WO2002099896A9

WO2002099896A9 - Infrared radiation imager

Info

Publication number: WO2002099896A9
Application number: PCT/US2002/018104
Authority: WO
Inventors: Feliks A Chudnovskiy; Michael A Gurvitch; Serge Luryi
Original assignee: Univ New York; Feliks A Chudnovskiy; Michael A Gurvitch; Serge Luryi
Priority date: 2001-06-05
Filing date: 2002-06-05
Publication date: 2003-04-10
Also published as: WO2002099896A1

Abstract

An infrared image signal can be read out optically and in parallel fashion using a sensor which includes an optical resonator structure with a thin layer of VO2 (17) on a mirror layer (13) disposed at a focal plane of IR optics (15) projecting the image. The resonator structure passes through a thermal cycle of rapid heating, for the VO2 to undergo a phase transition from a semiconductor to a metal, followed by a time interval in which the temperature is maintained constant, and then followed further by a time interval of cooling to the initial value. The total length of each such cycle can correspond to video rates. At a constant (bias) temperature, VO2 encodes an IR image via its temperature-dependent refractive index, and, by optical interference, the resonator structure reproduces the image via its temperature-dependent reflectivity. Optical memory characteristics of the hysteretic phase transition in VO2 facilitate retaining the encoded image for readout by illuminating the resonator with coherent polarized light. Reflected light can be optically processed and read out with a CCD (118) or similar device.

Description

INFRARED RADIATION IMAGER

Technical Field

The invention is concerned with infrared detection and imaging and, more particularly, with generating an optical output signal in response to infrared input.

Background of the Invention

Infrared (IR) detector devices have been developed using room- temperature arrays of resistive bolometers. Into a focal plane of an IR optical system that projects an IR image, a detector surface is placed which is divided into small, e.g. 20 μm by 20 μm pixels, with each pixel having a pair of electrically conducting leads for sampling its electrical resistance. Each pixel is suspended on a membrane over a vacuum gap separating it from a bulk substrate, with the leads and the membrane serving as a mechanical support for the pixel. When a pixel is heated by the projected IR radiation, its resistance changes. In an imaging camera the number of pixels may reach 10⁵ and more. Resistance changes in individual pixels are sequentially read out via a special electronic circuit called Read-Out Integrated Circuit (ROIC) which is integrated with a device silicon substrate, with some circuit elements placed beneath the suspended sensor membranes. After irradiation, increased pixel temperature and associated resistance change decay and disappear upon heat dissipation by conduction through the support and electrical lead structure and radiation to the surroundings. The longest time constant achievable by minimizing heat loss in known technology is about 30 ms, using vacuum micro-electro-mechanical systems (MEMS) fabrication.

Sequential readout places stringent requirements on the pixel scanning rate and thus on ROIC, for the complete detector readout to be completed within a time frame of 10 ms to 30 ms. The ROIC and electrical connections to each pixel make for a reduced fill factor, in that the fraction of total sensor surface area which has sensing functionality is about 60% to 80%.

One of the materials of choice for resistance-change sequential readout technology is vanadium dioxide, VO₂, or, more precisely, a mixture of vanadium oxides containing mainly VO₂; it is often designated as VO_x. However, after a decade of development effort, serious difficulties are reported as to uniformity, reproducibihty and yield, in part because the VO₂ sensor films are deposited on membranes rather then a bulk substrate, and also because of intricate MEMS technology used.

VO₂ has been utilized near room temperature, where it is a narrow-gap semiconductor with a temperature coefficient of resistance α = 1/R (dR dT) in the range of 2% to 4%. In the semiconductor phase, VO₂ is essentially transparent to IR radiation with wavelength between 8 mm and 14 mm, i.e. in the window of IR transparency of the atmosphere and the wavelength range of main interest in IR imaging. Thus, such detectors have to rely on auxiliary layers for the absorption of radiation of interest.

Summary of the Invention

Obviating the need for electrical connection to pixel elements, a thermal pattern on a sensor element can be read out optically. A corresponding infrared sensor device has one or several thermally insulated sensor elements such as pixels for exposure to thermal radiation and exposure also to readout illumination. The sensor elements include a material whose refractive index is temperature dependent.

Brief Description of the Drawing

Fig. 1 is a schematic of an imager, image sensor or detector-converter in accordance with a preferred first embodiment of the invention. Fig. 2a is a schematic and Fig. 2b a graph for illustrating applicable optical resonator principles.

Fig. 3a, 3b and 3c are graphs for illustrating applicable hysteretic behavior of refractive index.

Fig. 4a and 4b are graphs for illustrating optical memory based on hysteretic behavior.

Fig. 5 is a graph of irradiative heating power and attendant temperature in a VO₂ layer as a function of time.

Fig. 6 is a schematic of an imager in accordance with a preferred second embodiment of the invention. Fig. 7 is a schematic for illustrating applicable spatial Fourier analysis of an image.

Detailed Description

Optical Conversion and Readout Fig. 1 in its lower part shows IR-sensor layers on a substrate 11 of silicon, for example. The sensor includes a layer 14 of a material such as VO₂ typically having a thickness in the range from 50 nm to 100 nm, disposed on an optical mirror layer 13, e.g. a film of aluminum about 50 nm thick. VO₂ is suitable as a sensor material on account of a strongly temperature-dependent refractive index, between a metallic phase and a semiconductor/insulator phase. There are many other materials undergoing phase transition with desired change of optical properties, including V₂O₃, titanium oxides, and magnetite (Fe₃O₄). VO₂ is exemplary here for having a particularly suitable phase transition temperature, near 65 C, and temperature-dependent refractive index in the near- infrared and visible spectral range. The layer 13 is separated from the substrate 11 by a layer 12 of thermal insulation, e.g. of SiO₂ aerogel or V₂O₅ aerogel a few microns thick. The layers 12, 13 and 14 are laterally divided into square "passive pixels" without requiring electrical connectivity, with minimal lithography-limited gaps between them. Alternatively, the layer 12 can be left continuous, with only the layers 12 and 13 being pixellated. Pixel size can be 20 μm by 20 μm, for example, and the gap size 1 μm to 2 μm, yielding a fill factor of approximately 91% to 95%ι. Such subdividing avoids rapid lateral spreading or "blooming" of temperature variations ΔT to be expected along continuous VO₂ and aluminum or similar uninterrupted films.

Fig. 1 in its upper part shows additional elements such as IR optics or lens 15 for focusing an IR signal onto the detector surface, near-IR or visible illumination 16 which is uniform, coherent and polarized, e.g. as from a laser with beam expander, analyzer and Fourier optics elements 17 for optical processing, and a charge-coupled device (CCD) 18 or similar matrix for detecting processed light and converting it into an electrical signal. In device use an IR image is projected onto the VO₂ surface 14 which simultaneously is illuminated with coherent light 16 of a shorter wavelength. The coherent light is subject to interference due to reflection from the two VO₂ film surfaces, and the reflected light is optically processed further by analyzer and Fourier components 17. The processed light is read out by the CCD or similar matrix 18.

The sensor layer 14 is placed at the focal plane of the detector. As in resistive bolometers, after incident IR radiation has been given sufficient time, e.g. 1 ms, a spatially dependent incident intensity or irradiance induces corresponding spatial temperature variations ΔT(x,y) over the plane of the sensor film 14. The ΔT(x,y) actually comprises. a discreet set of different pixel ΔT's, with each pixel having an essentially uniform temperature. Such discreteness can be indicated by the notation ΔT(i, j), where i, j are integer row and column pixel indices.

Device functioning may be attributed to first-order structural and metal- insulator phase transition in VO₂. It is known in the art that single-crystal VO₂ undergoes a sharp hysteretic metal-insulator transition as a function of temperature at about 65 C, and that in films the transition is broadened. Transition temperature is sensitive further to the presence of impurities, and doping of vanadium oxide especially with tungsten or titanium can be used for adjustment in an approximate range from 30 C to 90 C. Typically, doping with tungsten lowers the transition temperature, and conversely for doping with titanium.

In the transition region, VO₂ exhibits variation of optical properties, specifically of the refractive index and absoφtion coefficient. A detector-converter of the invention can be viewed as including an optical resonator comprising a VO₂ film over a mirror, offering a capability to visualize minute variations of optical coefficients. For at least part of the frame or exposure time, the resonator is at a constant or bias temperature inside the hysteresis loop of VO₂. By virtue of the phase transition, the temperature variations ΔT(i, j) produce changes in the VO₂ optical refractive index, Δn = n(T₀ + ΔT) - n(T₀), thus encoding an IR image via Δn(i, j) changes. Simultaneously, the sensor surface is uniformly illuminated by polarized laser light either in the visible, e.g. with wavelength of 0.4 μm to 0.7 μm, or in the near IR, e.g. with wavelength of about 1 μm to 3 μm. Depending on its intensity, such illumination may or may not appreciably contribute to the thermal balance of the sensor surface, so that the further as described below. If both "top" and "bottom" mirror layers on layer 14 are made to be semi-transparent, and the substrate 11 is essentially transparent to illuminating radiation, the structure can operate in a transmission mode, with the analyzer and Fourier components 17 and CCD or similar matrix 18 for receiving transmitted illumination on the opposite side of layer 14.

The spatial resolution of the image is determined by the wavelength of IR radiation or by the pixel size, whichever is smaller. On account of their passive nature, the pixels can be made quite small, e.g. 10 μm by 10 μm, making them comparable to the wavelength of IR radiation of interest. In consequence, the use of passive pixels need not reduce the resolution of the imager.

Fig. 3a is the same as Fig. 2b, repeated here for convenience of following the temperature-hysteretic behavior of the reflectance R(T) for the λ, -minimum of the semiconducting phase per Fig. 3 b, and for the λ^-minimum of the metallic phase per Fig. 3c. Intensity variations induced by the Δn(i, j) and ΔR(i, j) can be expected as rather small in comparison with the constant intensity of the incident illumination light, so that it is beneficial to process the reflected light to substantially subtract the constant background. For this purpose, for polarized monochromatic light a number of well- developed powerful optical techniques are available. The reflected, processed short- wavelength light is collected in a CCD or similar recording matrix, generating an electrical signal corresponding to the optical signal.

As described, device functionality includes two types of conversion, namely (i) from long- wavelength, 8-14 μm IR light to short- wavelength near-IR or visible light, and (ii) from incoherent, mixed-wavelength light to coherent, monochromatic light. The first type of conversion permits the use of existing, well- developed CCD or similar matrix detector technology with contactless, parallel readout, and the second allows the use of interference and application of powerful techniques of optical signal processing.

Once an image is written, optically recorded and read out as described above, the sensor needs to be refreshed/erased in order to prepare for the next image further as described below. If both "top" and "bottom" mirror layers on layer 14 are made to be semi-transparent, and the substrate 1 1 is essentially transparent to illuminating radiation, the structure can operate in a transmission mode, with the analyzer and Fourier components 17 and CCD or similar matrix 18 for receiving transmitted illumination on the opposite side of layer 14.

Fig. 3a is the same as Fig. 2b, repeated here for convenience of following the temperature-hysteretic behavior of the reflectance R(T) for the λ, -minimum of the semiconducting phase per Fig. 3b, and for the λ₂-minimum of the metallic phase per Fig. 3c. Intensity variations induced by the Δn(i, j) and ΔR(i, j) can be expected as rather small in comparison with the constant intensity of the incident illumination light, so that it is beneficial to process the reflected light to substantially subtract the constant background. For this purpose, for polarized monochromatic light a number of well- developed powerful optical techniques are available. The reflected, processed short- wavelength light is collected in a CCD or similar recording matrix, generating an electrical signal corresponding to the optical signal.

Once an image is written, optically recorded and read out as described above, the sensor needs to be refreshed/erased in order to prepare for the next image frame. Indeed, after a few milliseconds time following the image writing stage, the temperature differences ΔT will have vanished, but in each pixel the optical memory effect preserves the changed refractive index and reflectivity for as long as the sensor temperature remains within the hysteresis loop. Figs. 4a and 4b illustrate an effect of optical memory for the case of a small, short temperature pulse ΔT inside the hysteresis loop R(T) for λ,. Fig. 4a shows two closely positioned curves corresponding to R(λ) at T = T₀ and T = T₀ + ΔT. At λ„ reflectivity changes by ΔR. In Fig. 4b this changed value of reflectivity R + ΔR is "memorized" or recorded at a fixed bias temperature T₀. A written image can be erased by disrupting the energy input that caused heating of the VO₂ layer to the bias temperature T₀. The thermal decay or relaxation time of the VO₂ layer can be chosen sufficiently short for the heat to dissipate into the substrate in a few milliseconds. Indeed, under circumstances of minimal heat conduction, decay times are of the order of 30 ms. With an aerogel layer a few microns thick for insulation, estimated characteristic decay times drop to a few milliseconds. Thus, the temperature will quickly decrease to a value below the hysteresis. At the start of a next cycle, the source of radiation for heating of the VO₂ layer is turned on again, and the process repeats.

Fig. 5 shows a solid line of temperature of the VO₂ layer as a function of time when exposed to heating radiation having power as shown with a broken line. Peak power in the heating input corresponds to raising the VO₂ temperature from about 20 C to about 70 C. Then, the heating power is lowered to obtain a flat portion of T(t), and finally switched off to cool back to a temperature below the phase transition, e.g. 20 C. Numerical values in Fig. 5 are intended for guidance only, as it may be possible to erase a picture at temperatures above 20 C, the time domain numbers may differ from 10 ms, and the indicated power of 5000 W/m² merely represents an estimate for a typical geometry.

One function of illumination is to convert an IR signal into a shorter- wavelength, matrix-readable signal. Efficacy of such conversion depends on the responsiveness of the index of refraction of the VO₂ layer to temperature. Accordingly, the wavelength of illuminating light is chosen to provide for a significant temperature dependence of the refractive index. Measurements performed on VO₂ films have shown that Δn increases steadily from about 1 μm to at least 12 μm. If only for this reason it would be beneficial to use illuminating light of longer wavelength, e.g. at 12 μm. However, readily available detector matrices are sufficiently sensitive at shorter wavelengths only, near 2 μm, and such shorter illumination wavelengths also allow the use of thinner VO₂ films which are more responsive to IR radiation. Radiative Heating to the Bias Temperature

For combining functions of illumination to provide coherent light for optical readout and to heat the sensor material to a suitable bias temperature, a fairly powerful laser at a wavelength of about 1-2 μm is called for. For a 1-cm² sensor the 5000 W/m² for heating the resonator structure by 50 C in 10 ms amounts to 0.5 W. Required power is reduced further if the picture can be erased by lowering the temperature to just 40 C or possibly 50 C which may be achieved with narrow hysteresis loops. Such lasers can be controlled to produce suitably shaped power pulses, e.g. as shown in Fig. 5 or more intricate still. The laser beam can be spread to the desired area, e.g. 1 cm² by a cylindrical lens, for heating to result in essentially uniform temperature over that area.

For radiative heating separate from illumination, radiation can be absorbed from a broad-band heating lamp, for example. On a 10 ms time scale, corresponding to a 100 Hz frequency scale, heating power can be controlled either by controlling the actual power delivered by the lamp, or by including means for interrupting power, e.g. a chopper or variable-transparency filter. With such separate heating, laser illumination for pattern readout is reduced substantially.

Fig. 6 illustrates radiative heating by a lamp 61 from the back side of the resonator structure 13, 14, through a suitably transparent substrate 11 and aerogel 12. A layer 62 of "black" below the mirror 13 can be included for enhanced absoφtion of heating radiation.

Optical Data Processing

Principal goals include (i) reduction/elimination of background light from an illuminating laser while preserving and reading out small IR signal-induced variations, and (ii) elimination of an image of regular square pixels. The first goal can be met by using polarization filtering, and the second by using spatial Fourier transform techniques.

Polarization filtering depends on the fact that incident illumination is linearly polarized in a certain direction. Upon reflection from the VO₂/mirror resonator it will be elliptically polarized, with the major elliptical axis turned at an angle to the incident light. By placing an analyzer so that at the bias temperature T₀ the reflected light is largely absorbed, the background can be essentially eliminated. The light reflected at T₀ + ΔT, with corresponding different optical constants will predominantly pass. Fig. 7 illustrates spatial Fourier analysis of an image 71, with the diaphragm 73 in the focal or "frequency" plane of a lens 72 blocking out spots along the x- and y-axes corresponding to a square grid of pixels. The useful part of the image 71, represented by the two black rectangles, is reproduced on a screen 74, while the grid has been eliminated from the representation. Thus, the grid can be eliminated by forming a spatial Fourier spectrum of the reflected light, in the focal plane of a lens, and by blocking of the Fourier spectrum side bands or spots resulting from the rectangular grid. Additional optical processing can include spatial differentiation of the image for emphasizing image outlines, as well as other applicable coherent-light techniques.

Optical Responsiveness and Noise

Optical imager responsiveness depends in part on the shape of the reflectivity- versus-temperature curve and on the choice of the working point on that curve as resulting from the bias temperature T₀. In good-quality films, reflectivity in an optical resonator as described above can change by about 10% per degree C. Thus, for a 10-mK change in sensor temperature, reflectivity changes by about 0.1%). If the constant background can be subtracted, such a change in reflectivity may correspond to a very large number of photons, with actual numbers depending on the intensity of illuminating light. A sufficient number of photons per 10 mK or other desired temperature variation can be obtained readily for confident detection in the CCD or similar matrix.

For bolometric detectors it is known that, for a small pixel at 300 K, the fundamental thermal noise limit due to exchange of thermal energy with the environment is relatively small compared to other noise sources. By integrating over all frequencies we have found an upper limit of 0.1 mK for temperature fluctuations for our exemplary pixels. The major sources of noise in resistive bolometers have been identified as Johnson noise and random 1/f noise. Both have their origin in electrical resistance, or in the way electrical current flows through the system. Especially large noise is expected and observed in the resistance near percolation limit, i.e. in the resistive phase transition of VO₂. The relevant scale on which local irregularities are averaged in that case is the mean free path, which for VO₂ is in the nanometer range.

Among advantages of imagers of the invention is freedom from resistive noise and from concerns relating to percolation of an electrical current between micro- regions of the conducting phase. Instead, there is optical reflectance on a typical scale of about 1 -micrometer illumination wavelength. On such a scale the material can be expected to contain a large number of metal and semiconductor micro-domains that switch randomly. Such random switching is due to a small expected energy difference between the metal and semiconductor phase in the phase transition region, resulting in local fluctuations of the optical constants and attendant random telegraph noise on small length scales. On longer length scale of the illuminating wavelength much of this noise should average out.

Alternatives and Variations of Detector Design

Without requiring a separate aerogel layer, thermal isolation can be achieved when the VO₂ detector film material is included in aerogel form. Such a detector film may be formed by depositing an aerogel layer of V₂O₅ as described by J. Livage, "Vanadium Pentoxide Gels", Chem. Mater., Vol. 3, pp. 578-593 (1991), followed by annealing in a reducing atmosphere such as hydrogen for reduction to VO₂ in aerogel form. The resulting film has low thermal conductivity, with "blooming" of little or no concern even without pixellation. Thus, limited by diffraction only, the highest possible spatial resolution can be achieved with fill factor of 100%. Such a detector can be fabricated without any lithographic or microfabrication processes, and custom detector sizes up to full wafer size can be produced simply through selective dicing.

For a VO₂ detector layer in aerogel form, Formula (2) above is not directly applicable if n(T) is taken to represent the refractive index of bulk VO,. To maintain the same optical path through VO₂ , the required thickness of the VO₂ aerogel layer increases in proportion to the ratio of the VO₂ bulk density to the areogel density. The resulting thickness may be on the order of 2-3 micrometers.

As a detector layer, a VO₂ aerogel layer of a certain porosity p can be used on a SiO₂ aerogel thermal insulator layer having a higher porosity p', with the optical mirror then under both such layers.

Without reduction processing, V₂O₅ aerogel can be used also as a chemically similar thermal insulation material for a bulk, non-porous VO₂ detector film. Use of an active sensor layer in aerogel form- thus combining sensing and thermal insulation functions in one layer, can be made independent of readout mode, i.e. equally for devices with resistive readout in bolometric devices. Similar independence applies to use of V₂O₅ aerogel as thermal insulation under more dense VO₂, either in non- porous bulk form or at least having lower porosity.

With aerogel layer insulation, typical decay times as short as 1-3 ms may be undesirable in some instances. One related disadvantage lies in an associated loss of sensitivity, and another in shortened IR-radiation writing time. Additionally, aerogel tends to be brittle and micro-porous which may hinder deposition of optically smooth layers.

In lieu of thermal insulation by a layer in aerogel form or otherwise, VO₂- mirror pixel combinations can be supported by suspended membranes or micro-bridges over a vacuum gap, with the micro-bridges supported on a substrate. As there is no need for electrical contact to such pixels, fabrication is simplified over resistive bolometer technology.

For illumination, light sources other than a laser can be used. One suitable alternative includes an essentially monochromatic light source, e.g. including a wavelength filter, and a polarizer. Light reflected from the VO, film then reaches an analyzer filter disposed for eliminating the background. Temperature variations induced by the IR signal will change the angle of polarization, for this part of the reflected signal to pass through the polarizer. Use of a two-mirror interferometer-type structure as described above further enhances change of polarization, allowing for more efficient subtraction of background.

For positive temperature control of a resonator layer if desired, fast Peltier elements can be included to cycle the temperature between heating and erasing, e.g. with 30 ms frame time. Means for temperature control can be included also for stabilizing the substrate temperature.

For discrimination between the illumination background and the reflected image signal, a technique can be used as described in U.S. Patent No. 5,784,157, "Method and Apparatus for Identifying Fluorophores", issued to Gorfinkel et al. on July 21, 1998 which is incoφorated herein by reference. The amplitude of the IR input is modulated in the time domain, e.g. by chopping, while the illumination light is not modulated in this fashion. Then, components of the reflected signal other than those at the chopping frequency can be disregarded at the CCD/matrix level, for example. For enhanced sensitivity, the readout matrix can further include photon-counting light- sensitive elements such as photomultipliers and/or avalanche photodiodes.

Claims

1. An infrared sensor device comprising at least one thermally insulated sensor element for exposure to first radiation having a first wavelength in an infrared wavelength range, and further for exposure to second radiation having a second wavelength which is shorter than said first wavelength, wherein said sensor element comprises a material which at a first temperature has a first refractive index for said second radiation, and at a second temperature has a second refractive index for said second radiation.

2. The device of claim 1, comprising a plurality of thermally insulated sensor elements.

3. The device of claim 2, wherein the sensor elements are disposed as a 2- dimensional array of pixels.

4. The device of claim 3, wherein pixel fill factor is at least 90%.

5. The device of claim 1, wherein the sensor element comprises a material which has a phase transition temperature between said first temperature and said second temperature.

6. The device of claim 5, wherein phase transition is between a metallic phase and a semiconductor/insulator phase.

7. The device of claim 5, wherein the phase transition is hysteretic.

8. The device of claim 5, wherein the sensor element comprises a material which includes VO₂.

9. The device of claim 8, wherein the material further comprises an impurity for adjusting the phase transition temperature.

10. The device of claim 9, wherein the impurity is one of tungsten and titanium.

11. The device of claim 1, wherein the sensor element is substrate supported.

12. The device of claim 11, further comprising a thermal insulation region between the sensor element and the substrate.

13. The device of claim 12, wherein the thermal insulation region consists essentially of an aerogel.

14. The device of claim 13, wherein the aerogel includes SiO₂ aerogel andV₂O₅ aerogel.

15. The device of claim 1, wherein the sensor element is in aerogel form.

16. The device of claim 1, wherein the sensor element is included in a bridge structure over vacuum.

17. The device of claim 1, having at least one mirror layer for said second radiation on said sensor element.

18. The device of claim 17, having a top and a bottom mirror layer on said sensor element, wherein at least said top layer is semi-transparent to said second radiation.

19. The device of claim 18, wherein said bottom layer is semi-transparent to said second radiation, for device operation in transmission.

20. The device of claim 1, further comprising means for thermally biasing the sensor element.

21. The device of claim 1, further comprising means for optically processing second radiation reflected from the sensor element.

22. The device of claim 21, wherein said second radiation is monochromatic and polarized in an initial direction, and wherein said means for optically processing comprises means for optically subtracting an optical component which is polarized in said initial direction.

23. The device of claim 21, wherein said means for optically processing comprises means for suppressing pixellation grid lines.

24. An infrared sensor device comprising at least one thermally insulated sensor element for exposure to radiation having a wavelength in an infrared wavelength range, wherein said sensor element comprises a material in aerogel form.

25. An infrared sensor device comprising at least one thermally insulated sensor element for exposure to radiation having a wavelength in an infrared wavelength range, wherein said sensor element comprises VO₂ on a V₂O₅ thermal insulation.

26. A method for sensing an infrared radiation pattern, comprising the steps of

(i) having the infrared radiation pattern impinge on at least one thermally insulated infrared sensor element that comprises a material which at a first temperature has a first refractive index for an illuminating second radiation, and at a second temperature has a second refractive index for said illuminating second radiation; and (ii) illuminating the sensor element for said illuminating radiation to interact with said infrared sensor element to generate sensor-output radiation.

27. The method of claim 26, further comprising optically analyzing said sensor- output radiation.

28. The method of claim 27, wherein analyzing comprises use of Fourier optics.

29. The method of claim 26, further comprising generating an electrical signal from said sensor-output radiation.