances primarily associated with structures 423 and 424 or that
change most with deformations of an array in the Y direction.
[0029] Light radiated or output from the resonance associated with
plasmon oscillations in the X or Y direction may similarly have
characteristic polarization. Accordingly, selectively filtering the
polarization of the output light 140 may help to distinguish
plasmon resonances associated with oscillations in the X and Y
directions.
[0030] The polarization dependence of light-plasmon interactions
can thus be used to measure specific components of deformation of
an object or media containing an array of discrete plasmon
supporting structures. Returning to FIG. 3B, optical measurement
320 can thus include: illumination 322 of array 120 with input
light having selected and controlled characteristics, e.g., a
selected linear polarization; optical measurement 324 of a selected
component of output light 140, e.g., a selected linear
polarization, while input light 130 has the selected
characteristics; and repetition 326 of illumination 322 and
measurement 324 until all desired illumination and measurement
profiles have been exhausted, e.g., after illumination 322 and
measurement 324 have been performed with all of the linear
polarizations associated with symmetry axes in array 120. Optical
measurement 320 may thus provide more data that derivation 330 can
use to determine measurement results, e.g., to determine multiple
components of the strain in object 110. Further, rapid alternation
of the excitation light sources or rapid rotation or switching of
polarization filters can lead to essentially continuous deformation
measurement along multiple axes.
[0031] The specific configurations of arrays of discrete plasmon
supporting structures can be varied widely according to the desired
measurements and one implementation of a strain gauge uses a
hierarchical array, which may provide a wide dynamic range of
measurement results. FIG. 5A shows a plan view of one
implementation of a strain gauge 500 including a hierarchical array
of discrete plasmon supporting structures 510. Strain gauge 500 can
be used to measure or indicate deformation of an object or any
quantity that may be related to the deformation of the object.
Gauges that measure quantities that are dependent on such
deformations are sometimes referred to herein as strain gauges in
the general sense that a strain gauge may measure strain, torsion,
or any physical quantity that is related to the deformation of an
object. Strain gauge 500 includes discrete structures 510 that are
in or on a medium 540 and arranged to form assemblies 520 and 530
of different sizes. In the illustrated implementation, each
assembly 530 is an array of smaller assemblies 520, and each
assembly 520 is an array of discrete plasmon supporting structures
510. The implementation of FIG. 5A is thus one example of a
three-level hierarchical arrangement of discrete plasmon supporting
structures 510, but more generally, a strain gauge may have any
number of hierarchical levels, including just a single level or
arrangement of discrete plasmon supporting structures 510.
[0032] Each discrete plasmon supporting structure 510 is a
structure in which plasmons may reside. The plasmons in structures
510 may interact to varying degrees across the separations between
structures 510. The interaction of structures 510 depend upon the
sizes, geometries, and material characteristics of structures 510
and the surrounding materials (in particular the dielectric
properties of the materials between structures 510). The
interaction as noted above can shift the frequencies of plasmon
resonances or create additional, hybrid resonances. Plasmon
supporting structures 510 are discrete in that, absent structures
510, the gaps or separating material between plasmon supporting
structures 510 do not support strong plasmon resonances. For
example, each plasmon supporting structure 510 may be a plasmonic
nanoparticle surrounded by dielectric materials. However, plasmon
supporting structures 510 may be connected to each other in some
fashion to form arrays. For example, plasmon supporting structures
510 may be the tops of pillars or bumps that isolate or separate
areas in which plasmons reside even though the pillars or bumps
themselves are connected to each other through their bases.
[0033] In one implementation of strain gauge 500, medium 540 is a
stretchable medium made of a material such as a suitable plastics,
rubber, biopolymers, flexible metals, or fabric. Medium 540 may
further be an insulator or dielectric material and may be
transparent to frequencies of electromagnetic radiation
corresponding to the frequencies of some plasmon resonances used in
strain gauge 500 or may have a color, for example, black to absorb
visible input light. Medium 540 may be selected to attach to or
cover a surface of an object that undergoes a strain to be
measured. For example, medium 540 may be a thin layer of material
or tape with an adhesive that allows medium 540 to be attached to
the object so that medium 540 is able to stretch, compress, or
twist as the object undergoes the strain to be measured.
Alternatively, medium 540 may itself be the object that undergoes
the strain to be measured, or equivalently, medium 540 may be
eliminated and discrete structures 510 may be directly attached to
or deterministically positioned in or on an object for which strain
is to be measured or identified.
[0034] Discrete plasmon supporting structures 510 collectively
provide one or more useful plasmon resonances. In general, a
plasmon is a quantum of plasma oscillation, and one type of plasma
is made up of the free electrons near the surface of materials such
as a metal. Plasmons at the surfaces of metals or at interfaces are
commonly referred to as surface plasmons. A plasmon resonance of a
structure corresponds to a frequency of plasmon oscillation at
which the structure has a resonance for energy absorption or
radiation. The plasmon resonances of discrete structures 510
generally depend on the material, size, and shape of discrete
structures 510. Smaller structures, e.g., metal discs or spheres
having diameters less than about 200 nm, tend to have fewer
resonances and may have only one resonance in a desired frequency
range, e.g., the frequencies corresponding to visible light, or
have a few discrete resonances that are easily distinguishable.
Particles having linear dimensions smaller than a few hundred
nanometers, which may sometimes be referred to as nanoparticles,
are one example of discrete structures that may collectively
provide a suitable plasmon resonance. In particular, particles of a
metal such as platinum, gold, silver, copper, aluminum, or nickel
smaller than about 200 nm in width can be used to provide plasmon
resonances with frequencies corresponding to the frequencies of
visible light, but plasmon resonances may also be created at
suitable semiconductor interfaces, e.g., a silicon-air interface.
Discrete structures 510 thus may be separated semiconductor
interfaces or any plasmon supporting structures with separations
that can be determined during fabrication.
[0035] In the implementation of FIG. 5A, discrete structures 510
are arranged within assemblies 520 to have smallest inter-particle
gaps L1 aligned along a number of axes of interest, e.g., axes 522,
524, and 526. Assemblies 520 are arranged with smallest
center-to-center separations L2 in a rectangular grid in a
corresponding assembly 530, and assemblies 530 are arranged with
smallest center-to-center separations L3 in a rectangular grid on
medium 540. The plasmon resonances of discrete plasmon supporting
structures such as metallic nanoparticles have been shown to be
strongly dependent on interparticle spacing ranging from less than
a few nanometers up to hundreds of nanometers. The implementation
of strain gauge 500 of FIG. 5A can achieve a high dynamic range of
the plasmon resonance variation by providing critical dimensions on
a number of length scales. The different length scales in turn may
correspond to different scales of resonance frequencies. In array
500, separations L1 between discrete structures 510 within a single
assembly 520 are smallest, and separations L2 between adjacent
assemblies 520 may be larger than separation L1. In general, each
length scale will be most sensitive to changes on the order of that
length scale, and the existence of larger assemblies on smaller
scale resonances, e.g., higher frequency resonances, may primarily
be changes in the width of the higher frequency resonances.
[0036] In one specific implementation, discrete structures 510 are
about 5 to 200 nm in width, and separation L1 between closest
neighboring structures 510 is about 1 to 50 nm. Separation L2
between assemblies 520 may be about 100 to 300 nm. As a result,
strain gauge 500 may have plasmon resonance frequencies
respectively corresponding to separations L1 and L2. When array 500
has a plasmon resonance that corresponds to a frequency of light,
array 500 can become highly efficient at absorption or radiation of
light having that frequency and therefore have a color that depends
on the plasmon resonance. Changes in the plasmon resonances of
strain gauge 500 that result from changes in separation L1 and L2
can be measured as described above by illuminating strain gauge 500
and observing the resulting output light.
[0037] Assemblies 530 in one implementation are separated by
separations L3 that are larger than separations L1 or L2, and in
one implementation of strain gauge 500, separation L3 may be on the
order of the wavelength of light used for observation of strain
gauge 500. As a result, in addition to variation of the plasmon
resonance, the spacing of assemblies 530 may provide grating
effects, and the grating effect may vary with deformation of medium
540. The grating effect may, for example, shift the direction of
diffraction of specific wavelengths of light. Optical measurement
of output light 140 may thus include measuring angular differences
in output light 140, or wavelength of output light 140 at a given
angle. Such grating effects may also be used to effectively focus
or direct more of output light at a detector.
[0038] The plasmon resonances of individual structures 510 can
interact over distances on the order of their size or greater, but
typically the strongest interactions are over smaller length
scales. In array 500, the smallest plasmon supporting structures
510 may be some sort of particle that is less than or on the order
of a couple of hundred nanometers (e.g., down to about 5 or 10 nm).
For strong interactions, smallest separations of structures 510 (at
least when at their closest approach) may be less than the size of
a single structure 510. The interactions between assemblies 520 at
larger length scales would typically be weaker. Consequently,
dynamic range may improve by having some assemblies contain plasmon
supporting structures that are smaller and some other assemblies
contain plasmon supporting structures that are larger. FIG. 5B, for
example, shows an example of a hierarchical array 500B including
assemblies 520 of discrete plasmon supporting structures 510 as
described with reference to FIG. 5A and also assemblies 525 of
discrete plasmon supporting structures 515. Plasmon supporting
structures 515 may generally differ in size from plasmon supporting
structures 510 and may be of the same or different composition. As
a result, the plasmon resonances primarily in plasmon supporting
structures 515 may expected to be different from the plasmon
resonances primarily in plasmon supporting structures 510, whether
or not structures 510 and 515 have the same or similar separations.
The use of different size plasmon supporting structures 510 and 515
may provide array 500B with greater dynamic range in measurement of
deformations of media 540.
[0039] Strain gauges 500 and 500B of FIGS. 5A and 5B as described
above provides examples of an arrangements of discrete structures
510 or 515 to provide a measurement or indication of the strain or
deformation of an object. Many different types of arrangements are
possible. FIGS. 6A, 6B, and 6C illustrate respective
two-dimensional arrangements of discrete plasmon supporting
structures 611-614, 621-624, and 631-634 within respective
assemblies that provide sets of symmetry axes. In FIGS. 6A, 6B, and
6C, discrete structures 611-614, 621-624, and 631-634 may be but
are not required to be substantially identical. For example, in
FIG. 6A, discrete structures 611-614 may all be metal
nanoparticles, separated regions of a suitable interface, or
assemblies of such structures, and discrete structures 611-614 may
all have the same size and composition. Alternatively, different
types of plasmon supporting structures may be used within each
assembly of FIG. 6A, 6B, or 6C.
[0040] FIG. 6A shows an assembly with a rectangular arrangement of
plasmon supporting structures 611-614. Structures 611 and 612 are
separated from each other by a separation Lx in an X direction as
are structures 613 and 614. Similarly, structures 611 and 614 are
separated from each other by a separation Ly in a Y direction as
are structures 612 and 613. Separations Lx and Ly may be the same
or be different from each other. If separations Lx and Ly are
different, a plasmon resonance associated primarily with
X-direction oscillations and separation of structures may have a
different frequency from the frequency of a plasmon resonance
associated primarily with Y-direction oscillations and separations
of the structures. As described above, input and output light with
a linear polarization in the X-direction may most effectively
interact with plasmon resonances that oscillate along the
X-direction and that are supported by structures 611 and 612 or 613
and 614 separated in the X direction. Similarly, input and output
light with a linear polarization in the Y-direction may most
effectively interact with plasmon resonances that oscillate along
the Y-direction and are supported by structures 611 and 614 or 612
and 613 separated in the Y direction. Polarization filtering of
input or output light may thus be used to separately measure
deformation components.
[0041] FIG. 6B shows another example of a T-shaped arrangement of
discrete plasmon supporting structures 621-624 that provide plasmon
resonances corresponding to oscillation along orthogonal X and Y
axes. With the T-shaped configuration, discrete structures 621 and
622 are separated along the X axis by a separation Lx, and discrete
structures 623 and 624 are separated along the Y axes by a
separation Ly. If desired, structures 621 and 622 may be different
in size or composition or separation Lx may differ from Ly, so that
a plasmon resonance associated primarily with structures 621 and
622 has a different frequency from the frequency of a plasmon
resonance associated primarily with structures 623 and 624. Arrays
or assemblies of plasmon supporting structures with the arrangement
of FIG. 6B may thus provide information about two corresponding
components of strain.
[0042] FIG. 6C shows an arrangement of six discrete plasmon
supporting structures 631-636 that are arranged to provide
separations Lx, L60, and L120 along three directions within the
same plane. In particular, structures 631 and 632 are separated by
a distance Lx along the X axis. Structures 633 and 634 are
separated by a distance L60 along a direction 60.degree. from the X
axis. Structures 635 and 636 are separated by a distance L120 along
a direction 120.degree. from the X axis. Accordingly, three
different polarizations of input light and/or separate measurements
of output light having different polarizations can provide
information for determining separate components of strain within
the assembly of FIG. 6C.
[0043] FIGS. 6A, 6B, and 6C are examples of two-dimensional
patterns of plasmon support structures for assemblies that may be
used in two-dimensional arrays. However, three-dimensional arrays
of discrete plasmon supporting structures could alternatively be
employed and may be able to provide information about additional
strain components. FIG. 7 shows an example of an array of discrete
plasmon supporting structures 710 that are arranged at interfaces
of multiple layers 720 in order to form a three-dimensional array.
For example, plasmon supporting structures 710 can be plasmonic
nanoparticles and may be arranged in multiple flat deterministic
arrays such as described above, but in array 700, the flat
deterministic arrays are stacked to create a three dimensional
arrangement. In particular, separation Lx and Ly in X and Y
directions may be between structures 710 in one layer of structures
710, and each layer may be separated by a separation Lz in a Z
direction from an overlying or underlying layer of structures 710.
The plasmon resonances of array 700 will generally depend on
separations Lx, Ly, and Lz, so that deformation of array 700 in
three different directions may have measurable effects on the
plasmon resonances of array 700.
[0044] FIGS. 8A, 8B, 8C, and 8D illustrate a process for
fabrication of an array of discrete plasmon supporting structures.
The illustrated fabrication process can begin as shown in FIG. 8A
with fabrication of pillars 820 on a substrate 810 and deposition
of metal caps 830 on the tops of pillars 820. In general, substrate
810 can be made of any desired material but in one implementation
is a semiconductor substrate of a material such as silicon. Pillars
820 can be formed on substrate 810 using any fabrication techniques
that are capable of producing pillars of the desired widths, which
would typically be smaller than about 200 nm. For example, known
integrated circuit fabrication techniques such as nanoimprint
lithography can be used to achieve the desired feature size and a
pattern for pillars 820 corresponding to the array of plasmon
supporting structures to be created. Metal caps 830 can be made of
any metal capable of supporting surface plasmons, e.g., platinum,
gold, silver, copper, aluminum, or nickel caps, can be deposited on
pillars using physical vapor deposition (PVD), chemical vapor
deposition (CVD), electrochemical deposition, or sputtering, for
example. As an alternative to deposition of metal on top surface of
pillars 830, metal regions 830 may be formed from a layer of metal
formed overlying a layer from which pillars 820 are formed and
patterned at the same time pillars 820 are created.
[0045] Pillars 820 may be closely spaced, e.g., less than 200 nm
from a closest neighboring pillar 820 when originally formed, but
the spacing between pillars 820 (and therefore between metal caps
830) can be further closed or reduced as shown in FIG. 8B. For
example, when the pillars 820 are exposed to a volatile liquid
which is allowed to evaporate, microcapillary forces between
pillars 820 or metal caps 830 can pull metal caps toward their
nearest neighbors and reduce their separation, e.g., down to less
than about 1 nm. Molecules on the pillar surfaces can be used to
maintain the gaps between the tips of pillars 820 in the range of
1-10 nm. These molecules can either be coated on pillars 820 before
exposure to the volatile liquid, e.g. through vapor-phase
self-assembly, or can be contained in the volatile liquid itself,
coating the pillar surface before evaporation is completed.
[0046] A nanoimprinting process illustrated in FIGS. 8C and 8D can
be used to bond metal caps 830 with a new substrate 840. Substrate
840 is a flexible or deformable substantially flat substrate.
Transfer of metal caps 830 to substrate 840 occurs after separating
pillars 820 and substrate 810, provided that adhesion of caps 830
to substrate 840 is greater than the adhesion of caps 830 to
pillars 820. This bonding can be achieved in a number of ways, such
as through a chemical adhesive or through a curable polymer.
Suitable adhesive layers include chemicals which bind strongly to
substrate 840 while leaving one or more free groups to bond to
metal caps 830. For example, mercaptopropyl-trimethoxy-silane
(MPTMS) can be used to bind to oxide substrates, leaving a free
thiol group to bind to gold, silver or other suitable metals.
Suitable polymers include thermally curable materials such as
polymethyl methacrylate (PMMA) and polymide and UV curable
materials such as polydimethylsiloxane (PDMS). Substrate 840 with
attached metal caps 830 can thus form a desired array for use in a
strain sensor as described above or may be bonded to another layer
on which further metal caps 830 are bonded, e.g., during
fabrication of a three-dimensional array. In the case of transfer
to a curable polymer, for example PDMS, a thick enough layer may be
formed to peel off and apply to a new substrate if desired.
Substrate 810 and pillars 820 can be discarded or reused as a
template for fabrication of another array.
[0047] An article of Steven J. Barcelo, Ansoon Kim, Wei Wu, and
Zhiyong Li, entitled "Fabrication of Deterministic Nanostructure
Assemblies with Sub-nanometer Spacing Using a Nanoimprinting
Transfer Technique," VOL. 6, NO. 7, 6446-6452, ACSNano (2012)
further describes some suitable fabrication techniques and is
hereby incorporated by reference in its entirety.
[0048] The above described stress sensors that use optical
measurements of light output from an array of discrete plasmon
supporting structures can further employ additional techniques for
measuring strain. In particular, when an array includes the plasmon
supporting structures are metal or otherwise conductive, the
electrical resistance of the array may also depend on deformation
of the array and therefore may provide a secondary indication of
the deformation of the object. Also, when an array includes the
plasmon supporting structures that are ferromagnetic, the magnetic
properties of the array may depend on deformation of the array and
therefore may provide a different or a further secondary indication
of the deformation of the object. FIG. 9 is a block diagram of a
strain sensor 900 that employs an array 930 of discrete plasmon
supporting structure for measurement of strain in an object 110 or
within array 930. Array 930 may be substantially identical to array
130 described above, but for the particular implementation of FIG.
9, array 930 uses ferromagnetic conductive particles, e.g., nickel
nanoparticles, as the discrete plasmon supporting structures. Array
930 may further include additional structures such electrical
contacts for measurement of resistance in array 930.
[0049] Strain sensor 900 may employ lighting system 230 and an
optical sensor 240 in the same manner described above to measure
output light 140 and determine the optical response of array 930 to
input light 130. However, sensor 900 further includes an ohm meter
or other resistance measuring device 910 that measures the
electrical resistance across array 930. In general, the electrical
resistance may be expected to increase as the separations between
conductive structures in array increase. Sensor 920 also includes a
magnetic sensor 920 that detects the magnetic properties of array
930. The magnetic sensor 920 could scan the surface to measure the
magnetic field in a given area. In general, the magnetic field in a
given area may be expected to decrease as the material is
stretched, decreasing the effective density of magnetic
nanoparticles. Controller 950 in addition to performing the
functions of controller 250 described above can combine strain
measurements based on optical, resistive, and magnetic
characteristics of array 930 to improve accuracy of measurements of
strain or another physical quantity that depends on the strain in
array 930.
[0050] Although particular implementations have been disclosed,
these implementations are only examples and should not be taken as
limitations. Various adaptations and combinations of features of
the implementations disclosed are within the scope of the following
claims.
* * * * *