INFRARED IMAGE CONVERTER

Abstract: A device for converting a mid-wave infrared or long-wave infrared thermal image into a coherent near-infrared image includes a two-dimensional array of quantum-well-based optical modulators (500) and infrared photodetectors (100). Each modulator is integrated or hybridized with a respective photodetector, and the combination is connected to an electronic circuit. Variations in mid-IR or long-IR light intensity are converted by each photodetector into variations in a bias applied to its respective modulator. The bias variations modulate the intensity and/or phase of near-IR light illuminating the modulators.

Description: INFRARED IMAGE CONVERTER BACKGROUND This application relates to semiconductor quantum well devices for converting images from one spectral region into images in another spectral region. Much work has been done recently on a wide range of electro-optic devices based on the electric-field dependence of strong absorption resonances in semiconductor quantum wells (QWs). In a QW, a thin layer of one semiconductor material is sandwiched between cladding layers of a different material, with the electronic properties of the materials being such that an electric potential well (in the central layer) is formed between two electric potential barriers (in the cladding layers). The QW's small thickness, on the order of 100 A, results in quantization of charge-carrier motion in the thickness direction. Also, QWs exhibit the quantum-confined Stark effect, in which the wavelengths of the QW's peak optical absorptions associated with the creation of light- and heavy-hole excitons shift to longer wavelengths in response to an applied electric field. Since these peak excitonic absorptions have finite spectral widths due to electron/hole interactions with material impurities and phonons, the transmissivity of a QW at a wavelength near a peak varies as the applied field varies. These and other aspects of QW devices are described in commonly assigned U.S. Patent No. 5,047,822 to Little, Jr., et al., which is expressly incorporated here by reference. Because a single QW is so thin, devices are typically made by stacking a number of QWs, e.g., fifty, to obtain significant optical effects. Many aspects of multiple quantum well (MQW) devices are described in the literature, including C. Weisbuch et al. , Quantum Semiconductor Structures. Academic Press, Inc., San Diego, Calif. (1991). A simple MQW device is the absorption modulator, in which the excitonic absorption edge of the quantum wells is moved into and out of coincidence with the wavelength of a spectrally narrow light source, such as a laser, by varying an applied electric field. Thus, the intensity of the light transmitted or reflected by the modulator varies according to the applied electric field, or bias voltage, as noted above. One such absorption modulator, although based on Wannier-Stark localization rather than the quantum-confined Stark effect, is described in K.-K. Law et al., "Normally-Off High-Contrast Asymmetric Fabry-Perot Reflection Modulator Using Wannier-Stark Localization in a Superlattice", A pp lied Phvsics Letters vol. 56, pp. 1886-1888 (May 7, 1990); and K.-K. Law et al., "Self-Electro-Optic Device Based on a Superlattice Asymmetric Fabry-Perot Modulator with an On/Off Ratio > 100: 1", Applied Phvsics Letters vol. 57, pp. 1345-1347 (Sept. 24, 1990). In contrast to the QW's shift of the excitonic absorption peaks to longer wavelengths due to the quantum-confined Stark effect, Wannier-Stark localization leads to a shift to shorter wavelengths for increased electric field in superlattice structures. In general, a superlattice is a stack of interleaved thin barrier layers and QWs in which the QWs are resonantly coupled, causing the QWs' discrete charge-carrier energy levels to broaden into minibands. Applying an electric field destroys the resonance, misaligning the energy levels in neighboring QWs and localizing them over a few QWs. This changes the optical absorption spectrum from a smooth, miniband profile to a peaked, QW-excitonic profile and blue-shifts the absorption edge. As described in more detail below, Applicants' invention can be embodied using either MQW or superlattice structures. Also, it will be understood that such structures described in this application can be fabricated by a wide variety of semiconductor processing methods, e.g., metal-organic chemical vapor deposition, molecular beam epitaxy, and electrochemical deposition methods. See, e.g., J. Switzer et al., "Electrodeposited Ceramic Superlattices", _]___. vol. 247, pp. 444-445 (Jan. 26, 1990); and the above- cited Weisbuch et al. book. Simple MQW absorption modulators operating at room temperature can exhibit modulation depths, i.e., ratios of minimal to maximal absorptions, of about 10: 1 to 30: 1. These low modulation depths can be improved by combining an MQW structure with a suitable resonant optical cavity, such as an asymmetric Fabry-Perot etalon (ASFPE). An ASFPE is a resonant optical cavity formed by two planar mirrors that have different reflectivities. Such devices are described in commonly assigned U.S. Patent Application No. 08/109,550 filed August 20, 1993, by Terrance L. Worchesky and Kenneth J. Ritter, which is expressly incorporated here by reference. Another application of QWs is the quantum well infrared photodetector (QWIP). In the QWIP described in die literature, including Semiconductor Quantum Wells and Superlattices for Long-Wavelength Infrared Detectors. M. O. Manasreh, ed., pp. 55-108, Artech House, Boston, Mass. (1993), internal photoemission of electrons from bound states in GaAs QWs into high-mobility channels in the QWJ-P's cladding layers increases the conductivity of the QWIP in the presence of thermal light, i.e. , long-wave infrared (LWLR) wavelengths from about 8000 nm to about 12000 nm or mid-wave infrared (MWIR) wavelengths from about 3000 nm to about 5000 nm. The light is detected as an increase in the current flowing through the QWTP when operated at a fixed bias voltage. The characteristics of the QWIP (e.g., the peak-response wavelength, the optical bandwidth, and the electrical properties) are determined by the widths of the QWs (usually in the 4- to 8-nm width range) and the composition of the cladding layers (nominally thick layers of Al^-Ga^As, with x ranging from 0.2 to 0.6). Like a charge-coupled device (CCD) imager, arrays of QWJPs can be provided to form thermal images. In a conventional thermal imaging system, an array of detector elements is mated to a silicon multiplexer that reads out the current from each element sequentially in a "bucket brigade" fashion (i.e., the charge is collected from each element into a capacitor and then passed along a row of capacitors to a column-readout capacitor bank which passes it down to a single charge-measuring element on the multiplexer). The original position of each charge packet is tracked, and the image is reconstructed electronically, usually as a video image on a monitor. Multiplexers optimized for the electrical characteristics of QWLPs are not currently available. The multiplexers that have been used are not well suited for the relatively high dark current typical of QWTPs operating at temperatures around that of liquid nitrogen, and greatly increase the cost of the imaging system. Further, because the multiplexers are made from silicon instead of GaAs, the thermal-expansion-coefficient mismatch limits the arrays' physical sizes to well below the limit imposed by GaAs crystal- growth and processing technology. In addition, the multiplexers must also be cooled since they must be located as close to the detectors as possible, but the multiplexers' thermal mass and heat dissipation strain conventional cooling systems. Applicants have recognized that the current flowing through a QWLP can be used to provide the bias necessary for an MQW modulator. In such a device, the change in QWTP current due to a change in the amount of MWIR or LWIR light illuminating the QWTP will change the amount or phase of near-infrared (NIR) light, i.e., wavelengths from about 800 nm to about 2000 nm, reflected or transmitted from the MQW modulator. The change in intensity of LWLR or MWIR light is thus converted into a change in intensity or phase of NIR light. The publication, V. Gorfinkle et al., "Rapid Modulation of Interband Optical Properties of Quantum Wells by Intersubband Absorption", Applied Phvsics Letters vol. 60, pp. 3141-3143 (June 22, 1992), describes the theory of a doped MQW absorption modulator in which the interband absorption strength for NIR photons would be modulated by intersubband absorption of LWIR photons. The LWIR absorption would partially deplete the population of carriers in the ground state, thereby changing the density of final states for NLR absorption. A significant drawback of such a device for a purpose such as converting LWIR information into NIR information would be the interdependence of die operating LWTR and NTR wavelengths due to the absorptions occurring in the same MQW structure. Moreover, a very large LWTR flux and fabrication in a waveguide geometry are needed for significant NIR absorption modulation. SUMMARY Applicants' invention provides a device and a method for converting a mid-wave infrared (MWLR) or a long-wave infrared (LWTR) thermal image into a coherent near-infrared (NIR) image. The device includes a two-dimensional (2D) array of quantum-well infrared photodetectors (QWLPs), a 2D array of quantum-well optical modulators, and an electronic circuit. Variations in the intensity of LWTR or MWIR light incident on the array of QWIPs are converted into variations in the bias of the optical modulators. The bias variations result in changes in the intensity and/or the phase of NLR light reflected from (or transmitted through) the modulator array, thereby converting the LWIR or MWIR image into an NIR image. In one aspect of Applicants' invention, a hybrid image converter comprises an infrared detector array section disposed on a first substrate, an electronics section disposed on a second substrate, and an optical modulator array section disposed on a third substrate. The electronics section is sandwiched between the infrared detector section and the optical modulator section and serves to convert photocurrents generated in the pixels of the infrared detector array into voltages applied to respective pixels of the optical modulator array. In another aspect of Applicants' invention, an integrated image converter comprises an infrared detector array section and an optical modulator array section disposed on a first substrate (by disposing one of the sections on the substrate and disposing the other section on that section) and an electronics section disposed on a second substrate. Elements of the electronics section are electrically connected on a pixel-by-pixel basis to the elements of the integrated detector/modulator array using, for example, indium-bump bonding techniques. Applicants' image converter has the advantages of independently-selectable operating wavelengths, sensitivity to low light intensities, and planar (rather than waveguide) geometry, making it ideally suited for a two-dimensional staring array. Since the NIR light source can be a laser, the resulting coherent NIR image can be used as the input for an optical signal processor capable of performing complex image analysis such as pattern recognition or background clutter rejection. BRIEF DESCRIPTION OF THE DRAWINGS The features and advantages of Applicants' invention will be better understood by reading the following detailed description in conjunction with the drawings in which: Figure 1 is a schematic cross-section of a hybrid image converter including an infrared detector array, an electronics section, and an optical modulator array in accordance with Applicants' invention; Figure 2 is an electrical schematic diagram of an infrared detector section, an electronics section, and an optical modulator section of a hybrid image converter in accordance with Applicants' invention; Figure 3 is a schematic cross-section of an integrated image converter including an integrated infrared detector/optical modulator array and an electronics section in accordance with Applicants' invention; Figure 4 is an electrical schematic diagram of an integrated image converter including an infrared detector/optical modulator section and an electronics section in accordance with Applicants' invention; Figure 5 is a schematic cross-section of a single pixel of an integrated image converter connected to die electronics section in a first way that is in accordance with Applicants' invention; Figure 6 is a schematic cross-section of a single pixel of an integrated image converter connected to the electronics section in a second way that is in accordance witfi Applicants' invention; Figure 7 is an electrical schematic diagram of an integrated image converter connected to the electronics section in a second way that is in accordance with Applicants' invention; Figures 8a and 8b illustrate the conversion of variations in the intensity of MWIR light into variations in the intensity of NIR light using one embodiment of Applicants' invention; and Figures 9a and 9b illustrate the conversion of variations in die intensity of MWIR light into changes in the NIR absorption properties using a second embodiment of Applicants' invention. DETAILED DESCRIPTION Figure 1 shows a schematic cross-section of a hybrid image converter 10, comprising a QWJ-P pixel array 100 disposed on a substrate 200, electronic circuit pixels 300 disposed on a substrate 400, and QW optical modulator pixel array 500 disposed on a substrate 600. The hybrid image converter 10 is conveniently disposed on a substrate 20 that is transparent to NLR light and is a good tiiermal conductor (e.g., sapphire). The transparent substrate 20 is disposed on a cryogenic cold head 30 tiiat cools the substrate 20 and the hybrid image converter 10 to an advantageously low temperature (e.g., 77 Kelvin). In general, the optical modulators can operate at room temperature, but the QWLPs must be cooled to a temperature appropriate for the electronic circuits to generate enough contrast, viz., the QWIP dark current should be low enough for die QWIP photocurrent to be a reasonable fraction, e.g., at least 1 %, of that dark current. As a result of such cooling, the performance of the optical modulators actually improves. Each QWIP pixel in the array 100 advantageously comprises a plurality of n-type QWs disposed between two n-type contact layers that are 5 described in more detail below. The QWIP substrate 200 can be advantageously removed (using, for example, chemo-mechanical polishing) in order to trap LWIR or MWIR light in each pixel, thereby eliminating optical cross-talk among pixels in a large array. The removal of the substrate 200 will advantageously minimize strain in the QWTP array due to 10 thermal-expansion-coefficient mismatch between the QWIPs (that are composed of IH-V semiconductors, such as GaAs) and the electronics pixels 300 and substrate 400 (that are usually composed of silicon). This is important because the image converter 10 must cycle many times between room temperature and low (cryogenic) temperatures over the lifetime of the 15 device. The electronic circuit pixels 300 convert the photocurrents generated in die QWIP pixel array 100 into voltages that are applied to respective QW modulator pixels in the array 500. Via holes (not shown) are used to electrically connect the pixels on one side of die substrate 400 to their 20 respective pixels on the other side of the substrate 400. The electronic circuit pixels may be (but are not limited to being) conventional silicon transimpedance amplifiers (i.e., current-to-voltage converters) disposed on silicon substrates. These circuits and example variations consistent with Applicants' invention are described in more detail below. 25 Each QW optical modulator pixel in d e array 500 advantageously comprises a plurality of undoped QWs disposed between an n-type contact and a p-type contact to form a p-i-n diode or between two n-type contact to form an n-i-n modulator. The modulator substrate 600 can be advantageously removed using chemo-mechanical polishing if the substrate 30 600 is not transparent to light at the operating wavelengtfi of the QW modulator. The removal of the substrate 600 will also advantageously minimize strain in the QW optical modulator array due to tiiermal-expansion- coefficient mismatch between the modulator array and the electronics section, as described above for die QWIP. The QWIP pixels 100 and the QW modulator pixels 500 are electrically connected to die electronic circuits 300 by, for example, indium-bump bonds 700 using such techniques as described in die above-referenced U.S. Patent Application No. 08/109,550. During operation of die hybrid image converter 10 as illustrated in Figure 1, LWIR or MWIR light is incident through the substrate 200 (if it has not been removed) onto Λe QWIP pixels 100. (Variations in the intensity of the LWIR or MWIR light are represented in Figure 1 by variations in the thickness of the arrows.) A beam of uniform-intensity NIR light is directed tiirough d e transparent substrate 20 and die substrate 600 (if it has not been removed) onto die QW modulator pixels 500. Pixel-to-pixel variations in die bias on the modulator pixels 500 resulting from die response of die electronic circuit 300 to variations in the intensity of the LWIR or MWIR light cause the intensity or die phase of die NIR light reflected from die QW modulator pixels 500 to be modulated in proportion to die variations in die original LWIR or MWIR light. (Variations in die intensity or phase of the NIR light are also represented in Figure 1 as variations in the d ickness of the arrows.) Thus, a LWTR or MWIR image is converted into an NIR image. This embodiment of die image converter allows for the independent optimization of the properties of die QWIP array 100, the electronic circuit 300, and the QW optical modulator array 500. For example, since the QWJ-Ps 100 and the QW optical modulators 500 are disposed on different substrates, the growth conditions (e.g., substrate temperature during growtii) and die material compositions (e.g., die concentration of aluminum in die AlGaAs layers) of the two sections can be advantageously different to obtain the best performance from each section. Figure 2 shows an electrical schematic diagram of one pixel of the hybrid image converter 10, comprising a QWIP 100 (represented by a resistor), an electronic circuit 300, and a QW optical modulator 500 (represented by a diode). The electronic circuit 300 supplies a voltage bias, Vj e j, to the QWIP 100. A transimpedance amplifier 310 converts a photocurrent in the QWIP into a bias, V,,,,-,,, that is applied to die QW modulator 500. It will be appreciated that d e electronic circuit 300 shown in Figure 2 is merely a schematic representation of a wide variety of circuits that could perform die function of converting current into voltage. Modem semiconductor electronics techniques could be employed to realize more sophisticated circuits that use, for example, gain and offset corrections to die output of the transimpedance amplifier and multiple gain stages to obtain the voltages required to operate the QW modulator 500 (typically in the 1-10- volt range). Figure 3 shows a schematic cross-section of an integrated image converter 10', comprising an integrated QWTP/QW optical modulator pixel array 800 disposed on a substrate 200' and electronic circuit pixels 300' disposed on a substrate 400'. The QWTP/QW modulator pixels 800 are electrically connected to die electronic circuit pixels 300' using, for example, indium bumps 700', as described above. The integrated image converter 10' is disposed on a cryogenic cold head 30' tiiat cools die device to an advantageously low temperature (e.g., 77 Kelvin). The integrated QWTP/QW optical modulator pixels 800 and the electronic circuit pixels 300' are described in more detail below. It will be appreciated tiiat d e electrical connections represented in Figure 3 as one indium bump 700' per pixel may, in practice, be more tiian one electrical connection per pixel, as described below. As described above for d e hybrid image converter 10, the substrate 200' can be advantageously removed using chemo-mechanical polishing to reduce optical cross-talk and to minimize die strain due to die mismatch in thermal expansion coefficients between the detector/modulator section and die electronic circuit section of die integrated image converter 10'. During operation of die integrated image converter 10' as illustrated in Figure 3, LWIR or MWIR light is incident through die substrate 200' (if it has not been removed) onto die QWTP/QW modulator pixels 800. (Variations in die intensity of the LWTR or MWIR light are represented in Figure 3 by variations in the tiiickness of die arrows.) A beam of uniform-intensity NIR light is also incident through die substrate 200' (if it has not been removed) onto the pixels 800. Pixel-to-pixel variations in the biasses on the modulator sections of die pixels 800 resulting from die response of die electronic circuit 300' to variations in the intensity of the LWIR or MWIR light cause the intensity or the phase of the NIR light reflected from die pixels 800 to be modulated in proportion to die variations in the original LWIR or MWIR image. (Variations in die intensity or phase of the NIR light are also represented in Figure 3 as variations in die thickness of die arrows.) Thus, die LWTR or MWIR image is converted into an NIR image. Figure 4 shows an electrical schematic diagram of one pixel of me integrated image converter 10', comprising a QWIP 100' (represented by a resistor), an electronic circuit 300', and a QW optical modulator 500' (represented by a diode). Here, the QWIP 100' and the QW modulator 500' are shown electrically connected in series because, as explained in more detail below, die two sections are grown one after die otiier on a common substrate 200'. The electronic circuit 300' supplies the detector bias, V^, to the QWTP 100'. A transimpedance amplifier 310' converts a photocurrent in the QWTP into a voltage that is opposite in sign to die detector bias, V^, because a transimpedance amplifier such as this one inverts the output. This voltage is added to die detector bias, V^, using a voltage adding element 320. The summed bias is inverted witii a voltage inverting element 330, and applied to the QW optical modulator 500' as V-,^. The elements 310', 320, and 330 ensure tiiat die QW optical modulator 500' is reverse-biassed as required for operation. It will be appreciated that die electronic circuit 300' shown in Figure 2 is merely a schematic representation of a wide variety of circuits that could perform the function of converting current into voltage. Modern semiconductor electronics techniques could be employed to realize more sophisticated circuits tiiat use, for example, gain and offset corrections to the output of the transimpedance amplifier 310' and multiple gain stages to obtain die voltages required to operate the QW modulator 500' (typically in the 1-10-volt range). Figure 5 shows a schematic cross-section of me epitaxial layer structure of one pixel 800 of the integrated image converter, comprising a QWIP section 100', and a QW modulator section 500' disposed on a substrate 200'. As described in more detail below, die materials used in d e QWIP section 100' are similar to those used in d e QW modulator section 500'; tiius, sequential growtii (using, for example, molecular beam epitaxy) is not a problem. The pixel 800 is shown in Figure 5 electrically connected to the electronic circuit pixel 300' using indium bumps 700'. The QWIP section 100' advantageously contains a plurality of n-type QWs 110' disposed between two n-type contact layers 120' and 130'. In addition, the QWIP section 100' includes a grating 140' or otiier means for coupling LWIR or MWIR light of the appropriate polarization (i.e., perpendicular to the quantum well layers 110' as required by die polarization selection rules for intersubband optical absorption) into d e QWIP section 100'. The QW modulator section 500' advantageously contains a plurality of undoped QWs 510' disposed between an n-type contact layer 520' and a p-type contact layer 530', thus forming the intrinsic (i) region of a p-i-n diode. It will be appreciated that die contact layer 530' could be doped n-type to form an n-i-n optical modulator if it is necessary to modify die resistance of the modulator section as described below for other 5 embodiments of Applicants' invention. The contact layer 520' could be composed of die same material and doped to die same level as the QWIP contact layer 130'. In this case, the two contact layers 130' and 520' would form a single continuous layer tiiat electrically connects the QWIP section 100* to the QW modulator section 10 500'. In another embodiment, d e contact layer 520' could advantageously comprise a dielectric mirror that has high reflectivity (> 99%) at the NIR operating wavelengtii of die QW modulator section 500' (as described in the above-referenced K.-K. Law et al. papers). This would prevent die NIR light from entering the QWIP section 100' (where in some QWTP designs it 15 would be absorbed tiiereby degrading the performance of die modulator and producing unwanted photocurrents in the QWIP section 100', i.e., crosstalk). It would also prevent die NLR light from diffracting (or scattering) from the grating 140'. Diffraction (or scattering) of the NIR light at the grating 140' could degrade die quality of the NIR image. 0 The array of pixels 800 can be defined using standard semiconductor lid ography and etching techniques. A typical pixel would be a square having dimensions of about 50 micrometers (μm) by about 50 μm defined by etching away the material around die pixel to expose d e contact layer 530' nearest the substrate 200'. In most applications, e contact layer 530' is 5 common to all of the pixels of die array and tiius requires only one electrical contact instead of one per pixel. A second etch step is used to expose a small area of d e intermediate contact layers 130' and 520'. The grating 140' is usually etched into die contact layer 120' and coated witi a tiiin metal film. As shown in Figure 5, indium bumps 700' are used to connect the QWIP contact 120', the common intermediate contact (130' and 520'), and d e QW modulator contact 530' to the electronic circuit 300'. In this embodiment, an electronic circuit 300' such as die one shown in Figure 4 would be used to control the integrated image converter pixel 800. A typical QW section 510' of die optical modulator 500' comprises a plurality (e.g., 80) undoped GaAs quantum wells each witii a thickness of, for example, 10 nm and separated by cladding layers composed of, for example, AL^-Ga^^s layers each with a thickness of, for example, 5 nm. The mirror contact layer 520' comprises a plurality (e.g., 10) of alternating layers of, for example, Al< 03) Ga (0 ^s and AlAs. The contact layer 530' is typically composed of GaAs. An advantageous design for the QW section 110' of the QWIP 100' is described in commonly assigned U.S. Patent Application No. 07/906,417, filed June 30, 1992, by John W. Little, Jr., which is expressly incorporated here by reference. It is called the miniband- transport (MBT) quantum well infrared detector. An example MBT structure for operation in die LWIR band comprises a plurality (e.g., 40) of n-type GaAs QWs each witii a tiiickness of, for example, 8 nm, separated by superlattice barrier layers. The superlattice barriers comprise a plurality (e.g., 10) of alternating layers of, for example, GaAs and Al (0 . 3) Ga (07) As witii thicknesses (e.g., 1.5 nm and 4 nm, respectively) chosen so that a miniband of energy states is formed in d e barrier layers that serves as a high-mobility channel for carriers photoexcited out of die QWs. The MBT design is advantageous for use in die integrated image converter as compared with otiier QWIP designs because it allows more flexibility in the choice of material compositions. For example, me same aluminum concentration can be used in all of the AlGaAs layers in botii the QW modulator section 500' and die QWIP section 100' making sequential growth of these sections on a common substrate much less complicated than when d e aluminum concentrations are different. A particularly simple embodiment of die integrated image converter is illustrated by Figure 6, which shows a schematic cross-section of the epitaxial layer structure of one pixel having generally the same components as shown in Figure 5. Here, the electronic circuit 300' is a simple voltage source that is connected to die pixel 800 at only two points, the contact 120' and die contact 530' (i.e., the intermediate contacts 130' and 520' are not connected to die electronic circuit). An electrical schematic diagram of mis embodiment of die integrated image converter is shown in Figure 7. The QWIP section 100' and die QW modulator section 500' are connected in series, and a bias, V b l , is applied across die series circuit. Part of V b j will appear across the QWIP section 100' (and is labeled V^ in Figure 7), and part of V bii - will appear across the QW modulator section 500' (and is labeled V-^ in Figure 7). Photoinduced changes in the resistance of the QWTP section 100' cause changes in die distribution of the bias in the series circuit, thereby changing die bias on d e modulator section 500'. Such a voltage divider arrangement is effective only when a substantial fraction of die total current flowing through die QWTP is photocurrent rather tiian thermally-generated (dark) current (i.e., die QWIP is background limited). This requires low operating temperature or shorter-wavelength (MWIR) QWIP response. Because of the simplicity of the electronics, this mode would be useful for low-cost imaging systems such as the front end of an image intensifier for viewing MWLR scenes. Analysis of the electrical equivalent circuit shown in Figure 7 shows that die change in bias on the modulator, ΔV-.^, is related to d e fractional change in resistance of the QWIP, by the following expression: where V bii - is the total bias voltage on the pixel, and R^ and RJ_. are die effective resistances of the modulator 500' and die QWTP 100', respectively. For a fixed Under tiiese conditions, the bias on d e modulator is V bill /2, which gives die fractional change in die modulator bias according to die following: ΔV mαd V aod = -1/2 - ΔR^/R^ Eqn. 2 An example MWIR MBT comprises a plurality (e.g., 40) of n-type Inrø. D Gaφ 9 -As quantum wells with tiiickness of, for example, 4 nm separated by superlattice barriers comprising a plurality (e.g., 10) of alternating layers of Al (0 ^-G^ O/ DAS and Al^s-Ga^As with thicknesses of, for example, 2 nm and 3 nm, respectively. For an MWIR QWTP having me structure described above, resistances ranging from 1 x 10 10 ohms to 1 x 10" ohms have been measured at 80 K; these are comparable to me effective resistance of a reverse-biassed p-i-n diode, i.e., the resistance of die QW modulator section 500'. The fractional change in resistance of the QWTP 100' measured by changing die input image from a 300-K blackbody to a 500-K blackbody was found for one sample to be about 0.37 at a QWIP bias of four volts. When such a QWIP is connected in series witii a modulator and a bias of about ten volts is applied, die bias voltage would be divided approximately equally between the QWIP section 100' and die modulator section 500', and the bias change on die modulator for about 0.4 fractional change in detector resistance would be (from Eqn. 2) about one volt. For the relatively sharp absorption linewidm obtained in the QW modulators at 80 K, this should be sufficient to cause a reasonably large change in d e absorption of die NLR light in die modulator section. Figures 8a and 8b show experimental results of the conversion of variations in the intensity of MWIR light into variations in the intensity of NIR light using one embodiment of me integrated image converter biassed in series as shown in Figure 6. In this device, die QWs 510' in die modulator section were composed of In (0 . og) Ga ( o. 92) As with thicknesses of 10 nm. Figure 8a shows the transmission of the device in die NIR region of d e spectrum with and without light from an MWIR source incident on d e pixel. The dip in transmission around 880 nm for the "MWIR off" curve was due to exciton absorption. The sharp dip flattened out and shifted to longer wavelength when the MWIR source was turned on. This was die same behavior that was observed in die NIR spectrum when the bias was increased. This implied mat more bias appeared across die QW modulator section 500' when the MWIR light was on than when it was off. This was consistent with die description of die redistribution of voltages due to a decrease in die resistance of the QWTP in die presence of MWIR light. Figure 8b shows the transmission of the sample at a wavelength of 878 nm in me absence and presence of MWIR light. (The horizontal axis refers to the time at which a shutter was opened to allow die MWIR light to illuminate the pixel). The change in the intensity of the NIR light (indicated by die change in transmission) in response to the change in the intensity of the MWIR light is direct evidence of image conversion by d e integrated image converter pixel 800. The excitonic absorption did not remain sharp under application of an electric field in mis tested sample. This was presumably due to a problem with die sample quality caused by strain in the lattice-mismatched InGaAs quantum wells. Figures 9a and 9b show experimental results of image conversion in a second embodiment of me integrated image converter using me series-bias scheme shown in Figure 6 (biassed at -4 V and -5 V, respectively). In this sample, the QWs 510' were composed of high-quality GaAs with thicknesses of about 10 nm, and d e QWIP section was similar to mat described above for MWIR operation. Photocurrent I--- versus wavelengtii (in the NIR spectral region) was measured instead of transmission because the GaAs substrate was opaque to the operating wavelength of the modulator. In this type of measurement, the photocurrent signal is proportional to the absorption strength in die sample, and peaks in die photocurrent indicate exciton absorption. The left vertical axes in Figures 9a and 9b are relative photocurrent (labeled I,-.) and die right vertical axes are die ratios of die photocurrent witii MWIR light on the sample to die photocurrent with the MWIR light off. For both -4 V bias and -5 V bias (Figures 9a and 9b, respectively), the exciton absorption feature remained sharp and shifted to longer wavelengtii with the MWIR light on compared witii the MWIR light off, indicating a redistribution in the bias due to die presence of MWIR light as described above. The on/off ratios were in die range of 4 to 5, implying that NIR modulation depths of tiiis magnitude would be obtained for operation as an image converter. Applicants' invention has been described above in terms of specific embodiments. It will be readily appreciated by one of ordinary skill in die art, however, that die invention is not limited to those embodiments, and tiiat, in fact, die principles of die invention may be embodied and practiced in otiier devices and methods. Therefore, die invention should not be regarded as delimited by those specific embodiments but by the following claims.

Claims: CLAIMS 1. A device for converting variations in light having wavelengtiis in a first predetermined range into variations in light having wavelengths in a second predetermined range comprising: a substrate; an absorption modulator comprising a plurality of quantum wells having an absorption due to heavy-hole excitons that is maximal at wavelengths in die second predetermined range when a predetermined bias is applied to the modulator; a photodetector comprising a plurality of quantum wells and electrically connected to die absorption modulator; and means, electrically connected to d e absorption modulator and d e photodetector, for supplying biasses to d e absorption modulator and die photodetector; wherein die electrically connected photodetector and absorption modulator are disposed on die substrate and, when light having a first intensity and wavelengtiis in die first predetermined range is incident on die photodetector and light having a second intensity, a second phase, and wavelengtiis in the second predetermined range is incident on die absorption modulator, variations in the first intensity generate variations in die bias on die absorption modulator such tiiat at least one of d e second intensity and the second phase is varied. 2. The device of claim 1, comprising a two-dimensional array of me absorption modulators, a two-dimensional array of die photodetectors, and a plurality of the supplying means, wherein each photodetector is electrically connected to a respective absorption modulator, and each electrically connected photodetector and absorption modulator is electrically connected to a respective supplying means and is disposed on die substrate. 3. The device of claim 1, wherein me photodetector comprises a multilayer structure including die plurality of quantum wells, each quantum well having a bound ground energy state and a bound excited energy state, and a plurality of superlattice barrier layers interleaved between the quantum wells, each superlattice barrier layer having a miniband of energy states in resonance with d e excited energy states of its adjacent quantum wells, and the minibands and excited energy states provide an electrically continuous channel across the wells and layers for carriers excited from the ground energy states to the excited energy states and minibands by absorption of photons having wavelengtiis in d e first predetermined range. 4. The device of claim 1, wherein the photodetector is vertically integrated witii die absorption modulator. 5. An image converter for converting a first image having wavelengths in a first predetermined range into a second image having wavelengtiis in a second predetermined range comprising: an array of photodetectors arranged in a pixel format and disposed on a first substrate, each photodetector comprising a plurality of quantum wells and, when a first image incides on the converter, generating a photocurrent having a magnitude proportional to an intensity of a respective pixel of the first image; an electronics section disposed on a second substrate; and an array of optical modulators arranged in a pixel format and disposed on a third substrate, each optical modulator comprising a plurality of quantum well modulator layers having a predetermined thickness and an absorption due to heavy-hole excitons that varies at wavelengtiis in die second predetermined range in accordance witii a voltage applied to the optical modulator; wherein die electronics section is sandwiched between and electrically connected to d e photodetectors and d e optical modulators and is for converting, when a first image incides on die converter, photocurrents generated by the photodetectors into voltages applied to respective ones of the optical modulators to convert the first image into e second image. 6. The image converter of claim 5, wherein each photodetector comprises a multilayer structure including d e plurality of quantum wells, each quantum well having a bound ground energy state and a bound excited energy state, and a plurality of superlattice barrier layers interleaved between the quantum wells, each superlattice barrier layer having a miniband of energy states in resonance with die excited energy states of its adjacent quantum wells, and d e minibands and excited energy states provide an electrically continuous channel across the wells and layers for carriers excited from the ground energy states to die excited energy states and minibands by absorption of photons having wavelengths in die first predetermined range. 7. An integrated image converter for converting an infrared image having wavelengths in a first predetermined range into a second image having wavelengtiis in a second predetermined range comprising: a first substrate; an infrared detector array section comprising a plurality of infrared detectors arranged in a pixel format, each infrared detector for generating, when the infrared image incides on die converter, a photocurrent having a magnitude proportional to an intensity of a respective pixel of die incident infrared image; an optical modulator array section comprising a plurality of optical modulators arranged in a pixel format, each optical modulator for absorbing light having wavelengths in the second predetermined range in accordance witii a voltage applied to die optical modulator; and an electronics section disposed on a second substrate and comprising a plurality of means for converting a photocurrent generated by an infrared detector into a voltage; wherein the infrared detector array section and die optical modulator array section are disposed one on top of another and on d e first substrate, and die plurality of converting means are electrically connected on a pixel-by-pixel basis to one of the infrared detector array section and die optical modulator array section to apply the voltages converted from d e photocurrents to respective ones of the optical modulators and convert die infrared image into the second image. 8. In a semiconductor device comprising a plurality of quantum wells, a method of converting light having wavelengths in a first predetermined range into light having wavelengths in a second predetermined range comprising the steps of: converting the light having wavelengtiis in the first predetermined range into an electric current, the electric current having a magnitude determined by an intensity of the converted light; converting e electric current into an electric voltage having a magnitude proportional to the magnitude of die electric current; and controlling at least one of an intensity and a phase of die light having wavelengtiis in die second predetermined range in response to die electric voltage.

Inventors: AHEARN JOHN S, LITTLE JOHN W JR, AHEARN, JOHN, S., LITTLE, JOHN, W., JR.
Applicants: MARTIN MARIETTA CORP, MARTIN MARIETTA CORPORATION
Application year: 1995
Main classification: G02F2
IPC codes: B82B1, G02F1, G02F2