WO2012088117A1

WO2012088117A1 - Spectrometer including three-dimensional photonic crystal

Info

Publication number: WO2012088117A1
Application number: PCT/US2011/066167
Authority: WO
Inventors: Ioannis Kymissis; Jia Zhang; Marshall Cox; Nadia Pervez
Original assignee: Ioannis Kymissis; Jia Zhang; Marshall Cox; Nadia Pervez
Priority date: 2010-12-21
Filing date: 2011-12-20
Publication date: 2012-06-28

Abstract

A three-dimensional matrix can be configured to suspend diffracting particles (e.g., the matrix and diffracting particles providing a three-dimensional photonic crystal), the matrix including an adjustable volume fraction of the particles, the matrix configured to reflect an adjustable range of wavelengths, the adjustable range determined at least in part using the adjustable volume fraction of particles. A processor circuit can adjust the adjustable volume fraction of particles, and a detector can obtain information about at least a portion of energy reflected from the matrix, the portion corresponding to an amount of energy, within the adjustable range of wavelengths, present in incident optical energy reflected by the three-dimensional matrix.

Description

SPECTROMETER INCLUDING THREE-DIMENSIONAL PHOTONIC CRYSTAL

CLAIM OF PRIORITY

Benefit of priority is claimed to U.S. Provisional Patent Application Serial

Number 61/427,977, titled "SPECTROMETER INCLUDING THREE- DIMENSIONAL PHOTONIC CRYSTAL," filed on December 29, 2010 (Attorney Docket No. 2413.125PRV), and benefit of priority is also claimed to U.S.

Provisional Patent Application Serial Number 61/425,681 , titled "INFRARED PHOTONIC CRYSTAL SPECTROMETER," filed December 21 , 2010 (Attorney Docket No. 2413.127PRV), the entireties of both of which are hereby incorporated herein by reference.

BACKGROUND

Generally, commercially available spectrometers, such as molecular spectrometers, discriminate wavelengths via diffraction gratings. Light incident on a diffraction grating is reflected off of the grating, such as at an angle dependent on the wavelength of the incident light.

Differing wavelengths of light then spatially separate downstream of this grating, and are generally measured by a linear detector array. This spatially- resolved information is then converted to wavelength-resolved information using the geometry of the diffraction grating and the distance from the grating to the detector. To obtain high spectral resolution, the distance from the grating to the detector is generally quite long, resulting in physically large spectrometers.

One recent improvement to this diffractive grating approach is the use of the enhanced diffraction via the superprism effect in photonic crystals. However, this approach still scales in the same manner as a diffractive grating spectrometer. Smaller diffractive grating spectrometers generally sacrifice spectral bandwidth for spectral resolution. Cost is also an issue with diffractive spectrometers due to their tight manufacturing tolerances and after-production alignment, qualification, and calibration. OVERVIEW

A photonic crystal can reflect light including a specified range of wavelengths based on the parameters of the photonic crystal's structure, such as including a period of a lattice included in the photonic crystal, a refraction index difference, or one or more other parameters. Generally, a photonic crystal structure includes at least two materials having different refraction indices.

For example, in a three-dimensional (3D) photonic crystal, one material can be a matrix and the other can form "cores" suspended in the matrix, such as to provide an ordered periodic structure similar to atoms in a crystal lattice. The reflection of the specified range of wavelengths can be due at least in part to coherent Bragg optical diffraction within the photonic crystal, provided by the spatially-periodic modulation of the index of refraction between the cores and the matrix material (e.g., because the cores and matrix have different refractive indexes from each other).

In an example, the cores can be arranged in a non-close-packed face- centered cubic structure, and the matrix material can fill the region surrounding the cores. The period of such a structure can be adjusted, such as by controlling the amount of volume occupied by the cores as compared to the total volume of the cores plus the surrounding matrix material. Such a 3D photonic crystal can be included as a portion of an inexpensive or miniaturized spectrometer, such as to for use in characterizing an unknown source of light. Applications for such a spectrometer include color measurement, such as for LED testing, lighting testing, color matching in paint or cosmetics, or color calibration in printing. Other applications include analytical applications (e.g., chemical analysis, water testing, wine making, food/beverage manufacturing or monitoring, plasma etch process monitoring, thin film process monitoring, solar cell production process monitoring, oxygen sensing, pharmaceutical manufacturing). Still other applications can include medical or clinical diagnostic monitoring, such as skin testing or blood testing, among other uses. In an example, a three-dimensional matrix can be configured to suspend diffracting particles, the matrix including an adjustable volume fraction of the particles (e.g., to provide a 3D photonic crystal). For example, the matrix can be configured to reflect an adjustable range of wavelengths, the adjustable range determined at least in part using the adjustable volume fraction of particles. The three dimensional matrix including the diffracting particles can form a three- dimensional photonic crystal. As the volume fraction of diffracting particles is adjusted, the range of wavelengths reflected by the crystal can be swept across a desired or specified range of frequencies. Multiple three-dimensional matrices can be included on or within a single substrate, such as to provide coverage of a wider range of wavelengths than can be scanned by adjusting a single matrix.

A spectrum of an unknown source of light can be determined, such as by reflecting the unknown light off the matrix, while adjusting (e.g. "sweeping") the matrix to provide a specified range of reflected wavelengths, and comparing the resultant reflected responses with responses corresponding to a known source.

This overview is intended to provide an overview of subject matter of the present patent application. It is not intended to provide an exclusive or exhaustive explanation of the invention. The detailed description is included to provide further information about the present patent application.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates generally an example of an apparatus that can include a three-dimensional photonic crystal, such as optically coupled to a detector.

FIG. 2 illustrates generally an example of adjusting an inter-particle distance, such as including diffracting particles suspended in a matrix.

FIG. 3 illustrates generally an example of a technique that can include adjusting a volume fraction of diffracting particles in a photonic crystal.

FIG. 4 includes a plot of wavelength-dependent spectral responses for numerical aperture limited light extracted in the normal direction from a substrate including various illustrative examples of photonic crystal patterns. FIG. 5 illustrates generally an example of a photonic crystal spectrometer. FIG. 6 illustrates generally an illustrative example of a comparison between photonic crystal pattern response functions.

FIG.7 illustrates generally an example of propagating modes in a slab waveguide.

FIG. 8 illustrates generally an example of scattering of waveguide modes by a photonic crystal at an interface.

FIGS. 9A-B illustrate generally SEM images of a polydimethylsiloxane (PDMS) mold in FIG. 9A and imprint of a photonic crystal pattern in FIG. 9B, such as for 1.35 micrometer wavelength extraction.

FIGS. 10A-B illustrate generally SEM images of a Gallium Phosphide (GaP) photonic crystal cavity in FIG. 10A, and a broad-band reflectivity measurement of cavity resonance with Q =6000 in FIG. 10B.

FIG. 1 1 illustrates generally an example of photonic crystals transferred from a GaP wafer to another substrate such as via PDMS stamping.

FIG. 12 illustrates generally (GaAs) photonic crystal cavity indicating a linewidth below 1 nanometer in the near-infrared.

In the drawings, which are not necessarily drawn to scale, like numerals may describe similar components in different views. Like numerals having different letter suffixes may represent different instances of similar components. The drawings illustrate generally, by way of example, but not by way of limitation, various embodiments discussed in the present document.

DETAILED DESCRIPTION FIG. 1 illustrates generally an example of an apparatus 100 that can include a three-dimensional photonic crystal 108, such as optically coupled to a detector 124. The photonic crystal 108 can include a reflecting surface 104, such as configured to reflect a portion of incident optical energy 1 10. Such reflected optical energy 1 12 can include a specified range of wavelengths. The reflected optical energy 1 12 can be steered towards the detector 124, such as using a beam splitter 130 or other optics. One or more optical devices can be used, such as to stop, limit, or otherwise control the numerical aperture (NA) of optical energy 1 10 coupled to the photonic crystal 108, or the reflected energy 1 12 coupled to the detector 124.

In an example, the photonic crystal 108 can include a first three-dimensional lattice in a first region 120A. The first lattice can include diffracting particles (e.g., silica or polymer spheres), such as to provide a tunable opal structure. For example, a first, second, or third diffracting particle 106A-C can be suspended in a matrix 1 02, such as to provide a specified inter particle distance or a corresponding volume fraction of particles 106A-C. For example, the diffracting particles can include silica particles, such as silica microspheres having a different index of refraction than the surrounding matrix 108.

In an example, the incoming optical energy 1 10 can be incident

perpendicular to the surface 104 of the photonic crystal 108, or at least within a specified NA, and the reflected light (in the reversed direction also perpendicular to the surface 104) can provide an reflected intensity peak when wavelength of the incident optical energy 1 10 matches the condition shown in EQN. 1 (e.g., the Bragg equation for normal-incident energy impinging on a (1 1 1 ) plane of a face-centered cubic structure), below.

Λ = f ^ ) ^{l /:i} ( | ) "¾ ( ^0 - , ( 1 - " ) ) ¾? [EQN. 1 ]

In EQN. 1 , "D" can represent the core (e.g., diffracting particle 106A-C) diameter, "n_p" and "n_m"can represent the respective refraction indices of the core and the matrix 102 respectively, and " " can represent the volume fraction of the core. The volume fraction refers to the proportion of the volume of photonic crystal occupied by the cores or diffracting particles as compared to the total volume of the crystal.

The present inventors have recognized, among other things, that the volume fraction of the diffracting particles can be adjusted to cause the photonic crystal 108 to reflect a desired range of wavelengths. For example, the first region 120A can include a lattice configured to reflect a first range of wavelengths. The present inventors have also recognized that the materials used for one or more of the matrix 102 or the diffracting particles 106A-C can allow the inter-particle separation or volume fraction to be adjusted, such as discussed in the examples of FIG. 2. For example, a processor circuit 126 (e.g., a microcontroller, a field-programmable gate array (FPGA), an application specific integrated circuit (ASIC), or a

microprocessor) can be coupled to the photonic crystal 108. The processor circuit 126 can modulate or otherwise adjust the volume fraction of the diffracting particles 106A-C, such as to "sweep" the peak value of the reflected wavelength range, corresponding to EQN. 1 , across a desired range of wavelengths. Such a sweep can be performed in discrete steps, or continuously. For example, in FIG. 4, a plot of response curves can be obtained corresponding to discrete ranges of wavelengths that can be reflected by the photonic crystal 108.

In an example, such as to adjust the range of reflected wavelengths, the processor circuit 126 can be coupled to one or more of an electric field generator (e.g., one or more plates or other electrodes located in contact with or near the matrix 102), a magnetic field generator (e.g., a coil or other structure configured to couple a magnetic field into the matrix 102), a heating or cooling apparatus (e.g., to alter or control the temperature of the matrix 102), or one or more mechanical actuators such as to induce strain, stretch, or compress the matrix 102.

A second region 120B can provide a second lattice configuration, such as to reflect a second range of wavelengths. In an example, if a range of available adjustable reflected wavelengths does not cover an entirety of a desired range of wavelengths, the second region 120B can be configured for tuning across a different range of wavelengths than the first region 120A. In an example, such ranges of wavelengths can overlap, or can be specified according to an application for the spectrometer (e.g., to capture various peak emission ranges of a gaseous species, etc.). For example, the detector 124 can obtain information about the intensity of the reflected energy 1 12, or the location of the reflected energy 1 12 along the surface 104 of the photonic crystal 108, such as for determination of a spectrum of an unknown source of the incident optical energy 1 10. FIG. 2 illustrates generally an example of adjusting an inter-particle distance, such as including diffracting particles suspended in a matrix 102 as shown in FIG. 1. For example, increasing an inter-particle distance in the lattice can decrease the volume fraction of particles as compared to a total volume of matrix plus particles, thus red-shifting the range of reflected wavelengths. Similarly, decreasing an inter-particle distance in the lattice can increase the volume fraction of particles, thus blue-shifting the range of reflected wavelengths.

Various materials can be used for the matrix 102. For example, the matrix 102 and cores can be formed lithographically (e.g., using e-beam lithography), such as to achieve a desired index contrast between the core regions (e.g., the diffracting particles 106A-C) and the surrounding matrix 102. However, such lithographic fabrication techniques can be time-consuming.

In another approach, the present inventors have also recognized that a self- assembling colloid can be used to provide the photonic crystal 108. For example, a portion of the photonic crystal of FIGS. 1 -2 can include one or more materials or techniques mentioned in Kim et al., "Integration of Colloidal Photonic Crystals toward Miniaturized Spectrometers," Advanced Materials, Vol. 22, pp. 946-950 (2010), which is hereby incoiporated by reference herein in its entirety, including its discussion of non-close-packed colloidal crystals such as including silica particles dispersed in ethoxylated trimethylolpropane triacrylate (ETPTA).

The present inventors have also recognized that some polymer materials exhibit a controllable strain or displacement behavior, such as in response to one or more of an applied electric or magnetic field, a temperature change, an addition of a charged species or a solvent, or using one or more other techniques. For example, in the illustrative example of FIG. 2, showing a plane of a face of a three- dimensional lattice, a first state 230A can include a first inter-particle distance between particles 206A-B, such as included as a portion of a face-centered cubic lattice (or other structure). The distance between nearby diffractors can be adjusted, such as reduced as shown in the second state 230B, or increased. In an example, the distance can be further reduced, such as to provide a close-packed structure (or a nearly close-packed structure), such as shown in the third state 230C. Such a change in inter-particle distance can correspond to a change in the volume fraction of the cores (e.g., the particles 206A-B) in the 3D lattice, changing the photonic bandgap and thus affecting the range of reflected wavelengths.

In an example, the matrix 102 can include an electrically-tunable

polyfeiTocenylsilane (PFS), such as configured to suspend silica or other polymer spheres to provide a tunable photonic crystal. For example, a portion of the photonic crystal of FIGS. 1 -2 can include one or more materials or techniques such as mentioned in Arsenault et al., "Photonic-Crystal Full-Colour Displays," Nature Photonics, Letters, Vol. 1 , August 2007, doi: 10.1038/nphoton.2007.140, or

Arsenault et al., United States Patent No. 7,826, 131 , "Tunable Photonic Crystal Device," both of which are hereby incorporated by reference herein in their respective entireties, including their discussion of photonic crystal structures including PFS.

In an example, the matrix 102 can include a polymer material, such as a ferroelectric polymer that can provide an electrostrictive response. Such an electrostrictive material can provide controllable strain, such as in response to an applied electric field. In an example, a ferroelectric relaxor terpolymer can fonn a portion of the matrix 102, such as including a poly(vinylidene fluoride- trifluoroethylene-chlorofluoroethylene) ("P(VDF-TrFE-CFE") terpolymer, such as available from Ktech Corporation, Albuquerque, New Mexico, United States of America.

FIG. 3 illustrates generally an example of a technique 300 that can include, at 302, adjusting volume fraction of diffracting particles, as discussed in the examples above. The particles can be suspended in a three-dimensional matrix, to reflect an adjustable range of wavelengths, the adjustable range of wavelengths determined at least in part using the volume fraction of diffracting particles. At 304, the technique 300 can include obtaining information about at least a portion of the energy reflected from the matrix, the portion corresponding to an amount of energy, within the adjustable range of wavelengths, present in incident optical energy reflected by the three-dimensional matrix. The technique 300 can be performed such as using apparatus or materials discussed above in the examples of FIGS. 1 -2, or FIG. 4.

The technique 300 can optionally include estimating a spectrum of incident optical energy using the information obtained about the energy reflected from the matrix during a first or a second duration. The information obtained, such as by a detector, can include one or more of the location of the reflected energy along a surface of the matrix or an intensity of reflected energy detected during a specified duration.

FIG. 4 includes a plot of wavelength-dependent spectral responses 1 -9 for numerical aperture limited light extracted in the normal direction from a substrate including various illustrative examples of photonic crystals. It is believed the responses 1 -9 of FIG. 4 can be conceptually similar to the responses of a tunable three dimensional photonic crystal that can be adjusted to reflect a desired range of wavelengths. In an example, the responses 1 -9 can each correspond to optically detected intensity responses for respective tuned wavelengths on or within the photonic crystal, each range including a specified volume fraction of diffracting cores (e.g., particles) to provide a desired wavelength response peak, as discussed in the examples above. Because the response functions can overlap, information provided by a detector coupled to the photonic crystal can be used to provide a estimate of an input spectrum across the range of wavelengths provided by the overlapping responses 1 -9.

A spectrometer using a photonic crystal can map such intensities in a pattern

(or temporal) response space to a wavelength space such as using information known about the response functions for each tuned wavelength range. Using matrix arithmetic, the response of the system can be described as Ax— b + delta, where A can be an ///><« matrix including information about the intensities of /// tunable ranges at n wavelengths, .v can be a wavelength space representation of the input optical energy, and b can be a pattern response space (or a temporal response) representation of the input optical energy (such as corresponding to an intensity pattern or intensity time-series detected by an optical detector). In an example, delta can represent an optional error parameter such as used during estimation. The /// responses can peak at different wavelengths (as shown in FIG. 4), A can be full rank and thus its Moore-Penrose pseudoinverse is a valid right inverse, which can be used to solve for the spectral response of the input to provide an estimate of the input spectrum, x. For example, a matrix projection operator can be a function of both the number of response elements and the number of wavelengths used to characterize a pattern. Such a projection operator, P, can be represented by

and the recovered spectrum (e.g., an estimate of the spectrum x), can be represented by

Examples that can include a two-dimensional (2D) photonic crystal, and examples that can include infrared (IR) applications

FIG. 5 illustrates generally an example of a photonic crystal spectrometer. For example, a compact infrared photonic crystal spectrometer can be used to detect wavelengths between 1-14 micrometers, such as for use in high vacuum

environments or at cryogenic temperatures. The photonic crystal spectrometer can use leaky modes of an array of photonic crystals patterned on a multimode waveguide to selectively extract wavelength-specific light such as to a 2D detector array (e.g. an imager). The pattern detected by the imager can be analyzed to reconstruct the spectrum.

The present inventors have recognized that using photonic crystal-based spectrometer is different than diffractive grating technologies. For example, the photonic crystals can be arrayed over two or three dimensions such as including many small patterned regions allowing a single detector to image many patterns. Unlike grating-based technologies, the spectrometer resolution can be independent of the distance between the diffractive elements and the detector. Also, unlike a Fourier transform infrared (FTIR) spectrometer, the photonic crystal spectrometer requires no moving parts. These characteristics also yield a

spectrometer with the potential for high vacuum, low temperature, vibration, or particle tolerance, unlike other technologies. The scaling law that governs photonic crystal structures can allow miniaturization of such photonic crystal-based spectrometers; the resulting module size can be constrained by the dimensions of the detector.

A photonic crystal-based spectrometer can be used in high-resolution infrared (IR) applications, such as where moving parts are undesirable. The photonic crystal spectrometer's geometry scales in a different manner than a diffractive grating spectrometer, which can help break the dependency of resolution on the spatial separation between the diffractive element (in this case, the photonic crystal array) and the detector.

Infrared spectroscopy can be used in applications in a wide variety of settings, including industrial, agricultural, scientific, health, and defense-related applications. Due to the abi lity of infrared spectroscopy to detect chemical signatures of extremely small molecules such as carbon dioxide, it can be used for identification of air and water-bom contaminants in defense, law enforcement, and public health applications. Such applications can include rapid and portable biological/chemical analysis and sensing (e.g., for use such as by the United States Department of Defense, the United States Department of Homeland Security, the United States Centers for Disease Control, the United States Food and Drug Administration, the United States Environmental Protection Agency, or other agencies or entities located in the United States or elsewhere), rapid and portable molecular analysis for forensics (e.g., such as for use by the United States Federal Bureau of Investigation, or other state or local law enforcement), or for emissions testing and analysis. A compact infrared photonic crystal spectrometer can be used to detect wavelengths between about 700 nanometers -14 micrometers (or another range of wavelengths), such as for use in high vacuum environments or at cryogenic temperatures. The photonic crystal spectrometer can use the leaky modes of an array of photonic crystals patterned on a multimode waveguide to selectively extract or reflect wavelength-specific light such as to a 2D detector array (e.g. an imager). The pattern detected by an imager can be analyzed to reconstruct the spectrum.

For example, the substrate can be transparent to a near-infrared range of wavelengths, such as including wavelengths from less than 700 nanometers to more than 14 micrometers, such as encompassing a majority of a near-infrared range of wavelengths. The substrate can include one or materials such as zinc sulfide, fused silica, silicon oxide, cesium iodide, cesium fluoride, calcium chloride, potassium chloride, thallium bromo-iodide, or that can include one or more other materials.

In an example, the spectrometer can include an optical detector sensitive to the farthest infrared wavelength of interest. Since semiconductor detector materials are sensitive to light of equal or higher energy than their bandgap, and the photonic crystal array converts spectral composition information to spatial location information, a 2 -dimensional imager sensitive to the farthest IR wavelength of interest can be used in the photonic crystal spectrometer for wavelengths from the deep IR to the visible. The IR detector can include one or more of a bolometer, a pyroelectric detector, a subband detector, or another IR detector, such as including a solid-state detector. In an example, the use of high quality-factor (Q) photonic crystals in an infrared application can provide enhanced spectral resolution. For example, the Q factor of photonic crystals can be dependent on photonic crystal materials and dimensions.

In an example, the spectrometer can include a layer or structure between or comprising a portion of the photonic crystal or the optical detector, such as for conversion of energy from outside the detector's sensitive range of wavelengths into a region inside the detector's sensitive range such as using one or more of a non- linear optical region, a phosphor, a 15 fluorophore, a charge-discharge material, an organic dye, an organic crystal, or quantum dots.

The present inventors have recognized that using photonic crystal-based spectrometer is different than diffractive grating technologies. For example, the photonic crystals can be arrayed over two dimensions such as including many small patterned regions allowing a single detector to image many patterns. Unlike grating-based technologies, the spectrometer resolution can be independent of the distance between the diffractive elements and the detector, Also, unlike a Fourier transform infrared (FTIR) spectrometer, the photonic crystal spectrometer requires no moving parts. These characteristics also yield a spectrometer with the potential for high vacuum, low temperature, vibration, or particle tolerance, unlike other technologies. The scaling law that governs photonic crystal structures can allow miniaturization of such photonic crystal-based spectrometers; the resulting module size can be constrained by the dimensions of the detector.

The examples discussed above and below can include or can use apparatus or techniques, such as discussed above with respect to the examples of FIGS. 1 -4. For example, an illustrative example of response function curves discussed above in the example of FIG. 4 can be used, such as via an inverse matrix operation, to reconstruct the spectrum under test, for either a reflective 3D photonic crystal structure, or a structure configured to extract light from a waveguide via a 2D photonic crystal structure.

In the example of FIG. 5, a photonic crystal spectrometer can include three components: an optical waveguide that can feed light into the system, a photonic crystal pattern (or patterns) that can selectively extract bands of wavelengths from the waveguide, and a detector (e.g., including optics).

FIG. 6 illustrates generally an illustrative example 600 of a comparison between photonic crystal pattern response functions, including a first response function 602A, and a modified response function 602B. The response curves discussed above, such as in the example of FIG. 4, can be narrowed such as to provide improved extraction efficiency or response function quality factor, as shown in the example of FIG. 6. For example, a quality factor can be increased, or a signal-to-noise ratio can be decreased, via material selection, photonic crystal array layout, or a reduction of the numerical aperture of the optics between the photonic crystal array and detector. In an example, an increase in quality factor can improve a signal to noise ratio, such as for coupling to a detector having a fixed dynamic range. Such an increase in quality factor, or decrease in signal-to-noise ratio can reduce an integration time used during detection, such as providing an improvement in detection speed or an enhancement in a frequency resolution of detection.

FIG. 7 illustrates generally an example of propagating modes in a slab waveguide. FIG. 8 illustrates generally an example of scattering of waveguide modes by a photonic crystal at an interface. As discussed above, wavelength selective extraction in a photonic crystal fabricated on top of an optical waveguide can occur because the presence of the photonic crystal can alter the phase matching at an interface between the optical waveguide and free space (or another dielectric material that contrasts with the optical waveguide).

Introduction of a periodic potential variation scatters guided wavevectors by reciprocal lattice vectors that can be represented by "G" into extracted wavevectors, such as illustrated in FIG. 8. The range of wavelengths extracted by a photonic crystal pattern can be determined at least in part by the lattice constant of the pattern or the modes supported by the waveguide. In an illustrative example, a thick glass coverslip supporting thousands of modes can be used as a waveguide, simplifying the design relationship between peak extraction wavelength and lattice constant. The range of wavelengths imaged at the normal by the camera can be a subset of the extracted wavelengths, such as determined by the numerical aperture of the optics interfacing the camera with the photonic crystal array.

The present inventors have recognized, among other things, that adapting the photonic crystal spectrometer concept from visible wavelengths to IR wavelengths can include scaling up the photonic crystal dimensions (as compared to the dimensions use for extraction of visible light), using a different material for the patterned waveguide, or using a different optical detection technique (e.g., a different detector).

In an example including an IR spectrometer, factors that can influence performance can include index of refraction and resulting index contrast in the patterned region of a two-dimensional photonic crystal, or within a volume of a three-dimensional photonic crystal. A relatively high index contrast is desirable for an IR spectrometer application.

Materials suitable for transmission of 1-14 micrometers include zinc sulfide, potassium chloride, potassium bromide, silicon oxide, fused silica, silver chloride, silver bromide, thallium bromo-iodide, calcium chloride, cesium fluoride, and cesium iodide, for example.

In a photonic crystal coupled to an optical waveguide, a periodic potential formed by spatial variation in the relative permittivity of a medium interacts with electromagnetic radiation allowing for scattering by reciprocal lattice vectors, which in turn leads to the extraction of a specified range of wavelengths of light. The band structure can be determined by the choice of lattice, the basis of such a lattice formed by the shape and size of the features included in a patterned array, the thickness of the patterned layer including the array, or the contrast in the spatial variation. For example, the energy scale for the band structure can be set by the lattice constant. Using such parameters, a photonic crystal can be configured for extraction of 1-14 micrometer radiation (or some other desired range of

wavelengths), such as including response functions appropriate determination of an unknown input spectrum at a wavelength of about 0.1 micrometer or finer.

FIGS. 9A-B illustrate generally SEM images of a PDMS mold in FIG. 9A and imprint of a photonic crystal pattern in FIG. 9B, such as for 1.35 micrometer wavelength extraction. In an example, a fabrication process for the photonic crystal patterns compatible with substrate materials, such as discussed above, can include photolithography, electron beam lithography (EBL), or nanoimprint lithography (NIL), or one or more other fabrication techniques. For example, a pattern can etched into the substrate or created in an adjacent material layer above the substrate. The processes employed can depend on the substrate chosen as various materials exhibit differing degrees of sensitivity to light, humidity, and process chemicals.

In an example, a fabrication process for a photonic crystal can include a PDMS stamping process, such as to provide one or more features with a 900 nanometer lattice constant, such as on glass, such as for extraction of 1.35 micrometer light, as shown in the illustrative example of FIG. 9A. Such a stamping process can be used to provide the photonic crystal pattern shown in FIG. 9B.

FIGS. 10A-B illustrate generally an SEM image of a GaP photonic crystal cavity in FIG. 10A, and a corresponding broad-band reflectivity measurement of such a cavity resonance showing a quality factor (Q) of approximately 6000 in FIG. 10B. In an example one or more high quality factor photonic crystal cavities can be fabricated, such as using one or more techniques shown in the illustrative examples of FIGS. 9A-B, or FIG. 1 1 or using other techniques.

FIG. 1 1 illustrates generally an example, such as a technique, that can include transferring a photonic crystal pattern from a GaP wafer to another substrate, such as using a PDMS stamp, or using one or more other materials. For example, at 1 102, a photonic crystal pattern can be fabricated on a first substrate, such as a GaP wafer. A second material, such as PDMS, can be formed on a surface of the first substrate, such as to provide a "stamp" including "pillars" corresponding to the GaP wafer cavity locations. At 1 104, the stamp can be lifted off the first substrate, such as for transferring a pattern of cavities on the first substrate onto a second substrate. At 1 106, a photonic crystal pattern can be formed on a surface of a second substrate using the PDMS stamp, such as corresponding to the photonic crystal pattern provided on the first substrate.

FIG. 12 illustrates generally an example of polarization-resolved

measurement of a GaAs photonic crystal cavity indicating a linewidth below I nanometer in the near-infrared. FIG. 12 illustrates generally the result of a polarization-resolved reflectivity measurement on a device, such as shown in the illustrative example of FIG. 1 1. In an example, one or more imaging detectors sensitive to 1-14 micrometers can be included as a portion of the IR photonic crystal spectrometer. Such a detector can include a 2D focal plane array such as including a microbolometer, a photodetector, a pyroelectric detector, or a thermoelectric element. A high- resolution detector need not be used, as sensitivity improves as the size of the patterned regions increases.

In an example, one or components or portions of the IR photonic crystal spectrometer can be characterized such as using one or more of a high-vacuum or low-temperature environment, such as including an analysis of any outgassing provided by the components under test. During development, crystal patterns configured to extract a target wavelength range can be fabricated and characterized such as using a scanning electron microscope (SEM). The response functions of such patterns can be assessed such as using light extraction measurements. For example, a sensitivity can be estimated using light extraction measurements and a calibrated IR photodetector.

FIGS. 1 3A-B illustrate generally an illustrative example of a 9-pattern photonic crystal spectrometer that can be used to calculate a spectrum using wavelength selectively extracted waveguided light from a glass slide. For example, a patterned portion of an array can selectively extract a range of wavelengths. An illustrative example of response function curves corresponding to the nine patterns is shown in FIG. 13 A. These response functions can be used, such as via an inverse matrix operation, to reconstruct the spectrum under test similar to the techniques discussed above in relation to FIG. 4.

In an illustrative example, as shown in FIG. 13B, a white LED light source can be measured using the 9-pattem photonic crystal spectrometer. The measured spectrum can be compared to both the results obtained with a commercial spectrometer and the projection of the spectrum on the 9 element basis formed by the photonic crystal pattern response functions. Simulations can show that the observed deviation between the measured and actual spectrum can be due to the number of photonic crystal patterns. For example, by increasing the number of patterns, the spectrometer resolution can be increased.

In an example, an analysis of the basis formed by the photonic crystal response functions can be used to determine a minimum distance between two delta functions in wavelength that can still allow for them to be resolved as separate entities. In addition to narrowing the response functions (e.g., increasing a quality factor associated with a response function corresponding to a pattern in the photonic crystal), an increased number of response functions can be used such as to increase resolution.

Numerical aperture (NA) can influence response function width. For example, in a spectrometer without imaging lenses, a reduced NA is used as compared to a spectrometer with lenses. In an example, measurements can be made such as using an adjustable iris in the infinity space of an infinity-corrected lens system to assess the impact of NA adjustment on the measured response functions.

In an example, the photonic crystal pattern can extract not only the target wavelengths, but also multiples of the frequencies included in the range of the target wavelengths, thus order sorting taking advantage of such an effect can be used to provide a spectrometer sensitive to a range of wavelengths from 1 micrometer to 20 micrometers, or some other range of frequencies (e.g., spanning more than an octave).

Additional Notes

The above detailed description includes references to the accompanying drawings, which form a part of the detailed description. The drawings show, by way of illustration, specific embodiments in which the invention can be practiced. These embodiments are also referred to herein as "examples." Such examples can include elements in addition to those shown or described. However, the present inventors also contemplate examples in which only those elements shown or described are provided. Moreover, the present inventors also contemplate examples using any combination or permutation of those elements shown or described (or one or more aspects thereof), either with respect to a particular example (or one or more aspects thereof), or with respect to other examples (or one or more aspects thereof) shown or described herein.

All publications, patents, and patent documents referred to in this document are incorporated by reference herein in their entirety, as though individually incorporated by reference. In the event of inconsistent usages between this document and those documents so incorporated by reference, the usage in the incorporated reference(s) should be considered supplementary to that of this document; for irreconcilable inconsistencies, the usage in this document controls.

In this document, the terms "a" or "an" are used, as is common in patent documents, to include one or more than one, independent of any other instances or usages of "at least one" or "one or more." In this document, the term "or" is used to refer to a nonexclusive or, such that "A or B" includes "A but not B," "B but not A," and "A and B," unless otherwise indicated. In this document, the terms

"including" and "in which" are used as the plain-English equivalents of the respective terms "comprising" and "wherein." Also, in the following claims, the terms "including" and "comprising" are open-ended, that is, a system, device, article, or process that includes elements in addition to those listed after such a term in a claim are still deemed to fall within the scope of that claim. Moreover, in the following claims, the terms "first," "second," and "third," etc. are used merely as labels, and are not intended to impose numerical requirements on their objects.

Method examples described herein can be machine or computer- implemented at least in part. Some examples can include a computer-readable medium or machine-readable medium encoded with instructions operable to configure an electronic device to perform methods as described in the above examples. An implementation of such methods can include code, such as microcode, assembly language code, a higher-level language code, or the like. Such code can include computer readable instructions for performing various methods. The code may form portions of computer program products. Further, in an example, the code can be tangibly stored on one or more volatile, non-transitory, or non- volatile tangible computer-readable media, such as during execution or at other times. Examples of these tangible computer-readable media can include, but are not limited to, hard disks, removable magnetic disks, removable optical disks (e.g., compact disks and digital video disks), magnetic cassettes, memory cards or sticks, random access memories (RAMs), read only memories (ROMs), and the like.

The above description is intended to be illustrative, and not restrictive. For example, the above-described examples (or one or more aspects thereof) may be used in combination with each other. Other embodiments can be used, such as by one of ordinary skill in the art upon reviewing the above description. The Abstract is provided to comply with 37 C.F.R. § 1.72(b), to allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the

understanding that it will not be used to interpret or limit the scope or meaning of the claims. Also, in the above Detailed Description, various features may be grouped together to streamline the disclosure. This should not be interpreted as intending that an unclaimed disclosed feature is essential to any claim. Rather, inventive subject matter may lie in less than all features of a particular disclosed embodiment. Thus, the following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separate embodiment, and it is contemplated that such embodiments can be combined with each other in various combinations or permutations. The scope of the invention should be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.

Claims

THE CLAIMED INVENTION IS:

1. An apparatus comprising:

a three-dimensional matrix configured to suspend diffracting particles, the matrix including an adjustable volume fraction of the particles, the matrix configured to reflect an adjustable range of wavelengths, the adjustable range determined at least in part using the adjustable volume fraction of particles;

a processor circuit configured to adjust the adjustable volume fraction of particles; and

a detector configured to obtain information about at least a portion of energy reflected from the matrix, the portion coiTesponding to an amount of energy, within the adjustable range of wavelengths, present in incident optical energy reflected by the three-dimensional matrix.

2. The apparatus of claim 1 , the processor circuit is configured to adjust the adjustable volume fraction using one or more of an adjustable magnetic field applied to the matrix, an adjustable electric field applied to the matrix, an adjustable temperature of the matrix, or via an applied mechanical manipulation of the matrix.

3. The apparatus of claim 2, wherein the matrix includes an elastomeric material, and wherein the processor circuit is configured to adjust the adjustable volume fraction via stretching or compressing the elastomeric material in at least one axis.

4. The apparatus of claim 2, wherein the matrix includes an electrostrictive material, and wherein the processor circuit is configured to adjust the adjustable volume fraction via application of a voltage differential across at least a portion of the matrix.

5. The apparatus of claim 4, wherein the matrix is configured to reduce an inter-particle distance of the particles in response to an applied voltage differential.

6. The apparatus of claim 1 , wherein the matrix includes a P(VDF-TrFE-CFE) terpolymer.

7. The apparatus of claim 1 , wherein the matrix includes polyferrocenylsilane (PFS).

8. The apparatus of claim 1 , wherein the suspended particles include silica microspheres.

9. The apparatus of claim 1 , wherein the three-dimensional matrix is configured to reflect a first specified range of wavelengths during a first duration using a first volume fraction of the particles and a second specified range of wavelengths during a second duration using a second volume fraction of the particles, and wherein the detector is configured to obtain information about energy reflected from the matrix during the first duration and during the second duration.

10. The apparatus of claim 9, wherein the processor circuit is configured to estimate a spectrum of incident optical energy using the information obtained by the detector during the first and second durations.

1 1. The apparatus of claim 10, wherein the information obtained by the detector includes one or more of the location of the reflected energy along a surface of the matrix or an intensity of reflected energy detected during a specified duration.

1 2. A method, comprising:

adjusting a volume fraction of diffracting particles, suspended in a three- dimensional matrix, to reflect an adjustable range of wavelengths, the adjustable range of wavelengths determined at least in part using the volume fraction of diffracting particles;

obtaining information about at least a portion of energy reflected from the matrix, the portion corresponding to an amount of energy, within the adjustable range of wavelengths, present in incident optical energy reflected by the three- dimensional matrix.

13. The method of claim 12, wherein the adjusting includes applying an adjustable magnetic field to the matrix, applying an adjustable electric field to the matrix, adjusting a temperature of the matrix, or mechanically manipulating the matrix.

14. The method of claim 13, wherein the matrix includes an elastomeric material, and wherein the adjusting includes stretching or compressing the elastomeric material in at least one axis.

15. The method of claim 13, wherein the matrix includes an electrostrictive material, and wherein the adjusting includes applying a voltage differential across at least a portion of the matrix.

16. The method of claim 15, wherein the matrix is configured to reduce an inter- particle distance of the particles in response to applying the voltage differential.

1 7. The method of claim 12, comprising:

reflecting a first specified range of wavelengths during a first duration using a first volume fraction of the particles; reflecting a second specified range of wavelengths during the second duration using a second volume fraction of the particles;

and wherein the obtaining information about at least a portion of energy includes obtaining energy reflected from the matrix during the first duration and during the second duration.

1 8. The method of claim 17, comprising estimating a spectrum of incident optical energy using the information obtained about the energy reflected from the matrix during the first and second durations.

19. The method of claim 18, wherein the information obtained about the energy reflected from the matrix includes one or more of the location of the reflected energy along a surface of the matrix or an intensity of reflected energy detected during a specified duration.

20. A processor-readable medium comprising instructions that, when executed by the processor cause the processor to:

adjust a volume fraction of diffracting particles, suspended in a three- dimensional matrix, to reflect an adjustable range of wavelengths, the adjustable range of wavelengths determined at least in part using the volume fraction of di ffracting particles; and

obtain information about at least a portion of energy reflected from the matrix, the portion corresponding to an amount of energy, within the adjustable range of wavelengths, present in incident optical energy reflected by the three- dimensional matrix.

21 . A spectrometer, comprising:

a three-dimensional matrix configured to suspend diffracting particles, the matrix including an adjustable volume fraction of the particles, the matrix configured to reflect an adjustable range of wavelengths, the adjustable range determined at least in part using the adjustable volume fraction of particles, the three-dimensional matrix configured to reflect a first specified range of wavelengths during a first duration using a first volume fraction of the particles and a second specified range of wavelengths during a second duration using a second volume fraction of the particles, and wherein the detector is configured to obtain

information about energy reflected from the matrix during the first duration and during the second duration;

a detector configured to obtain information about at least a portion of energy reflected from the matrix, the portion con'esponding to an amount of energy, within the adjustable range of wavelengths, present in incident optical energy reflected by the three-dimensional matrix; and

wherein the processor circuit is configured to estimate a spectrum of incident optical energy using the information obtained by the detector during the first and second durations.