WO2005004245A2

WO2005004245A2 - Photoluminescent infrared source

Info

Publication number: WO2005004245A2
Application number: PCT/GB2004/002709
Authority: WO
Inventors: Mohammed Reza Taghizadeh; John Graham Crowder; Charles Thomas Elliot
Original assignee: Heriot-Watt University
Priority date: 2003-06-28
Filing date: 2004-06-24
Publication date: 2005-01-13
Also published as: GB0315177D0; WO2005004245A3

Abstract

A photoluminescent radiation source, particularly for generating infrared radiation. The source is made from a layer of semiconducting material which, when optically pumped, emits radiation in a desired wavelength range, and an output array of micro-lenses attached to an output surface of the semiconducting layer. The output surface of the semiconducting layer is optically immersed by the output array of microlenses. In the preferred embodiment a further, input microlens array is attached to an input surface of the semiconducting layer to focus pump laser irradiation on to the semiconducting layer. The source has many applications, for example in gas sensing, active thermal imaging and infrared spectrometry.

Description

PHOTOLUMINESCENT INFRARED SOURCE The present invention relates to the generation of radiation and, more specifically, though not exclusively, to an improved infrared source for generating infrared radiation up to about 12 microns wavelength. Infrared sources are required for many applications, for example in gas sensing applications, in spectrometers and in active thermal imaging. Traditionally, Light Emitting Diodes (LEDs) , lamps or globars have been used in these applications. At room temperature and at infrared wavelengths up to about 12μm LEDs generally perform poorly in terms of, for example, power output and efficiency. Thermal sources have relatively slow response times and may not be very safe in explosive environments . In order to increase the power and/or directionality of radiation emitted from an LED it is known to attach mechanically (by gluing) a hemispherical immersion lens 2 to the emitting face of the LED 1, as illustrated in Fig.l. However, this requires a skilled and time-consuming mounting operation to be performed individually for every LED to which it is desired to attach an immersion lens. It is an aim of the present invention to provide an infrared source which avoids or minimises one or more of the foregoing disadvantages. According to a first aspect of the invention there is provided a photoluminescent radiation source comprising: a layer of semiconducting material which, when optically pumped, emits radiation in a desired wavelength range; and an output array of micro-lenses attached to an output surface of the semiconducting layer, wherein said output surface of the semiconducting layer is optically immersed by said output array of micro-lenses. Preferably, the semiconducting material emits infrared radiation when optically pumped by a near-infrared laser source, for example a laser diode emitting radiation of a shorter wavelength than the required infrared emission wavelength. Typically, this may be a laser diode emitting in the range of 800 to lOOOμm, such as a Gallium Arsenide (GaAs) laser diode (which emits laser radiation of about 810nm) . However, it will be appreciated that a strong emitter laser emitting outside this region, for example emitting at 700nm or llOOnm, could equally be used. The invention enables a compact, bright source of infrared radiation to be provided, giving improved power output and efficiency as compared with prior art infrared sources. Preferably, the array of micro-lenses is integrally formed with the semiconducting layer. For example, the array of micro-lenses may conveniently comprise a substrate having an output surface on which a multiplicity of micro-lenses are fabricated and an input surface onto which the semiconducting layer is deposited or "grown". This enables wafer-scale integration of the luminescent layer and the micro-lens array, which makes the device suitable for mass fabrication. Multiple such photoluminescent radiation sources may thus be fabricated in the same wafer which may then be diced up prior to packaging as separate devices. In a preferred embodiment, the photoluminescent radiation source further includes an input micro-lens array which is attached to an input surface of the semiconducting layer, said input and output surfaces of the semiconducting layer being opposing surfaces. The input micro-lens array may conveniently be attached to the semiconducting layer by, for example, adhesive means. Advantageously, for every micro-lens in the output micro- lens array there is a respective micro-lens in the input micro-lens array, the two arrays being formed and arranged such that the focal point of each micro-lens in the output array is substantially the same as the focal point of the respective micro-lens in the input array, whereby the input micro-lens array focuses incoming pump laser radiation onto a plurality of regions in the semiconducting layer and the output micro-lens array receives and collimates radiation emitted from the same said regions. The micro-lenses in each micro-lens array may be diftractive or refractive lenses. Advantageously, the micro- lenses in the input array are hexagonally close packed. This optimises the field of view for both the input and output arrays. The materials of which the semiconducting layer and the output micro-lens array are made preferably have the same, or substantially the same, refractive index. Advantageously, the material of which the input micro-lens array is made also has substantially the same said refractive index. Preferably, the micro-lenses in the output array are Fresnel lenses which may conveniently be formed using Reactive Ion Etching (RIE) on a layer of infrared transmitting material such as, for example, silicon, Germanium or Gallium Arsenide. The input array of micro-lenses may conveniently be formed using RIE to etch a layer of fused silica (Si0₂) . Alternatively one or both of the micro-lens arrays may be formed using other processing techniques such as photolithography, holographic lithography, direct e-beam writing, or any other suitable micro-fabrication technique. The repeat distance between the Fresnel lenses in the output micro-lens array may be in the range of 20 to

2000μm, preferably approximately 200μm. The thickness of the semiconducting layer is preferably less than, but may in some cases be approximately equal to, the absorption length of the semiconducting material for the emitted radiation. The thickness of the semiconducting layer is desirably greater than, or approximately equal to, the absorption length for the exciting (i.e. pump) radiation. The semiconducting layer may, for example, be InSb where the desired wavelength range of radiation emitted by the photoluminescent source is 4 to 7 μm, or Cadmium Mercury Telluride (CMT) for emitted radiation of up to 12μm wavelength. Advantageously, a reflective coating may be provided on the input surface of the semiconducting layer, the coating being designed to reflect radiation in at least a portion of the spectral range emitted by the semiconducting layer. (It will be appreciated that this reflective coating must allow the pump laser radiation to be transmitted therethrough.) Alternatively, multi-layer dielectric stacks may be provided on both the input and output surfaces of the semiconducting layer to form a resonant cavity therebetween. Additionally, or alternatively, photonic band gap structures (for example a layer of material having a wider photonic band gap than the semiconducting material) may be provided on one or both said surfaces of the semiconducting layer and/or throughout the thickness of the semiconducting layer. These measures may increase the photoluminescent emission from the device. Optionally a passivation layer may be provided at the interface between the semiconducting layer and the first micro-lens array. Optionally, to obtain broader spectral emission from the device, the semiconducting layer may comprise a plurality of regions of different semiconductor material, each said material emitting radiation in a different spectral region, when optically pumped. The semiconducting layer may thus comprise a plurality of interlaced grids of different materials. These grids may be irradiated simultaneously or separately, by differently angled pump laser beams. For example, a spectrally narrow output may be obtained by irradiating one grid, or a spectrally broad output may be obtained by irradiating a number of grids simultaneously. The differently angled pump laser beams may be produced in various ways, for example by several differently angled lasers, or by a single pump laser incident on a Diffractive Optical Element (DOE) . Alternatively, the input micro-lens array can be designed such that when illuminated by a single pump laser beam one or more of the different grids are irradiated simultaneously, whereby a plurality of different wavelengths are emitted from the output micro-lens array. In another possibility, if no input micro-lens array is provided, illuminating the semiconducting layer with a single pump laser beam would cause all the grids to be irradiated, whereby again a plurality of different wavelengths would be emitted from the output micro-lens array. Preferred embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings in which:

Fig.l is a side schematic view of a prior art device comprising an LED with a hemispherical immersion lens attached thereto;

Fig.2 is a side schematic part-view of a photoluminescent infrared source, according to one embodiment of the invention; Fig.3 is a plan schematic part-view of a semiconducting layer for use in an infrared source according to another embodiment of the invention; Fig.4 is a side schematic view of an infrared source incorporating the semiconducting layer of Fig.3; and

Fig.5 is a block diagram of a gas sensing system incorporating a photoluminescent radiation source. Fig.2 shows a layer 10 of semiconductor material sandwiched between two arrays of micro-lenses 8,9. The first or "output" array 8 of micro-lenses is in the form of a silicon substrate 12 approximately 0.5mm thick and having a multiplicity of Fresnel diffractive micro-lenses 13 (only two shown in Fig.2) fabricated, by Reactive Ion etching, on an output face 14 of the substrate 12. The semiconducting layer 10 is integrally formed on an input face 15 of this substrate 12, by deposition. Any suitable deposition process may be used, such as Molecular Beam epitaxy (MBE) , Chemical Vapour Deposition (CVD) , or Liquid Phase Epitaxy (LPE) . The second or "input" micro-lens array 9 is in the form of a layer of silica 16 having an array of convex refractive lenses 17 (only two shown in Fig.2) formed, also by Reactive Ion etching, in an input face 18 of the silica layer, an output face 19 of the silica layer being attached to an input surface 11 of the semiconducting layer 10 by a thin film of glue (not shown) . In this embodiment the repeat distance D between the micro-lenses 13 in the output array is of the order of 200μm. In use of the device, the semiconducting layer 10 is optically pumped by near-infrared radiation R (typically about 810nm wavelength) emitted from a GaAs laser diode (not shown) and directed so as to be incident upon the silica micro-lenses 17 of the input micro-lens array. (It will be readily appreciated that the afore-mentioned adhesive must be capable of transmitting the near infra-red pump radiation.) The input and output micro- lens arrays are configured and arranged such that the silica lenses 17 of the input array focus the incident laser radiation R_x onto respective regions 20 (typically of approximately 25μm in diameter) of the semiconducting layer 10, from which regions 20 photoluminescent infrared radiation emitted therefrom is collected by respective ones of the Fresnel micro-lenses 13 of the output micro-lens array and output therefrom as collimated infrared output radiation R_Q. Three paths Pι,P₂,P₃ of typical light rays are traced through the device in Fig.2, to illustrate this. It will be appreciated that, by being in optical contact with the silicon substrate 12, the output face 15 of the semiconducting layer is optically immersed by the silicon substrate 12 having the Fresnel diffractive micro-lenses fabricated therein. This optimises the power extracted from the luminescent layer. Also, if the micro-lenses are designed to be hyperspherical the directionality of the beam of output radiation emitted from the device can be improved. Optical immersion techniques are well understood in the art, for example as described in "Immersed radiation detectors", Jones R C, Appl. Opt. Vol. 1, pp607-13 (1962), and "Mid-infrared gas detection using optically immersed, room-temperature, semiconductor devices2, Crowder J G, Hardaway H R and Elliot C T, Meas. Sci. Technol. Vol 13, pp882-884 (2002). In the embodiment of Fig.2 the materials of the semiconducting layer 10 and the substrate 12 are chosen to have substantially the same refractive indices. The greater any mismatch between these refractive indices the lower will be the extraction efficiency of the device, so the closer these refractive indices are the better. This invention presents for the first time a combination of a diffractive micro-lens array and optical immersion, in order to provide an improved photoluminescent radiation source. The thickness of the semiconducting layer is less than the absorption length of the semiconductor material for the radiation R₀ emitted therefrom. Also, the thickness of the semiconducting layer is greater than or about equal to the absorption length for the near-infrared pump radiation R_x . This means that as many as possible of the exciting photons are absorbed while as few as possible of the emitted photons will be re-absorbed. In the embodiment of Fig.2 the semiconducting layer is made of InSb which emits radiation of wavelength 4 to 7μm, and a typical device thickness may be about 1 micron. However, in some cases where surface recombination is expected to be very high it would be desirable to make the semiconducting region as thick as possible without causing a lot of re-absorption of photons and so in such cases it may be preferable to make the thickness of the semiconducting layer approximately equal to the absorption length for the emitted radiation R₀. It will be appreciated that the above embodiment is well suited for mass fabrication at wafer level. A plurality of the output micro-lens arrays can be readily fabricated in one silicon wafer 12 and a semiconducting layer deposited (e.g. by Chemical Vapour Deposition, or otherwise) on the input face 15 thereof. The silica layer 16 containing the required respective input arrays of micro-lenses would then be assembled to the exposed input surface 11 of the semiconducting layer 11, by a layer (or beads) of adhesive. The assembled structure would then be diced up to form individual photoluminescent source chips, each chip having a desired size of output microlens array. (For simplicity all chips from one wafer would preferably be of the same array size.) In an alternative possibility, the chips may be diced from the wafer before the silica input micro-lens arrays are attached thereto, but this is less preferred as it would require individual post-processing assembly of each chip. Various modifications and improvements to the above- described embodiment are possible without departing from the scope of the invention. For example, the input face 19 of the semiconducting layer may be coated with a reflective coating designed to transmit the near-infrared pumping radiation but reflect longer wavelength radiation emitted by the semiconducting material. This would provide a near factor of two improvement in efficiency since this modification reflects forward the infrared radiation which would otherwise be lost through the back of the device. In an alternative embodiment, multi-layer dielectric stacks may be provided on both the input and output surfaces of the semiconducting layer, designed to form a resonant cavity, thereby increasing the output power at a specific (desired) wavelength. A further possible embodiment may alternatively or additionally incorporate a photonic band gap structure in the semiconducting layer 10, designed to improve power output at a specific (desired) wavelength. Such band gap structures would be formed, for example, photolithographically in or on the semiconducting layer. In another possible embodiment, surface recombination effects, which would reduce the power output from the device, are reduced by arranging a low recombination interface at the output surface 15 of the semiconducting layer 10. There are two types of methods in common use to form such an interface. One is the use of a surface passivation layer which produces an accumulated surface at which the energy bands are bent, resulting in an electric field which opposes minority carriers from reaching the surface. Examples are an anodic film on n- type CMT or InSb or a ZnS layer on p-type CMT. The other general method is to use a wider gap alloy at the surface, for example CdTe evaporated onto CMT and annealed to interdiffuse it. Alternatively, or additionally, the materials used to create the reflective coating or resonant cavity coatings in the afore-mentioned possible embodiments may be used to provide a low recombination interface. It will be appreciated that the semiconducting layer need not always be made of InSb. Alternative semiconducting materials could be used depending on the desired spectral output of the device. For example, CMT could be used to give emitted radiation of up to 12μm wavelength. Depending on the radiative efficiency required for any particular application, other possible candidates are indium arsenide (emitting up to 3.4 microns), Indium arsenide antimonide (emitting out to about 9 microns) , Lead tin telluride, mercury zinc telluride and mercury manganese telluride (emitting out to beyond 12 microns . ) In another embodiment, the semiconducting layer 10 is made up of a plurality of regions of different semiconducting materials. Each material emits infrared radiation in a different spectral region, when optically pumped. Fig.3 is a schematic part-view of such a layer which has a honeycomb-like construction made up of a series of interlaced grids, each grid comprising a multiplicity of regions of one semiconducting material, so that the different regions A,B,C,D,E... are hexagonally close packed. Typically, the hexagonal regions may be 25μm in diameter. By changing the angle of illumination of the incident pump laser beam one can change which honeycomb regions A,B,C,D,E... are illuminated, and thus change the spectral range of the infrared radiation output from the semiconducting layer. In one possible embodiment a plurality of differently angled pump laser beams P1,P2 may be used to simultaneously, or separately, illuminate the device, as illustrated in Fig.4. It will be appreciated that this embodiment offers a choice of output spectrum, for example a broad-band spectral output for spectrometers and a narrow-band output for sensing a particular gas. This embodiment may open up the possibility of new small portable infrared spectrometers. In a further modification, where a number of pump lasers are being used, each pump laser (illuminating a respective one of the interlaced grids of different semiconductors) could be individually modulated at a distinct frequency. This feature would be useful in, for example, gas sensing applications where it would enable separate measurements to be made simultaneously in several distinct spectral regions e.g. to sense different gases. It will be appreciated that the above-described invention has many different possible applications. It is particularly well suited for gas sensing applications since most gases absorb radiation strongly in the wavelength range from 4 to 12 microns. E.g. Carbon dioxide (used in the medical, brewing, diving and horticulture industries), NOx and SOx (pollutants from vehicle exhausts and power station flues), hydrogen sulphide (oil exploration) , carbon monoxide (mining and waste disposal sites) , and ethylene (perishable fruit transport) . The above-described invention will improve the performance and cost-effectiveness of gas sensing instrumentation in these areas, compared with prior art sensing instrumentation previously used (such as chemical sensors). Moreover, as the inventive source operates at room temperature it is particularly suitable for use in explosive environments, where safety is paramount. Fig.5 shows schematically an example gas sensing arrangement incorporating a photoluminescent infrared source like that of Fig. 2. The arrangement comprises a gas cell 30 having inlet/outlet ports 31,32 and sealed at each end by a respective window 33,34. The infrared source 40 according to Fig.2 is disposed between one such end window 33 and a lens 35 arranged to focus the beam from a pump laser diode 36 provided for pumping the infrared source 40. At the other end of the gas cell 30, optical filters 37,38 and disposed between the cell end window 34 and detectors 39 for detecting predetermined wavelengths. Another possible application of the above-described embodiments is in active thermal imaging. The inventive device will provide fast powerful sources for active thermal imaging e.g. range-gated systems for penetrating scattering media such as fog, and improved robotic vision for smoky and/or mist- filled environments.

Claims

1. A photoluminescent radiation source comprising: a layer of semiconducting material which, when optically pumped, emits radiation in a desired spectral range; and an output array of micro-lenses attached to an output surface of the semiconducting layer, wherein said output surface of the semiconducting layer is optically immersed by said output array of micro-lenses.

2. A radiation source according to claim 1, wherein the semiconducting material emits infrared radiation when optically pumped by a near-infrared laser source.

3. A radiation source according to claim 1 or claim 2, wherein the array of micro-lenses is integrally formed with the semiconducting layer.

4. A radiation source according to claim 3, wherein the array of micro-lenses comprises a substrate having an output surface on which a multiplicity of micro-lenses are fabricated and an input surface onto which the semiconducting layer is deposited.

5. A radiation source according to any preceding claim, further including an input micro-lens array.

6. A radiation source according to claim 5, wherein said input micro-lens array is attached to an input surface of the semiconducting layer, said input and output surfaces of the semiconducting layer being opposing surfaces.

7. A radiation source according to claim 5 or claim 6, wherein the input micro-lens array is attached to the semiconducting layer by adhesive means.

8. A radiation source according to any of claims 5 to 7, wherein for every micro-lens in the output micro-lens array there is a respective micro-lens in the input micro-lens array, and the two arrays are formed and arranged such that the focal point of each micro-lens in the output array is substantially the same as the focal point of the respective micro-lens in the input array, whereby, in use, the input micro-lens array focuses incoming pump laser radiation onto a plurality of regions in the semiconducting layer and the output micro-lens array receives and collimates radiation emitted from the same said regions.

9. A radiation source according to any preceding claim, wherein the micro-lenses in at least one of the input and output micro-lens arrays are diffractive lenses.

10. A radiation source according to any preceding claim, wherein the micro-lenses in at least one of the input and output micro-lens arrays are refractive lenses.

11. A radiation source according to any preceding claim, wherein the micro-lenses in the input array are hexagonally close packed.

12. A radiation source according to any preceding claim, wherein the materials of which the semiconducting layer and the output micro-lens array are made have substantially the same refractive index.

13. A radiation source according to any preceding claim, wherein the micro-lenses in the output array are Fresnel lenses etched on a substrate of infrared transmitting material .

14. A radiation source according to claim 13, wherein the repeat distance between the Fresnel lenses in the output micro-lens array is in the range of 20 to 2000μm.

15. A radiation source according to any preceding claim, wherein the input array of micro-lenses is formed using RIE on a layer of fused silica (Si0₂) .

16. A radiation source according to any preceding claim, wherein the semiconducting layer emits infrared radiation in the range of 4 to 7μm when optically pumped with radiation in the range of 800 to lOOOnm.

17. A radiation source according to any of claims 1 to 15, wherein the semiconducting layer emits infrared radiation of up to 12um wavelength when optically pumped with radiation in the range of 800 to lOOOnm.

18. A radiation source according to any preceding claim, further including a reflective coating provided on the input surface of the semiconducting layer and formed and arranged to reflect radiation in at least a part of said desired spectral range.

19. A radiation source according to any of claims 1 to 17, further including multi-layer dielectric stacks provided on both the input and output surfaces of the semiconducting layer to form a resonant cavity therebetween.

20. A radiation source according to any preceding claim, further including photonic band gap structures for increasing photoluminescent emission from the device.

21. A radiation source according to any preceding claim, further including a passivation layer film provided at the interface between the semiconducting layer and at least one of the input and output micro-lens arrays.

22. A radiation source according to any preceding claim, wherein the semiconducting layer comprises a plurality of regions of different semiconductors, each said semiconductor emitting radiation in a different spectral region, when optically pumped.

23. A radiation source according to claim 22, wherein the semiconducting layer comprises a plurality of interlaced grids of different semiconductors.

24. A radiation source according to any preceding claim, further including pump laser means.

25. A radiation source according to claim 24, wherein the pump laser means comprises a laser diode.

26. A radiation source according to claim 22 or claim 23, further including a plurality of laser diodes arranged so that their respective output beams of pump laser radiation are incident on the semiconducting layer at different angles.

27. A radiation source according to claim 26, wherein each said laser diode is modulated at a distinct frequency.

28. A gas sensor including a photoluminescent radiation source according to any of claims 1 to 27.

29. A portable gas sensing system including a gas sensor according to claim 28.

30. A thermal imaging system including a photoluminescent radiation source according to any of claims 1 to 27.

31. A portable spectrometer including a photoluminescent radiation source according to any of claims 1 to 27.

32. A method of fabricating photoluminescent radiation sources comprising the steps of: providing a wafer substrate capable of transmitting radiation in a desired spectral range; fabricating on one side of said wafer substrate a multiplicity of arrayed micro-lenses; depositing on an opposite side of the wafer substrate a layer of semiconducting material which, when optically pumped, emits radiation in said desired spectral range, the semiconducting layer being optically immersed on one side by said multiplicity of micro-lenses fabricated on the wafer substrate; and dicing the wafer so as to release a plurality of separate chips therefrom, each chip comprising a respective array of said micro-lenses having a semiconducting layer attached thereto.

33. A method according to claim 32, wherein, prior to said dicing step, a substrate comprising a further multiplicity of arrayed micro-lenses is attached to an exposed surface of the semiconducting layer.

34. A method according to claim 32, wherein a respective input array of micro-lenses is then attached to an exposed surface of the semiconducting layer of each chip.

35. A method according to any of claims 32 to 34, wherein the micro-lenses fabricated in the wafer substrate are Fresnel lenses formed by Reactive Ion Etching (RIE) of the substrate.