Abstract:
A multi-spectral photodetector for detecting two or more different bands of infrared radiation is described. The photodetector includes a diffractive resonant optical cavity that resonates at the two or more infrared radiation bands of interest. By detecting infrared radiation at two or more discrete applied biases and by generating a spectral response curve for the photodetector at each of these biases, the response to each of the individual bands of infrared radiation can be calculated. The response to each band of infrared radiation can be found by deconvolving the response at each bias. The photodetector finds many uses including military and medical imaging applications and can cover a broad portion of the infrared spectrum.

Description:
BACKGROUND OF THE INVENTION 
   In the field of infrared (IR) imaging, the current objective is to provide high pixel count imagers at low cost with high performance. InSb, HgCdTe and quantum well infrared photodetector (QWIP) technologies have demonstrated high performance large area imagers. Each of these technologies has various strengths and weaknesses. InSb photodetectors offer high performance, ease of fabrication and operation at wavelengths of less than 5 μm, but must be cooled to approximately 80 K. HgCdTe photodetectors can be designed to operate in the important long wavelength IR (LWIR) band corresponding to a wavelength range of 8 to 12 μm and the middle wavelength IR (MWIR) band corresponding to a wavelength range of 3 to 5 μm. HgCdTe photodetectors require very tight tolerances in material and fabrication uniformity, especially in the LWIR band, to ensure high performance. QWIP photodetectors have been demonstrated in both the MWIR and LWIR bands. Because of the maturity of the GaAs/AlGaAs material system used in QWIP photodetectors, tight tolerances in both material and fabrication uniformity are readily obtained. QWIP photodetector sensitivity is generally lower than that achieved by InSb or HgCdTe photodetectors in their respective wavelength bands. 
   Dual-band or multi-spectral detection is increasingly desirable as a method to increase the probability of detection under various environments. As an example, objects that are only slightly above room temperature, such as a person, are most easily detected in the LWIR corresponding to the peak IR radiation emission band for near room temperature objects. In contrast, a hot object, such as an automobile exhaust pipe, can be readily detected in the MWIR corresponding to the peak IR radiation emission band for objects having a temperature of more than 600 K. Thus, a system that provides high performance with either of these objects should be sensitive to both wavelength bands. 
   In military applications, it is possible to camouflage an object such that the object emits little radiation in a particular portion of the IR spectrum. A dual-band or multi-spectral photodetector with appropriately selected sensing wavelengths therefore provides a means of detecting objects that have been camouflaged in this manner. 
   Additional applications may use dual-band or multi-spectral photodetectors for discriminating one object from another. As two objects at different temperatures emit different amounts of IR radiation at different wavelengths, a dual-band or multi-spectral photodetector can more readily discriminate between the objects. As an example, two identical cars may be parked next to each other. If one has recently been driven while the second has not been operated for several hours, a dual-band or multi-spectral detector could readily sense the subtle differences in emissivities corresponding to temperature differences of less than a degree. 
   Medical applications can also benefit from the additional discrimination that can be achieved with a dual-band or multi-spectral photodetector. In particular, detection of cancerous lesions using infrared imaging has shown great promise. The sensitivity of such systems can be increased by imaging at two or more wavelengths to remove any artifacts in the image, such as might be caused by birthmarks, scars, tattoos, etc. The use of two or more wavelengths will also increase sensitivity as smaller temperature differences can be detected. 
   Conventional IR detector technologies have proven difficult to adapt to this current demand for dual-band or multi-spectral detection. As noted above, high performance single band detectors and imaging arrays have been demonstrated using HgCdTe, InSb and QWIP technologies. Of these, dual-band or multi-spectral detection is possible only with the HgCdTe and QWIP technologies. The dual-band and multi-spectral HgCdTe photodetectors demonstrated to date have suffered significantly from both non-uniformity in the HgCdTe material and the fabrication process. While dual-band and multi-spectral QWIP photodetectors do not place as stringent requirements upon the starting material, the fabrication process has similarly proven quite challenging. Further, both IR detector technologies have suffered from reduced performance in dual-band or multi-spectral photodetectors in comparison to single band performance. Lastly, operation in more than one wavelength band has generally required at least one electrical connection between the photodetector and the external electronics for each wavelength band. 
   In view of the desirability of dual-band and multi-spectral IR photodetectors, there exists a need for a design that places fewer and/or less stringent requirements upon the starting material and/or the fabrication process. Such photodetectors should also be highly producible. It is also desirable to develop a photodetector technology that requires fewer electrical connections between each photodetector and the external electronics. Furthermore, it is desirable to readily change from detecting in one wavelength band to another wavelength band, even alternating consecutive images between two or more wavelength bands. 
   SUMMARY OF THE INVENTION 
   In one embodiment of the present invention, a multi-spectral IR photodetector comprises in order, a top electrical contact, a series of elongate IR absorbing elements for absorbing two or more bands of IR radiation, a bottom electrical contact and a reflector. These elements form a diffractive resonant optical cavity (DROC) for the two or more bands of IR radiation. This is in contrast to previous DROC designs that required multiple cavity designs to support multiple bands of IR radiation, such as the designs found in U.S. Pat. No. 6,452,187. A photoresponse is sensed by applying an external bias between the top and bottom contacts and measuring the resulting current. By changing the magnitude and/or polarity of the applied bias, the ratio of the photoconductive response between the two or more bands of IR radiation changes. Therefore, the relative magnitude of the IR radiation detected in each of the two or more bands can be established by changing the applied bias. 
   In another embodiment of the present invention, a multi-spectral IR photodetector comprises in order, a top electrical contact, a series of elongate IR absorbing elements for absorbing two or more bands of IR radiation, a bottom electrical contact and a reflector. These elements form a doubly periodic DROC. The doubly periodic DROC has a period in a first direction to diffract at least one of the bands of IR radiation and a different period in a perpendicular direction to diffract at least one of the other bands of IR radiation. Again, the relative magnitude of the IR radiation detected in each of the two or more bands can be established by changing the bias applied between the top and bottom contacts. 
   In yet another embodiment of the present invention, a dual-band IR imager includes an array of pixel elements, with each pixel element comprising in order, a top electrical contact, a series of elongate IR absorbing elements for absorbing two bands of IR radiation, a bottom electrical contact and a reflector. These elements form a DROC for the two bands of IR radiation. The dual-band IR imager further includes readout electronics that bias the pixel elements at two different applied voltages. The readout electronics then multiplex the resultant photoresponse signals at the two different applied biases. 
   In each embodiment, the applied bias is placed across the elongate IR absorbing elements via the top and bottom contacts such that a resulting current flow is along an axis of the elongate IR absorbing elements. A magnitude of the resulting current flow is indicative of the quantity of IR radiation absorbed by the elongate IR absorbing elements. Changing the magnitude and/or polarity of the applied bias results in non-linear changes in the response magnitudes of each band of IR radiation. Therefore, the relative magnitude of the IR radiation detected in each band can be established by changing the applied bias and through using external signal processing circuitry. The number of individual bands being sensed must be less than or equal to the number of different applied biases to allow establishing signals that are proportional to each individual band of IR radiation absorbed. 
   Photodetectors comprising a single multi-spectral IR photodetector, a one-dimensional line array of photodetectors and a two-dimensional area array of photodetectors are envisioned. The one- and two-dimensional arrays of photodetectors are readily amenable to forming imagers for various applications. Depending upon the desired bands of IR radiation, a number of different material systems may be used to form the IR absorbing elements. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is described in reference to the following Detailed Description and the drawings in which: 
       FIG. 1  is a cross-sectional view of the starting material for first and second embodiments of the present invention, 
       FIGS. 2   a-d  are energy band diagrams for different starting materials for use in the present invention, 
       FIGS. 3   a-c  are IR absorption mechanisms for different starting materials for use in the present invention, 
       FIG. 4  is a cross-sectional view of the first embodiment of the present invention, 
       FIG. 5  is a perspective view of the first embodiment of the present invention, 
       FIG. 6  is a top down view of the first embodiment of the present invention, 
       FIG. 7  is a spectral response of a photodetector corresponding to the first embodiment of the present invention, and 
       FIG. 8  is a top down view of the second embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Various embodiments of the present invention are described in detail with reference to the drawings with corresponding elements having corresponding numbers throughout the drawings. While the following description will generally discuss a dual-band IR photodetector, altering the design of the IR absorbing layer can lead to absorption in three or more IR wavelength bands. 
     FIG. 1  is a cross-sectional view of the starting material  100  used in fabricating a dual-band IR photodetector in accordance with the two embodiments. The starting material  100  includes a series of laminae formed in succession. A top contact  102  is formed of doped semiconductor material. The top contact  102  is preferably heavily doped to reduce contact resistance and has a thickness of approximately 0.2 to 0.5 μm. An IR absorbing layer  104  is formed of semiconductor material that absorbs IR radiation in first and second IR wavelength bands. The IR absorbing layer  104  preferably has a thickness of between 0.4 and 1.0 μm. A bottom contact  106  is formed of doped semiconductor material. The bottom contact  106  is preferably heavily doped to reduce contact resistance and has a thickness of approximately 0.2 to 0.5 μm. A reflector  108  that is highly reflective to both the first and second bands of IR radiation completes the starting material  100 . The reflector  108  is preferably metallic and formed of gold, aluminum or an alloy of two or more metals. The thickness of the reflector  108  is preferably between 0.1 and 0.25 μm. Alternatively, the reflector  108  can be formed of a Bragg reflector designed to be highly reflective to both the first and second bands of IR radiation. Further, the reflector  108  is preferably planar. A bottom ohmic contact  110 , is in electrical contact with the bottom contact  106  such that the signal may be provided to the external electronics. The bottom ohmic contact  110  is preferably formed of an alloy of Ni/Au/Ge or Au/Sn/Au and has a thickness of 0.1 μm. 
   The IR absorbing layer  104  can be formed of several different materials and material systems, only one of which will be examined in detail hereinafter. The preferred material comprises n-type multiple quantum wells (MQWs) formed of GaAs and its alloys such as AlGaAs and InGaAs. QWIPs and Enhanced QWIPs (EQWIPs) have demonstrated high levels of performance using GaAs/AlGaAs and InGaAs/AlGaAs MQW IR absorbing layers. 
     FIGS. 2   a  through  2   d  illustrate the conduction band energy diagram for four different possible MQW-based IR absorbing materials.  FIG. 2   a  illustrates MQW material  200  having isolated or uncoupled quantum wells  202   a,b  and  204   a,b . Isolated or uncoupled means that the ground state energy levels  206   a,b  in quantum wells  202   a,b  do not interact with the ground state energy levels  208   a,b  of quantum wells  204   a,b . This is accomplished by having relatively thick barriers  210   a-e . Typical barrier widths are 300-500 Å. The quantum wells  202   a,b  and  204   a,b  preferably comprise GaAs and will have a width of 20-50 Å depending upon the IR radiation band to be absorbed. As illustrated in  FIG. 2   a , quantum wells  202   a,b  are narrower than quantum wells  204   a,b , and thus absorb the longer of the two IR radiation wavelength bands. The different width quantum wells need not be interleaved as illustrated in  FIG. 2   a , but may form two groups of equal width quantum wells. If two groups of equal width quantum wells are used rather than interleaved quantum well widths, it is preferable that the quantum wells for absorbing the shorter IR radiation wavelength band be placed closer to the reflector  108 . The barriers  210   a-e  preferably comprise Al X Ga 1−X As, where 0.1≦X≦0.6 depending upon the IR radiation bands to be absorbed. 
   A second starting material  220  design is illustrated in  FIG. 2   b . In contrast to the isolated or uncoupled quantum wells in MQW material  200 , the quantum wells  222   a,b  and  224   a,b  are weakly coupled. In this case, the ground state energy levels  226   a,b  of quantum wells  222   a,b  slightly interact with the ground state energy levels  228   a,b  of quantum wells  224   a,b , causing each to broaden. The weakly coupled quantum wells are separated by narrow barriers  230   a,b , while each pair of weakly coupled quantum wells is separated by a broad barrier  232   a-c . Preferred narrow barrier widths are 100-200 Å while the preferred broad barrier widths are 300-500 Å. As with MQW material  200 , the quantum wells within each coupled pair are of different widths. As illustrated in  FIG. 2   b , quantum wells  222   a,b  are broader than quantum wells  224   a,b  and will therefore absorb the shorter of the two IR radiation wavelength bands. The quantum wells  222   a,b  and  224   a,b  preferably comprise GaAs and will have a width 20-50 Å depending upon the IR radiation bands to be absorbed. In addition, like MQW material  200 , the barriers  230   a,b  and  232   a-c  will comprise Al X Ga 1−X As, where 0.1≦X≦0.6 depending upon the IR radiation bands to be absorbed. 
     FIG. 2   c  illustrates strongly coupled MQW material  240 . The ground state energy levels  246   a,b  of quantum wells  242   a,b  strongly interact with the ground state energy levels  248   a,b  of quantum wells  244   a,b , causing each to significantly broaden. Under the appropriate applied bias, the ground state energy levels  246   a,b  will align with the ground state energy levels  248   a,b  leading to potentially higher IR absorption and dark current. The coupled quantum wells are separated by narrow barriers  250   a,b , while each pair of coupled quantum wells is separated by a broad barrier  252   a-c . Preferred narrow barrier widths are 20-75 Å while the preferred broad barrier widths are 300-500 Å. As with MQW material  200 , the quantum wells within each coupled pair are of different widths. As illustrated in  FIG. 2   c , quantum wells  242   a,b  are broader than quantum wells  244   a,b  and will therefore absorb the shorter of the two IR radiation wavelength bands. The quantum wells  242   a,b  and  244   a,b  preferably comprise GaAs and will have a width 20-50 Å depending upon the IR radiation bands to be absorbed. In addition, like MQW material  200 , the barriers  250   a,b  and  252   a-c  will comprise Al X Ga 1−X As, where 0.1≦X≦0.6 depending upon the IR radiation bands to be absorbed. 
   A fourth type of MQW material  260  suitable for absorbing IR radiation is shown in  FIG. 2   d . This MQW material  260  uses quantum wells  262   a,b  formed of GaAs, while quantum wells  264   a,b  are formed of InGaAs. By using InGaAs as the quantum well material, the ground state energy levels  268   a,b  of quantum wells  264   a,b  are lower, allowing the absorption of shorter IR wavelengths than the ground state energy levels  266   a,b  of quantum wells  262   a,b . As with MQW material  200 , the quantum wells  262   a,b  and  264   a,b  can be either interleaved or grouped. The quantum wells  264   a,b  comprise In Y Ga 1−Y As, where 0.0≦Y≦0.15 depending upon the IR radiation bands to be absorbed. The barriers  270   a-e  will preferably have a thickness of 300-500 Å and will comprise Al X Ga 1−X As, where 0.1≦X≦0.6 depending upon the IR radiation bands to be absorbed. 
   A number of IR absorption mechanisms are feasible with the above four MQW material types. First is a bound to continuum (BTC) type material  300  as shown in  FIG. 3   a . In BTC type material, the IR radiation is absorbed when an incident photon  302  excites a ground state electron  304  from a GaAs quantum well  306  into the energy continuum  308  above the AlGaAs barrier energy  310 . Due to quantum mechanical selection rules, the incident photon  302  must have an electric field component that is normal to the plane of the quantum well  306  if the incident photon  302  is to be absorbed by lattice matched MQW material  200 ,  220 ,  240  and  260 . By applying an electric field across the BTC type material  300 , the excited electron  312  is swept from the IR absorbing material  104 , collected in either the top or bottom contact  102 ,  106  depending upon the applied polarity and detected in an external circuit. For absorption in the MWIR, the barriers of the BTC type material  300  will preferably have an aluminum content X in the range of 0.4 to 0.6. For absorption in the LWIR, the barriers of the BTC type material  300  will preferably have an aluminum content X in the range of 0.1 to 0.4. 
   A second IR absorption configuration is bound to quasi-continuum (BTQC) type material  320  and is illustrated in  FIG. 3   b . In BTQC material  320 , the IR radiation is absorbed when an incident photon  302  excites a ground state electron  322  from a quantum well  324  into an energy state  326  approximately equal to the barrier energy  328 . By applying an electric field across the BTQC type material  320 , the excited electron  330  is swept from the IR absorbing material  104 , collected in either the top or bottom contact  102 ,  106  and detected in an external circuit. Example material parameters for the BTQC type material  320  are quite similar to the BTC type material  300 , with the exception of the quantum well thickness. A slightly wider quantum well  324 , having a thickness of 25-50 Å, is required to create the energy state that is approximately equal to the barrier energy  328 . Like the BTC type material  300 , the BTQC type material  320  IR absorption wavelengths are typically controlled by the specific quantum well thickness and barrier composition. 
   A third IR absorption configuration illustrated in  FIG. 3   c , uses two bound energy states, and thus is known as bound to bound (BTB) type material  340 . In BTB type material, the IR radiation is absorbed when an incident photon  302  excites a ground state electron  342  from a quantum well  344  into a bound energy state  346  less than the AlGaAs barrier energy  348 . By applying an electric field across the BTB type material  340 , the excited electron  350  tunnels through a portion of the adjacent barrier and is swept from the IR absorbing material  104 , collected in either the top or bottom contact  102 ,  106  and detected in an external circuit. Example material parameters for the BTB type material  340  are generally similar to the BTC type material  300 , with the exception of the quantum well width. The quantum well width must be greater than that of similar BTC type material  300  as two bound energy states must be created. Typical quantum well widths in BTB material  340  range from 30 to 60 Å. 
   In any of the BTC, the BTQC and the BTB type materials  300 ,  320 ,  340 , the quantum well can alternatively be In Y Ga 1−Y As, wherein 0.0&lt;Y&lt;0.15. A further alternative is the use of a material system such as InGaAs/InAlAs, InGaAs/InP or various other III-V or II-VI based material systems. Yet another alternative is the use of p-type MQW material in which a ground state energy hole is excited during absorption of the incident photon. It should be noted that p-type MQW material places no requirements on the polarization of the photon&#39;s electric field. Molecular beam epitaxy (MBE) or metal-organic chemical vapor deposition (MOCVD) can be used to form the various type IR absorbing materials from the different III-V and II-VI material systems. 
   The conduction band energy diagrams of  FIGS. 2   a-d  illustrate only two quantum well widths, thus leading to two different absorption bands. For a dual-band IR photodetector, the designs illustrated in  FIGS. 2   a-d  are sufficient. However, for a multi-spectral IR photodetector that absorbs in three different bands, three different well widths would be required. Due to the very precise nature of MBE and MOCVD, forming IR absorbing material with quantum well widths that differ by 5 Å is possible. Therefore, an IR absorbing material for a triple-band IR photodetector could have quantum well widths of 25 Å, 30 Å and 38 Å. 
   A dual-band IR photodetector  400  according to a first embodiment of the present invention is illustrated in FIG.  4  and can be formed from any of the various starting materials described above. The first embodiment is formed through a process comprising an etching step and a metal deposition step thereby fabricating the dual-band IR photodetector  400 . The etch process removes a portion of the top contact  102  ( FIG. 1 ) and the dual-band IR absorbing layer  104  down to the bottom contact  406 , resulting in top contact elements  402   a-c , and IR absorbing layer elements  404   a-c . The resulting width of the top contact elements  402   a-c  and the dual-band IR absorbing layer elements  404   a-c  is from 0.8 to 1.6 μm depending upon the two desired IR wavelength bands. Note that while top contact elements  402   a-c  appear to be separate in  FIG. 4 , top contact elements  402   a-c  are electrically interconnected as shown in  FIGS. 5 and 6 . The reflector  408  is deposited on the side of the bottom contact  406  opposite the etched portion. The fabrication process need not be conducted in this sequence. 
   While the above etching processes may appear difficult, two different possible approaches to the processes have been developed. Both processes are based upon the difference in etch rates of different materials. In the simplest solution, the etch rate of the bottom contact  406  is significantly lower than the etch rate of the dual-band IR absorbing layer  104 . In this case, the etch can be timed to ensure the dual-band IR absorbing layer  104  is completely removed with little fear of removing much of the bottom contact  406 . The second solution would be used in the case where the etch rate between the dual-band IR absorbing layer  104  and the bottom contact  406  is similar. In this case, an etch stop layer (not illustrated) is placed between the dual-band IR absorbing layer  104  and the bottom contact  406 . The selected etch stop layer material preferably has an etch rate that is significantly lower than the etch rate of the dual-band IR absorbing layer  104 . As an example, the etch rate of Al 0.6 Ga 0.4 As is significantly less than the etch rate of Al 0.3 Ga 0.7 As. As the dual-band IR absorbing layer  104  may comprise GaAs/Al 0.3 Ga 0.7 As MQW material, an etch stop comprised of Al 0.6 Ga 0.4 As is feasible. 
   Two alternative structures are also possible. As seen in  FIG. 4 , the etching is stopped at the interface between the dual-band IR absorbing layer  104  and the bottom contact  406 . The first alternative, not illustrated, is to etch partially into the bottom contact  406 . The second alternative, also not illustrated, is to etch completely through the bottom contact  406  to the reflector  408 . These two alternatives provide an additional degree of design freedom. The second alternative also offers the advantage that as the reflector  408  preferably comprises a metal such as gold, which has an extremely low etch rate, the etch process is simplified. 
   The dual-band IR photodetector  400  illustrated in  FIGS. 5 and 6  forms a DROC that resonates at two different wavelengths for IR radiation incident on the dual-band IR photodetector  400  from the top contact elements  402   a-c  side. The two resonant wavelengths are controlled in part by the period of the top contact elements  402   a-c  and the elongate dual-band IR absorbing elements  404   a-c , by the width of the top contact elements  402   a-c  and the dual-band IR absorbing elements  404   a-c  and by the thicknesses of the top contact elements  402   a-c , the bottom contact  406  and the dual-band IR absorbing elements  404   a-c . Lastly, the two resonant wavelengths are controlled in part by the material design of the dual-band IR absorbing layer elements  404   a-c.    
   It must be noted that in contrast to conventional dual-band IR photodetectors, the IR radiation is incident on interleaved IR absorbing quantum wells or on the grouped longer wavelength IR absorbing quantum wells. In conventional dual-band IR photodetectors, if the IR radiation were incident on the longer wavelength IR absorbing layer, this longer wavelength IR absorbing layer would absorb the shorter wavelength IR radiation as well. This would result in significant cross-talk within the longer wavelength signal and little short wavelength signal. However, electromagnetic field modeling of the first embodiment showed the longer wavelength radiation generated high electric field regions closer to the top contact elements  402   a-c  corresponding to the longer wavelength absorbing material when the quantum wells are grouped rather than interleaved. Likewise, the shorter wavelength radiation generates high electric field regions closer to the bottom contact  406  corresponding to the shorter wavelength absorbing material when the quantum wells are grouped. 
   The removal of a portion of the dual-band IR absorbing layer  104  provides several advantages. By creating the DROC, the photoresponse of the dual-band IR photodetector  400  is enhanced as IR radiation of the appropriate wavelength resonates within the cavity increasing absorption. Thus, the cavity improves the signal generated or quantum efficiency of the dual-band IR photodetector  400 . Secondly, the generated dark current is reduced. The dark current is generated within the dual-band IR absorbing layer elements  404   a-c . By removing a significant portion of the dual-band IR absorbing layer  104 , a reduction in dark current is observed. The dark current generates noise within the signal from the dual-band IR photodetector  400 . As this dark current induced noise is the primary source of noise under certain operating conditions, reducing the dark current is important to improving the sensitivity of the dual-band IR photodetector  400 . As the DROC increases quantum efficiency and reduces dark current and therefore noise, the dual-band IR photodetector  400  has a higher signal to noise ratio or sensitivity when compared to other dual-band IR photodetector technologies. 
     FIG. 7  shows the spectral response for a dual-band IR photodetector made in accordance with this first embodiment. As can be seen, the dual-band IR photodetector exhibits strong photoresponse in two narrow bands of LWIR radiation. Each of the response peaks is approximately 0.5 μm in width with the first band centered at 8.7 μm and the second band centered at 11.1 μm. 
   The relative strength of the photoresponse for each of the two bands is also clearly illustrated in FIG.  7 . For negative applied biases, the 8.7 μm band has a photoresponse approximately 3 times as large as the 11.1 μm band. For positive applied biases, the photoresponses are approximately equal. Therefore, the ratio of the photoresponse of the 8.7 μm band to the photoresponse of the 11.1 μm band is a relatively strong function of applied bias. 
   External electronics can be used to separate the photoresponses of the two bands due to their strong function of applied bias. By generating spectral response curves for the dual-band photodetector at two known applied biases, preferrably using a blackbody radiation source, the photoresponse due to radiation emitted by a target in each band can readily be calculated. The photoresponse calculations generally correspond to two unknowns (the response to IR radiation at the two known applied biases) and two equations (the spectral response curve at each bias), which can then be easily solved. Algorithms that are more complex may be desirable for even greater sensitivity. Preferably, the algorithm deconvolves the two measured responses using the generated spectral response curves at the two known biases. This deconvolution algorithm is especially preferable when three or more spectral response curves at three or more corresponding known biases are used. When only two bands of IR radiation are detected, the two known applied biases are preferably of opposite polarity. Note that the number of different applied biases must be equal to or greater than the number of infrared bands to be deconvolved. Therefore, a triple-band IR photodetector would require the application of at least three biases. 
   The external electronics used in conjunction with a single dual-band IR photodetector or an array of photodetectors in accordance with the first embodiment can be relatively complex. If an array of dual-band IR photodetectors is created for use in an imager, a silicon readout integrated circuit (ROIC) can be used to perform many of the required functions. The ROIC is preferably indium bump bonded to the array of photodetectors to provide electrical, thermal and mechanical connections. The ROIC can be used to provide the two or more applied biases required to collect the excited electrons from the array of photodetectors. These collected electrons will typically be used to charge a separate capacitor for each photodetector for a given integration time, thereby producing a signal voltage. If space permits, a separate capacitor for each bias for each photodetector is preferable. A separate capacitor for each bias for each photodetector reduces the memory requirements for the ROIC. 
   Once a scene has been imaged for the integration time at each bias, the resulting signal voltages are deconvolved using the stored spectral response curves. The stored spectral response curves are based upon the photodetectors&#39; responses under the two or more applied biases, preferrably when illuminated by blackbody radiation. Upon deconvolution, the signals for each of the two or more bands can be multiplexed to an external system for further manipulation and/or display. While the above functions would preferably be performed by the ROIC, the ROIC may be limited to merely providing the two or more biases, integrating the charge and then multiplexing the resultant signal voltages to an external system. In this case, the external system would then deconvolve the signal voltages based upon the stored spectral response curves, thereby simplifying ROIC design. 
   A dual-band IR imager using the present invention could operate in two different modes. For an application requiring rapid detection of an object, the output from the bias resulting in the greatest sensitivity would be used. Alternatively, the sum of the signals at each bias could be used. Once the object had been detected, the dual-band IR imager would be switched into an object identification mode. In this object identification mode, the infrared spectral emissions at two wavelengths would be detected. As many objects have distinctive thermal signatures, i.e., they do not emit the same amount of radiation at all wavelengths, the target could be identified. This object identification mode would require storing the thermal signatures of a number of possible objects to improve the likelihood of object identification. 
   While the first embodiment was illustrated with equal periods in both the X and Y directions as seen in  FIG. 6 , this need not be the case. For an application that requires broader first and second band spectral response, a different period may be used in the X and Y directions. An initial design, such as that illustrated in  FIG. 6 , has a period of 5.9 μm in both the X and Y directions. This leads to a dual-band IR photodetector having first and second band spectral response peaks at 8.7 μm and 11.2 μm respectively. By increasing the period in the Y direction by 24% relative to the period in the X direction, the first band spectral response could include resonances at 8.7 and 11.2 μm while the second band spectral response could include resonances at 9.9 and 12.7 μm. Thus, slightly varying the periodicity in the two perpendicular directions can broaden the spectral response, though the magnitude of the photoresponse will likely decrease. 
   The first embodiment was illustrated with dual-band IR absorbing elements  404   a-c  having equal widths for elements oriented in both the X and Y directions. Equal widths in both the X and Y directions are not required. The use of different width dual-band IR absorbing elements  404   a-c  in the X direction relative to the Y direction provides an additional degree of freedom when designing the dual-band IR photodetector  400 . 
   A dual-band IR photodetector  500 , according to a second embodiment of the present invention, is illustrated in FIG.  8 . In the second embodiment, the periodicity in the X and Y directions is different as seen in FIG.  8 . The shorter wavelength band is more strongly diffracted by the DROC in the X direction (smaller period) while the longer wavelength band is more strongly diffracted by the DROC in the Y direction (greater period). A dual-band IR photodetector designed to respond in the MWIR and the LWIR could have a 3.0-4.0 μm period in the X direction while having a 7.0-8.0 μm period in the Y direction. The design process is simpler in the second embodiment as the periodicity and width in each direction can quickly be optimized through electromagnetic field modeling. Note, however, that truly optimized designs will require both electromagnetic field modeling and experimental results. As each wavelength band is most efficiently coupled in only one direction corresponding to one polarization, photoresponse corresponding to the second polarization is reduced. Thus, an engineering trade-off is required when selecting between the first and second embodiments. 
   As indicated above, when using MQW material for the dual-band IR absorbing layer  104 , the IR radiation must have an electric field component that is perpendicular to the plane of the quantum well. It has been found, both experimentally and through electromagnetic field modeling, that a DROC of the type found in the first and second embodiments is efficient at rotating the plane of electric field polarization of the incident IR radiation. This is important, as the incident IR radiation will typically be normally incident upon the dual-band IR photodetector  400 . The normally incident IR radiation will have its electric field polarized in the plane of the quantum well and thus little IR radiation would be absorbed. Therefore, use of the DROC design of the first and second embodiments is quite advantageous. 
   An important factor in the described embodiments is a non-linear change in the spectral response of the dual-band or multi-spectral IR absorbing layer  104  as a function of bias. If the output signal for the photodetector does not exhibit any spectral dependence upon applied bias, then the signal cannot be deconvolved to establish a signal for each of the two or more spectral bands. The underlying physics of the spectral bias dependence depends on at least the ground state electron population within the quantum wells and electron transport within the photodetector. The ground state electron population depends in part upon doping the quantum well with n-type dopants. Wider quantum wells, such as those required for shorter IR wavelength bands can more easily be doped to higher levels due to their width. The quantum well ground state electron population must be kept high to ensure electrons are present to absorb an incident IR photon. The refilling of the quantum well comes in part from the dark current within the photodetector. In the weakly coupled quantum wells of IR absorbing material  220 , this dark current will tend to collect in the first quantum well encountered. That is, electrons that are transported through the barrier will most likely scatter into the first quantum well after the barrier. In  FIG. 2   b , this would correspond to scattering into quantum wells  222   a,b  under one polarity and quantum wells  224   a,b  under the opposite polarity. This is shown in  FIG. 7  where the response for a negative applied bias shows filling of the broader quantum well is favored, i.e., the shorter wavelength response is enhanced relative to positive applied bias. 
   Electron transport differences are pronounced when the quantum wells are grouped according to well width. Experiments have shown that at low biases, longer wavelength response is typically enhanced as little of the applied bias is dropped across the shorter wavelength group of quantum wells. With little applied bias being dropped across the shorter wavelength group of quantum wells, excited electrons are much more likely to scatter into the longer wavelength group of quantum wells. At higher biases, more of the applied bias is dropped across the shorter wavelength group of quantum wells and the excited carriers are more likely to be collected at the contact. 
   In the strongly coupled MQW material  240 , the ground state electron populations are a function of bias. At certain applied biases, the ground states of the coupled quantum wells are at the same energy. This tends to equalize the response from the coupled quantum wells. At other biases, one or the other of the quantum wells will be lower and tend to have a majority of the ground state electrons leading to greater response for the higher populated quantum well. For BTB type material  340  using strongly coupled quantum wells, having the excited state in one quantum well aligned with either the ground state or the excited state of the other quantum well allows excited carriers to more readily tunnel out of the first quantum well and be collected at the contact. For these reasons, strongly coupled MQW material  240  exhibits a very strong bias dependence. This advantage is at least partially offset due to the generally larger dark current exhibited by strongly coupled MQW material  240 . 
   Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. Therefore, such changes and modifications should be construed as being within the scope of the invention.