Infrared dynamic scene simulating device based onlight down-conversion

V.K. Malyutenko, K.V. Michailovskaya, O.Yu. Malyutenko, V.V. Bogatyrenko and D.R. Snyder

Abstract: The pixelless dynamic scene projector successful in generating high-speed (microsecondrange) broadband infrared (�ho < Eg; where v is light frequency, �h is Planck’s constant and Eg isthe band gap value of the screen material) scenarios through shorter wavelength optical pumpingð�ho > EgÞ of a semiconductor screen was developed, fabricated, and tested. The device operationprinciple is based on a possibility of dynamically modulating the apparent temperature (or poweremitted in the spectral range of free carrier absorption–emission processes) of an image opticallygenerated on a semiconductor screen (light down-conversion process). The device successfullymonitors both hot and cold images (compared to a scene temperature) as well as erase-image anddisplay-hidden-image processes. The results of an experimental study of a germanium screen(300 K , T , 500 K) in the 8–12mm spectral range are reported for the first time.

1 Introduction

Dynamic infrared (IR) scene projectors (DISPs) are tools forthe evaluation of electro-optical seekers and thermalimaging cameras by simulating IR scenarios composed oftarget-on-background pictures. Until now, two-dimensional(2-D) thermal resistor arrays (devices based on electricallyheated pixels [1, 2]) have been the most promising devicesin the field. Major drawbacks to these DISPs originate fromtheir pixel technology. First, emitter arrays are characterisedby fixed-pattern noise due to variations in electrical andemissive properties of individual pixels and, therefore,demand for non-uniformity correction. Secondly, the pixeltechnology results in low fill-factor (F) values (for example,F ¼ 46% for 39 � 39 mm2 pixel pitch [1]). Thirdly, thedimensions of single pitch limit the spatial resolution of thisdevice. But the major disadvantage of conventional DISPtechnologies is their inability to simulate cold targets.

The purpose of this paper (which is the second in the field[3]) is to demonstrate further progress towards thedevelopment of pixelless high-speed DISPs capable ofsimulating images which are hot and cold compared to thebackground temperature. We show that the emissivitydifference between a screen made of a semiconductor(covered with a thin film) and an image (which is part of thisscreen, free of film) can be monitored by an optical down-conversion process. We show that a ‘visible’ light projector(�ho > Eg; where o is light frequency, �h is Planck’s constantand Eg is the band gap value of the screen material) capableof generating free charge carriers in a semiconductor screen

due to interband electron transitions, creates a controllableapparent temperature ðTaÞ pattern across the screen free offilm. Ta is determined as the temperature of a black body ofequal power emitted in the spectral range of interest. Thelast is due to so-called free carrier absorption–emissionprocess (intraband electron transitions), which are active ina wide spectral region beyond the fundamental absorptionrange ð�ho < EgÞ: Also, we show that this down-conversion-of-light approach does not only permit exposing hot andcold targets, and an additional advantage of our approach isthe ability to demonstrate erase-image and display-hidden-image dynamic processes. Experimental tests are performedon germanium screens (300 K , T , 500 K) with ‘visible’light being the source of real visible or near-IR radiationðl < 2mmÞ; whereas converted light is the IR 2-D dynamicscenario captured by a thermal imager operating in the 8 to12mm spectral range.

2 Down-conversion approach for DISPs

The spectral value of the thermal emission (TE) power Poof a scene with thickness d, reflection coefficient R andabsorption coefficient K kept at a given temperature T is

Po ¼ 1oJoðTÞ ð1Þ

Po is the product of two factors, namely, JoðTÞ; which is thePlanck distribution of black-body radiation and a screenemissivity 1o; which can be expressed as

1o ¼ ð1 � RÞð1 � �Þð1 � R�Þ�1 ð2Þ

where

� ¼ exp �Z d

0KðxÞdx

� �ð3Þ

is a factor of light transmission through the screen [4].As one can see from (1), there are two ways to monitor

power emitted by a screen.(a) The best known approach is a screen temperature controlmethod. Indeed, the temperature determines both spectraland integral values of power emitted by the scene.

IEE Proceedings online no. 20030846

doi: 10.1049/ip-opt:20030846

V.K. Malyutenko, K.V. Michailovskaya, O.Yu. Malyutenko and V.V.Bogatyrenko are with the Institute of Semiconductor Physics, 45, ProspectNauki Kiev 03028, Ukraine

D.R. Snyder is with the Air Force Research Laboratory, Eglin AFB,FL 32542, USA

Paper first received 17th October 2002 and in final revised form 21st July2003

IEE Proc.-Optoelectron., Vol. 150, No. 4, August 2003 391

The operation principle is based on electrically driven localheating up of single pixels (Joule heating). The maximumapparent temperature values conventional DISPs cansimulate ðT < 800KÞ look acceptable for practical appli-cations. However, long temperature rise–fall time (somemilliseconds for DISPs available in the market) limits theapplication of these devices to a 200 Hz frame rate, which isnot acceptable for testing high-speed IR cameras.(b) The alternative way to monitor IR images is based on thepossibility of controlling local screen emissivity 1 at a giventemperature by modulating the parameters this valuedepends on (d, R or K; see (2)). Indeed, by increasing theabsorption coefficient of initially transparent ðKd � 1Þ andtherefore low-emissivity ðPmin

o Þ screen, the scene may betransformed into opaque media ðKd � 1Þ: This processresults in a remarkable increase of the TE value up to Pmax

o ;which as a matter of fact is the power emitted by a blackbody with allowance for scene reflectivity. The maximumand minimum signals are related as

Pmaxo =Pmin

o ¼ ð1 � RÞ=Kd ð4ÞTo effectively utilise this approach two issues should beaddressed. First, the reflectivity of a screen should beminimised; this may be done, for example, by use of atransparent coating. Secondly, the power emitted by theambient should be much less than the TE of a thin screen. Ifthese conditions are not met, then Pmin

o increases by thevalue of background radiation reflected by and transmittedthrough the screen. Increasing the temperature differencebetween the screen and the surroundings effectivelyminimises this ‘ambient noise’.

The absorption coefficient of an IR screen can be easycontrollable provided this screen is made from a wide-bandsemiconductor, such as Ge or Si. As the lattice absorption inthese non-polar semiconductors is negligible in the near-and mid-IR, the only medium capable of effectivelyinteracting with radiation in the spectral range well beyondthe fundamental absorption is free current carriers. Thecarrier absorption coefficient (K) is connected to the freeelectron (n) and hole ( p) concentrations by K ¼ �nn þ �pp(�n; �p are free carrier absorption cross-sections of aquantum of given frequency o). Thus, photo-excitation ofthe scene with quantum energy �ho > Eg; followed by theincrease of free carrier concentration, results in modulationof a scene TE power in the longer wavelength range�ho < Eg (down-conversion) provided that the screen isinitially transparent ðKd � 1Þ:

3 Measurements and discussion

There are two ways to simulate an IR image by a down-conversion process. The image itself can be projected as avisible light pattern on an initially uniform pixellesstransparent screen (background) kept at a given temperatureTb: ‘Visible’ light evolution in time and space is replicatedon the screen as an IR image, whose temporal characteristics(rise–fall time) and spatial resolution depend on carrierlifetime t and diffusion length [3]. As ‘visible’ light causes alocal apparent temperature increase, this approach iscapable of simulating only hot images ðTi > TbÞ:

Another method, which is the subject of this paper, is acombination of a uniform visible light projector and aninitially non-uniform semiconductor screen. This non-uniformity is due to the emissivity pattern created on thescreen, for example by combining regions covered with a thinfilm insensitive to ‘visible’ light (background with emissivity1b) and a free screen surface (an image, which emissivity

1i depends on a visible light power). We show below that thisapproach appears to be promising way of simulating aninitially cold image, and erase-image or display-hidden-image processes. Consider the details of this approach.

A semiconductor screen ð18 � 18 � 4mm3Þ is made ofan optically polished and chemically etched n-Ge ð� ¼0:1�10 O:cmÞ slab with the front surface partly coveredwith a 0.5mm thick Bi ð1b ¼ 0:4Þ film deposited by vacuumdeposition. The image (free surface) is made by aconventional photolithography process and the relation1i 1b holds. To minimise initial free carrier absorptionand 1i values, the screen was n-doped (as �p > �n; thenachieving IR transparency is a compromise betweenn-doping level and intrinsic hole concentration [3]).A miniature heater maintains the screen real temperatureup to 2008C.

The spectral distribution of power emitted by the image isshown in Fig. 1, curves (i) and (ii). These results wererecorded by modulating the TE signal with a mechanicalchopper kept at 300 K. As one can see, the initial integral TEpower, which the image radiates (curve (i)) is as small as10% of the black-body radiation (curve (iii)). This screensignal is combined of ambient noise ðT ¼ 300KÞ; freecarrier emission, and lattice emission. The lattice emissionis clearly seen at longer wavelengths ðl 12mmÞ andoriginates from weak many-phonon lattice absorption [5].Visible light of moderate level (150 W incandescent lamp)causes a remarkable increase in image TE (curve (ii)),which is mostly due to the free carrier absorption–emissionprocess. In particular, the hole absorption–emission processresulting from direct hole transitions between sub-bands ofthe valence band is responsible for the non-regularity ofspectra at shorter wavelengths ðl < 6mmÞ [6]. Non-directphonon-assisted electron and hole transitions, whoseeffectiveness increases approximately as the square of thewavelength, form the remaining part of the spectra. Furtherincrease of visible light intensity leads to gradual opaque-ness of the image and results in saturation of TE spectral andintegral power values. This is illustrated in Fig. 2 where the‘visible’ light power dependence of the integral IR TEpower emitted by an image demonstrates a tendency tosaturation. The light source is a Nd:YAG laser operated inpulse mode at l ¼ 1:06mm: More specifically, the plot inFig. 2 is difference between irradiated (P) and initial ðP0Þ

Fig. 1 Spectral distribution of emitted power for n-Ge

r ¼ 3O:cm; l ¼ 3:5mm;T ¼ 80 �C(i) TE spectra of initially transparent image(ii) Down-conversion (moderate visible power) results in remarkableincrease of image TE(iii) Black body radiation

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image states ðDP ¼ P � P0Þ and is therefore connected tothe device performance. The Planck distribution factor isresponsible for the increase of this saturation level withtemperature increase.

The temperature dependencies of the IR TE power of theimage and the free carrier lifetime of the screen are shown inFig. 3. The striking result is that an optimum operatingtemperature exists for a given screen thickness and dopinglevel and at higher temperatures the efficiency of down-conversion catastrophically degrades. This degradationresults from intense thermal carrier generation followedby gradual initial opaqueness of the image. The decrease inCarrier lifetime (at T > 150 �C) also contributes to theperformance degradation at higher temperature.

It is now shown how the image could be modified with‘visible’ light. A uniform light beam is incident on the frontside of the screen and a calibrated IR thermal imagingcamera (HgCdTe cooled photodetector, 8–12mm spectralrange, 400ms frame duration) images the back surface of thescene and maps the 2-D emissivity pattern created by excesscarriers. Parameters of interest are the image excessapparent temperature (DT) and IR power emitted (DP)values, the signal differences between illuminated andshadowed scenes. It is important to note that the camerameasures radiance differences but not temperatures; there-fore, Ta and P values are connected by a calibrated signaltransfer function. The camera is synchronised with visiblelight pulse in such a manner that the picture of interest

appears at the second frame whereas first frame captures thenon-illuminated scene. By subtracting these frames one canget an apparent temperature difference DT map stimulatedby visible light. It was carefully verified that ‘visible’ lightpulses did not change the real temperature of the screenduring the tests.

The erase-image process is shown in Fig. 4. In the initialstate, the IR camera monitors the image as the region withlower apparent temperature ðTi < TbÞ; which is due to thesmall initial emissivity value compared to that for theremaining part of the screen covered with Bi film ð1i < 1bÞ:Free current carriers generated by the light projectorincrease both 1i and the image apparent temperature, andfinally erase the cold target. For this reason, the camera isunable to track this image at light level, which equalises theemissivity across the whole scene ð1i � 1bÞ: For the sceneof interest this happens when the apparent temperatureincreases by 15.8 8C. Higher power light transforms a coldimage into a hot one ð1i > 1bÞ:

Figure 5 illustrates display-hidden-image process. In theinitial state, the IR camera cannot register a hidden image,as the emissivities of image and the screen covered with a Bifilm are equal ð1i � 1bÞ: The emissivities are equal as aresult of a slight increase of screen operating temperature.Increasing the free carrier concentration by visible lightresults in the appearance of an image; the temperaturedifference captured by the camera is DT ¼ 11:9 �C: Moreintense light increases the image apparent temperature up toblack-body temperature with respect to the image reflectionvalue. It is important to note that all these light down-conversion processes happen with the rise–fall time, whichis as a minimum ten-fold shorter than these values forconventional DISP devices. As a matter of fact, carrierrecombination processes in semiconductors at T > 300Kare always shorter than hundreds of microseconds whereas

Fig. 2 Image integral TE against ‘visible’ light level

The dependence clearly indicates a saturation process, resulting fromdynamic opaqueness of the image(i) 68 8C; (ii) 90 8C; (iii) 119 8C

Fig. 3 Temperature dependences of down-conversion efficiencyand scene carrier lifetime for n-Ge

� ¼ 0:1O:cm(i) Down-conversion efficiency(ii) Carrier lifetime

Fig. 4 Erase-image process captured by camera (stealth effectin IR)

T ¼ 90 �C; n-Ge, � ¼ 0:9O:cmLeft: cold image is seen at hot ðTb ¼ 48:8 �CÞ sceneRight: visible light erases the image

Fig. 5 Display-image process captured by camera

T ¼ 110 �C; n-Ge, � ¼ 0:9O:cmLeft: hidden image and scene simulate equal apparent temperature valuesðTb ¼ 70 �CÞRight: ‘visible’ light displays hidden image

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heat dissipation thermal processes are in the millisecondrange.

4 Conclusion

We have shown experimentally that the apparent tempera-ture difference between a screen (made of semiconductorcovered with thin film) and an image (which is part of thisscreen, free of film) may be easy monitored by light down-conversion processes. Shorter wavelength stimulation(�ho > Eg; ‘visible’ light) is capable of free carriergeneration in a screen material, whereas screen and imageparameters are measured in the near- and mid-IR ð�ho < EgÞ[7].

The physical concept behind this new approach involvesfree carrier absorption–emission processes resulting in adynamically controllable local TE change inside an IRscene. Contrary to conventional IR scenes made of 2-Dpixel architecture operated by a current, the screendeveloped is driven by light and is therefore free of pixelsand electrical contacts. Instead of using slow (millisecondrange) local screen Joule heating and cooling processes(contact temperature control), the DISP developed utilises afast (microsecond range) emissivity modulation processcontrolled by free electron and hole recombination pro-cesses (remote optical control, whose spatial resolution islimited by carrier diffusion length). Other than that, the mainadvantage of down-conversion pixelless DISPs is theirability to simulate cold targets, as well as dynamicallyerase-target (stealth effect in IR) and display-hidden-targetprocesses, which are very important for practicalapplications.

The ‘visible’-to-IR down-conversion concept appears tobe promising way to dynamically monitor hot and coldimages provided the screen is made of single (homopolar)wide-band-gap semiconductors such as Ge or Si. Weakmulti-phonon absorption permits reproducing initially lowtemperature images even at high screen temperature,whereas relatively long carrier lifetime requires low-levelvisible light to monitor a scene in erase-display-targetmodes. There are no fundamental operational limitations forionic crystals. However, when it comes to heteropolarcompound crystals with strong single-phonon reststrahlabsorption, achieving initial image transparency in the3–16mm spectral range requires both ions to be as heavy aspossible and (or) the screen to be as thin as possible.

5 References

1 Robinson, R., Oleson, J., Rubin, L., and McHugh, S.: ‘MIRAGE: Systemoverview and status’, Proc. SPIE-Int. Soc. Opt. Eng., 2000, 4027,pp. 387–398

2 Pritchard, A.P., Lake, S.P., Balmond, M.D., Gough, D.W., Venables,M.A., Sturland, I.M., Crisp, G., and Watkin, S.C.: ‘Current status of theBritish Aerospace resistor array IR scene projector technology’, Proc.SPIE-Int. Soc. Opt. Eng., 1997, 3084, pp. 71–77

3 Malyutenko, V.K., Bogatyrenko, V.V., Malyutenko, O.Yu., Snyder,D.R., Huber, A., and Norman, J.: ‘Semiconductor screen dynamic visibleto infrared scene converter’, Proc. SPIE-Int. Soc. Opt. Eng., 2002, 4818,pp. 147–156

4 Malyutenko, V.K.: ‘Thermal emission in semiconductors. Investigationand application’, Infrared Phys., 1991, 32, pp. 291–302

5 Lord, R.C.: ‘Far infrared transitions of silicon and germanium’, Phys.Rev., 1952, 85, (1), pp. 140–141

6 Briggs, H.B., and Fletcher, R.C.: ‘New infrared absorption bands inp-type germanium’, Phys. Rev., 1952, 87, (6), pp. 1130–1131

7 Malyutenko, V.K., Malyutenko, O. Yu., Michailovskaia, E.V., Snyder,D.R., and Bogatyrenko, V.V.: Ukrainian Patent Disclosure Number2002064925, 14 July 2002

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