Abstract:
A plurality of quantum-grid infrared photodetector (QGIP) elements are concatenated to form a spectrometer. Each of the QGIP elements is adapted to detect light at a particular range of wavelengths. Additionally, each QGIP element is adapted to produce a photocurrent that is proportional to the amount of light detected at its respective range of wavelengths. This type of configuration permits spectrometry within a spectrum that spans the aggregate ranges of wavelengths of each QGIP element.

Description:
GOVERNMENT INTEREST 
     The invention described herein may be manufactured, used, and licensed by or for the United States Government. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates generally to spectrometry and, more particularly, to infrared spectrometry. 
     DESCRIPTION OF RELATED ART 
     Spectrometers are commonly used to characterize or identify chemicals and biological specimens in laboratories. Spectrometers are also used to characterized semiconductor materials and optical materials. 
     Many conventional spectrometers, such as Fourier Transform (FT) infrared (IR) spectrometers, are expensive, bulky, and fragile. Often, optical elements such as diffraction gratings or prisms are used in conventional spectrometers to separate incident light into its various wavelength components. For many conventional spectrometers, a source generates light across the spectrum of interest and a monochromater (e.g., diffraction grating or prism) separates the source radiation into its various wavelengths. A slit selects a collection of wavelengths that shine through a sample at any given time. The sample absorbs light according to its chemical properties, and a detector collects the radiation that passes through the sample. The detector outputs an electrical signal, which is normally sent directly to an analog recorder, thereby allowing recordation of energy as a function of frequency or wavelength. 
     For such spectrometers, the use of optical elements often results in optical attenuation, thereby reducing system sensitivity. Given these deficiencies in conventional spectrometers, it is desirable to have alternatives. For example, it is desirable to have spectrometers that are comparatively inexpensive, portable, and robust. 
     SUMMARY 
     The present disclosure provides spectrometers and related methods. 
     Briefly described, an embodiment of a spectrometer comprises a plurality of quantum-grid infrared photodetector (QGIP) elements. Each QGIP element is adapted to detect energy at a particular range of wavelengths and generate a photocurrent that is proportional to the energy detected at that particular range of wavelengths. 
     An embodiment of a method comprises the steps of detecting energy at a plurality of quantum-grid infrared photodetector (QGIP) elements and generating photocurrents in response to the detected energy. The detected energy is defined by a plurality of wavelengths that, in the aggregate, form a substantially continuous spectrum. Each photocurrent is proportional to the detected energy at one range of wavelengths. 
     Other systems, methods, features and/or advantages will be or may become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features and/or advantages be included within this description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Many aspects of the disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present disclosure. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
         FIG. 1A  is a diagram showing a spectroscopy system having a quantum-grid infrared photodetector (QGIP) spectrometer that performs infrared (IR) spectrometry. 
         FIG. 1B  is a diagram showing a top view of an embodiment of the QGIP spectrometer of  FIG. 1A . 
         FIG. 2A  is diagram showing a side view of a QGIP element in the QGIP spectrometer of  FIG. 1B . 
         FIG. 2B  is a diagram showing a perspective view of a QGIP element in the QGIP spectrometer of  FIG. 1B . 
         FIG. 2C  is a diagram showing an exploded view of a gridline from the QGIP element of  FIGS. 2A and 2B . 
         FIG. 3A  is a diagram showing a superlattice of  FIGS. 2A through 2C  in greater detail. 
         FIG. 3B  is a diagram showing the superlattice of  FIG. 3A  when a bias voltage is applied across the superlattice. 
         FIG. 4A  is a diagram showing the superlattice unit of  FIGS. 3A and 3B  in greater detail. 
         FIG. 4B  is a diagram showing an exploded view of a binary basis unit of  FIG. 4A  in greater detail. 
         FIG. 4C  is a diagram showing two quantum wells of  FIG. 4B  in greater detail. 
         FIG. 5  is a diagram showing a QGIP imager formed by concatenating multiple QGIP spectrometers. 
         FIGS. 6 through 8  are flowcharts showing embodiments of spectroscopic processes. 
     
    
    
     DETAILED DESCRIPTION 
     Quantum-grid infrared photodetector (QGIP) elements are relatively small and inexpensive devices that can be configured to detect energy at various wavelengths. QGIP elements typically detect light at a particular wavelength (or particular range of wavelengths) due to their inherent light-scattering properties. The properties of QGIP elements are discussed in detail in U.S. Pat. No. 5,485,015, which is incorporated herein by reference as if set forth in its entirety. Thus, only a truncated discussion of the properties of QGIP elements is provided here. For reasons discussed below, the characteristics of QGIP elements can permit construction of fairly cost-effective, portable, and robust spectrometers. Consequently, a spectroscopy system that employs a spectrometer having QGIP elements in robust and compact due to advantageous features provided by the QGIP elements. 
     Reference is now made in detail to the description of the embodiments as illustrated in the drawings. While several embodiments are described in connection with these drawings, there is no intent to limit the invention to embodiments disclosed herein. On the contrary, the intent is to cover all alternatives, modifications, and/or equivalents. 
       FIG. 1A  is a diagram showing an embodiment of a spectroscopy system  10  having a quantum-grid infrared photodetector (QGIP) spectrometer  100  that performs infrared (IR) spectroscopy. As shown in  FIG. 1A , the spectroscopy system  10  also includes an amplifier  102 , e.g., an amplifier circuit, a processor  104 , e.g., a processing circuit, and a display  106 . For reasons discussed below, the QGIP spectrometer  100  detects a spectrum of wavelengths and generates photocurrents that are proportional to the wavelengths (or ranges of wavelengths) detected by the QGIP spectrometer  100 . The generated photocurrents are provided to an amplifier circuit  102  that amplifies the photocurrents. The amplified photocurrents are provided to the processing circuit  104 , which measures and processes the amplified photocurrents. Upon processing, a spectrum  108  is displayed at the display  106 . 
     As is shown in  FIG. 1A , the displayed spectrum  108  depicts a distribution of detected wavelengths. In other embodiments, information acquired by the QGIP spectrometer  100  may be displayed in other formats. The amplifier circuit  102 , the processing circuit  104 , and the display  106  comprise part of a conventional spectroscopy system. In this regard, the QGIP spectrometer  100  may replace an IR detector from a conventional spectroscopy system. 
       FIG. 1B  is a diagram showing a top view of an embodiment of the QGIP spectrometer  100  of  FIG. 1A . In this embodiment, the QGIP spectrometer  100  comprises ten QGIP elements  110   a  . . .  110   j  (hereinafter simply referred to as “QGIP element(s)  110 ”). Each of the QGIP elements  110  is electrically coupled to a common substrate  225 . As described in greater detail below, each of the QGIP elements  110  is configured to detect energy (also referred to herein as radiation or incident light) over a particular range of wavelengths (λ). The common substrate  225  is electrically coupled to a metal layer  255  that is deposited around the QGIP elements  110 . The QGIP elements  110  are electrically coupled to top contact-bonding pads  140   a  . . .  140   j  (hereinafter simply referred to as “contact-bonding pad(s)  140 ”) using wire bonding  120   a  . . .  120   j  (hereinafter simply referred to as “wire bonding  120 ”). Additionally, the QGIP elements  110  are electrically coupled to a common bottom contact-bonding pad  130  via the substrate  225  and metal layer  255  using wire bonding  125 . The electrical coupling of the QGIP elements  110  to the contact-bonding pads  130 ,  140  facilitates detection of photocurrents that are generated by the QGIP elements  110  in response to detecting incident light at several different ranges of wavelengths. Thus, once the QGIP elements  110  generate photocurrents as a result of detecting the incident light, the generated photocurrents are provided to the amplifier circuit  102  via the contact-bonding pads  130 ,  140 . Since the operation of each QGIP element  110  is described in greater detail in  FIGS. 2A through 4C , only a truncated discussion of the operation of the QGIP elements  110  is provided with reference to  FIG. 1B . 
     In some embodiments, the area of each QGIP element  110  is approximately 150×1150 μm 2 . Thus, the QGIP spectrometer  100  of  FIG. 1  may be fabricated within a very compact area. The incident light detected by each QGIP element  110  is introduced through the substrate  225 . In other words, the incident light propagates through the substrate  225  prior to detection. Therefore, at least a portion of the substrate  225  is optically transparent with respect to the range of wavelengths that are to be detected. Hence, during fabrication, the metal layer  255  is deposited around the array of QGIP elements  110  so that the incident light is unobstructed when entering the QGIP elements  110  through the substrate  225 . Photocurrents from each QGIP element  110  are independently measured by current amplifiers (not shown), which reside in the amplifier circuit  102 . 
     Since the ten QGIP elements  110  detect energy at ten different ranges of wavelengths for the QGIP spectrometer  100  of  FIG. 1B , ten sets of photocurrents are generated, each of which is proportional to the energy at one of the ten different ranges of wavelengths. If the ten different ranges of wavelengths are relatively close to each other, then the aggregate of the wavelengths forms a substantially continuous spectrum. Consequently, measurement of the photocurrents generated by the QGIP elements  110  results in a spectroscopic measurement of the incident light. The compact nature of the QGIP spectrometer  100  provides a very compact and portable spectroscopy system for performing spectroscopic measurements. 
       FIG. 2A  is diagram showing a side view of a QGIP element  110  in the QGIP spectrometer  100  of  FIG. 1B . As shown in  FIG. 2A , the QGIP element  110  comprises a plurality of gridlines  200 . Each gridline  200  comprises a top metal layer  205 , a top contact layer  210 , a superlattice  215 , a bottom contact layer  220 , and a substrate  225 . For reasons provided below, the superlattice  215  detects light at a particular wavelength and generates a photocurrent that is proportional to the detected light. 
     Each QGIP element  110  comprises a grid pattern of metal that is first deposited on top of a quantum-well infrared photodetector (QWIP) material. Since QWIP materials are discussed in detail in U.S. Pat. Nos. 5,485,015 and Re 34,649, which are incorporated herein by reference as if set forth in their entireties, only a truncated discussion of QWIP material is presented here. The grid pattern of metal serves as the top metal layer  205  and also as a mask for etching (e.g., plasma etching, etc.). The QWIP material has a number of layers that constitute a superlattice active region. Using the mask, the QWIP material is etched vertically through the superlattice active region and down into a portion of the bottom contact layer. The etching process results in a number of gridlines  200  having superlattices  215 . An unetched portion  285  resides on one side of the QGIP element  110  for the purpose of wire bonding. Another layer of metal  255  is deposited on the bottom contact layer surrounding the QGIP element  110 . As described above, this layer of metal  255  is deposited around the QGIP elements  110  to provide unobstructed access of incident light  260  to the superlattices  215 . As is known, the propagating incident light  260  defines an optical-electric field  270  that is perpendicular to the propagation direction of the incident light  260 . The metal contacts  205 ,  255  are subsequently annealed to provide ohmic contacts. 
     The result of the fabrication process yields a lamellar grid pattern as shown in  FIG. 2A . In other words, the fabrication process yields a plurality of gridlines  200  that each has a fixed width (w), a fixed depth (t), and a fixed spacing (s). While a three-dimensional perspective view of the QGIP element  110  having a lamellar grid pattern is shown in  FIG. 2B , it should be appreciated that a cross-grid pattern or other equivalent patterns may also be used to generate the QGIP element  110 . 
     As shown in  FIG. 2B , the lamellar grid pattern of the QGIP element  110  includes gridlines  200  of a fixed length (l) that are arranged in a linear array across the breadth (b) of the QGIP element  110 . The two ohmic contacts (e.g., wire bonding  125 ,  120 ) are shown in  FIG. 2B  at one end of the QGIP element  110 . The length (l) of the gridlines  200  and the number of gridlines  200  in a QGIP element  110  do not significantly affect light-coupling characteristics (e.g., light absorption, resonant scattering of light, etc.) of the QGIP element  110 . However, the width (w) of the gridlines  200  affects light-coupling characteristics of the QGIP element  110 . In some embodiments, it is desirable to have the width (w) of the gridlines  200  be an odd multiple of the half-wavelength of the detection wavelength in the superlattice material. In other words, if the superlattice material is adapted to detect a wavelength of λ, then the width (w) of the gridline  200  should be approximately: 
               w   =       N   ⁢           ⁢     λ   s       2       ,           [     Equation   ⁢           ⁢   1     ]             
 
where N is an odd integer and λ s  is the incident wavelength in the material. The incident wavelength of the material is defined as: 
                 λ   s     =     λ     ɛ         ,           [     Equation   ⁢           ⁢   2     ]             
 
where λ is the wavelength in free space and ε is the dielectric constant of the material. Given Equations 1 and 2, the metal layer  205  on top of each gridline  200  acts as a resonant multi-pole antenna due to the selected width (w). The resonant properties of the metal layer  205  result in scattering of any incident light  260  that enters through the substrate  225 . In other words, the width (w) of the metal layer  205  on each gridline  200  defines a scattered field, which further defines the absorption characteristics of the QGIP element  110 .
 
     As shown in  FIG. 2C , the gridlines  200 , in addition to being the active absorbing material, also provide a two-fold function. First, the gridlines  200  behave as dielectric waveguides  265  for normal incident light  260 . Second the gridlines  200  behave as resonators for scattered light  275 . In operation, as incident light  260  enters through the substrate  225 , each gridline  200  directs the incident light  260  through the superlattice  215  to the top metal layer  205 . In this regard, the gridline  200  behaves as a dielectric waveguide  265  for the incident light  260 . Upon being directed to the top metal layer  205 , the light is reflected and scattered by the metal layer  205 . The reflected and scattered light  275  resonates through each of the gridlines  200  and is absorbed by the superlattice  215  structure of the gridlines  200 . 
     Due to the selected width (w), the wave-guiding properties of the gridlines  200  have fundamental modes of approximately 
           N   ⁢           ⁢     λ   s       2     .       
 
Since the fundamental modes of the waveguide  265  are integer multiples of half-wavelengths, these wave-guiding properties further strengthen the scattered field at the multi-pole resonant wavelengths and sharpen the absorption width for each QGIP element  110 .
 
     Electromagnetic simulations have shown that the optical absorption increases and the absorption bandwidth decreases when the height (t) of the gridlines  200  increases. Additionally, beyond a certain gridline height (t), the absorption decreases with increasing height (t). In other words, according to simulation results, an optimum wavelength-dependent gridline depth (t) exists. For example, for λ of approximately 9 μm, the optimum gridline depth (t) would be approximately 3.0 μm. Since each gridline  200  also affects the scattering field of other gridlines  200 , the spacing (s) between gridlines  200  should, preferably, be large. Simulations have shown that a gridline spacing of between approximately 3 μm to approximately 5 μm should be adequate to isolate scattering fields from adjacent gridlines  200 . 
     In one simulation, when the spectrometer  100  of  FIG. 1B  was designed to detect wavelengths (λ) of approximately:
 
λ={7.5, 8, 8.5, 9, 9.5, 10, 10.5, 11, 11.5, 12 μm}  [Equation 3],
 
each QGIP element was configured to have corresponding gridline widths (w) of approximately:
 
w={3.45, 3.74, 4.03, 4.32, 4.61, 4.9, 5.19, 5.48, 5.77, 6.06 μm}  [Equation 4],
 
a gridline spacing (s) of approximately 3 μm, and a gridline depth (t) of approximately 3 μm. The electromagnetic simulations showed that the full-width-at-half-maximum (Γ) of the detected peak was approximately 0.8 μm when the spectrometer  100  had a resonance of N=3.
 
     In another simulation, when the spectrometer  100  was designed to detect at wavelengths of approximately:
 
λ={6, 7, 8, 9, 10, 11, 12, 13, 14, 15 μm}  [Equation 5],
 
each QGIP element  110  was configured to have corresponding gridline widths (w) of approximately:
 
w={0.736, 0.942, 1.148, 1.354, 1.56, 1.766, 1.972, 2.178, 2.384, 2.59 μm}  [Equation 6],
 
and a gridline spacing (s) of approximately 5 μm. Here, the spectrometer  100  utilized an N=1 dipole resonance, and the calculated Γ was approximately 0.7 μm when the gridline depth (t) was approximately 3 μm.
 
       FIG. 3A  shows the superlattice  215  of  FIGS. 2A through 2C  in greater detail. As mentioned above, the superlattice  215  absorbs the incident light at the various wavelengths. The superlattice  215  comprises a number of superlattice units  310   a,    310   b  (simply referred to herein as “superlattice unit(s)  310 ”), which individually absorb the incident light at a given wavelength. Each superlattice unit  310  is defined by a lower miniband  430  of degenerate energy states and an upper miniband  420  of degenerate energy states. If each of the superlattice units  310  is substantially identical, then all of the superlattice units  310  will have similar electron-transfer characteristics. The electron-transfer characteristics of each of the superlattice units  310  permit detection of various wavelengths by each of the QGIP elements  110 . The structure of the superlattice unit  310  and details of the minibands  420 ,  430  are discussed in greater detail with reference to  FIG. 4A . 
       FIG. 3B  provides an example of electron transfer under an applied bias. As shown in  FIG. 3B , when the superlattice  215  is irradiated with incident light  260 , an electron in the lower miniband  430  may make an optical transition  330   a  to the upper miniband  420 . If a bias is applied to the superlattice  215 , then, upon transitioning  330   a  to the upper miniband  420 , the electron migrates  332   a,    334   a  across the upper miniband  420  from one superlattice unit  310   a  to another superlattice unit  310   b.  While the photocurrents flow in the upper miniband  420 , dark currents due to doped electrons flow in the lower miniband  430 . In order to block the flow of the dark currents between superlattice units  310 , superlattice barriers  460 ,  470  are placed at both ends of the superlattice unit  310 . The barrier heights of the superlattice barriers  460 ,  470  are chosen to be lower than the lower degenerate energy level ε7 of the upper miniband  420  but higher than the upper degenerate energy level ε6 of the lower miniband  430 . Thus, only the dark current is obstructed while the photocurrent is permitted to flow between the superlattice units  310 . The flowing photocurrents are then measured to determine the absorbed ranges of wavelengths and, consequently, the resulting spectrum. 
       FIG. 4A  is a diagram showing the superlattice unit  310  of  FIGS. 3A and 3B  in greater detail. While specific energy characteristics of components within the superlattice unit  310  are described in detail with reference to  FIGS. 4B and 4C , energy characteristics are discussed broadly with reference to  FIG. 4A  to provide an overview of the operation of the superlattice  310 . 
     As shown in  FIG. 4A , the superlattice unit  310  comprises a number of basis units  410   a,    410   b,    410   c  (hereinafter simply referred to as “basis units  410 ”). In some embodiments, the basis unit  410  is a binary basis unit that comprises two quantum wells  412 ,  414 . However, it should be appreciated that the basis unit  410  may be a single quantum well, a ternary quantum well, or a basis unit  410  having any integer value k as a basis. 
     The superlattice unit  310  further comprises several relatively thin inter-unit barriers (B 2 )  440   a  . . .  440   n  (hereinafter simply referred to as “inter-unit barrier(s)  440 ”). Each inter-unit barrier  440  separates adjacent basis units  410 . Additionally, each inter-unit barrier  440  permits energy coupling between adjacent basis units  410 . In the embodiment of  FIG. 4A , the basis units  410  are binary basis units  410  that each having a first quantum well  412   a  . . .  412   c  (hereinafter simply referred to as “first quantum well(s)  412 ”) and a second quantum well  414   a  . . .  414   c  (hereinafter simply referred to as “second quantum well(s)  414 ”). The first quantum well  412  and the second quantum well  414  may be manufactured using quantum-well infrared photodetector (QWIP) material. In each binary basis unit  410 , the first quantum well  412  is separated from the second quantum well  414  by an inter-well barrier (B 1 )  450   a  . . .  450   n  (hereinafter simply referred to as “inter-well barrier(s)  450 ”). Similar to the inter-unit barriers  440 , the inter-well barriers  450  are relatively thin barriers that permit well-to-well energy coupling. 
     The inter-well energy coupling and the inter-unit energy coupling result in a degeneration of energy levels. The degeneration of energy levels forms the minibands  420 ,  430 . The upper miniband  420  of energy levels are grouped together at approximately the upper energy level of the quantum well while the lower miniband  430  of energy levels are grouped together at approximately the lower energy level of the quantum well. Hence, the degeneration of energy levels manifests itself as a separation of energy (or wavelength) peaks in a spectrum. As shown in  FIG. 4A , for three binary basis units  410 , the lower miniband  430  has six degenerate energy levels that range from a lower degenerate energy level of ε1 through an upper degenerate energy level of ε6. Similarly, the three binary basis units  410  result in an upper miniband  420  having six degenerate energy levels that range from a lower degenerate energy level of ε7 through an upper degenerate energy level of ε12. These degenerate energy levels are described in greater detail by Choi et al. in “QWIP Structural Optimization,” (hereinafter “the SPIE reference”) presented at the 47th Annual Meeting of the SPIE, held on Jul. 7 through 13, 2002 in Seattle, Wash. The SPIE reference is incorporated herein by reference as if set forth in its entirety. Additionally, the formation of the degenerate energy levels is described in greater detail with reference to  FIGS. 4B and 4C . 
     The superlattice unit  310  further comprises superlattice barriers  460 ,  470 , which are located at each end of the superlattice unit  310 . As discussed above, the superlattice barriers  460 ,  470  are adapted to prevent electron transfer between the lower minibands  430  of the superlattice units  310  while permitting electron transfer between the upper minibands  420  of the superlattice units  310 . 
     A specific example of a broadband QWIP material may have parameters adapted to detect between an approximately 7 μm- to 15 μm-wavelength range. In one embodiment of a binary basis unit  410 , the wells  412 ,  414  are made of GaAs while the inter-well barriers  450  are made of Al 0.27 Ga 0.73 As. In that embodiment, the well thicknesses are approximately 70 Å and 75 Å, respectively, and the inter-well barrier thickness is approximately 25 Å. The inter-unit barriers are made of Al 0.27 Ga 0.73 As, and the inter-unit barrier thickness is approximately 25 Å. Given an absorption line broadening (σ) of approximately 11 meV, four basis units  410  are typically needed to obtain a smooth absorption spectrum. With approximately 11 meV in line broadening, the QWIP material maintains a relatively uniform absorption between approximately 7 μm to approximately 15 μm wavelength range. 
     An example embodiment of the superlattice  215  comprises eight periods of superlattice units  310 . Each superlattice unit  310  comprises four periods of binary basis units  410 , and each superlattice unit  310  is separated from an adjacent superlattice unit  310  by an approximately 600 Å undoped Al 0.19 Ga 0.81 As superlattice barrier  460 ,  470  and an approximately 25 Å undoped Al 0.27 Ga 0.73 As inter-unit barrier  440 . Each binary basis unit  410  comprises an approximately 70 Å GaAs well  412 , an approximately 25 Å Al 0.27 Ga 0.73 As inter-well barrier  450 , and an approximately 75 Å GaAs well  414 . Each binary basis unit  410  is separated from adjacent binary basis units  410  by an approximately 25 Å Al 0.27 Ga 0.73 As inter-unit barrier  440 . All of the barriers and the wells have a Si doping density of approximately 4×10 17  cm −3 . The eight-period superlattice units  310  are sandwiched between an approximately 1000 Å GaAs top contact layer  210  and an approximately 2.5 μm GaAs bottom contact layer  220 , both doped to approximately 4×10 17  cm −3 . The material layers are grown on a GaAs semi-insulating substrate. 
       FIGS. 4B and 4C  are diagrams that show energy characteristics of the binary basis unit  410  and the quantum wells  412 ,  414  of  FIG. 4A  in greater detail. As shown in  FIG. 4C , irradiation of the first quantum well  412  results in a finite absorption of radiation by the material of the first quantum well  412 . The absorption of the radiation results in a transition of electrons from a lower energy level (E 1 )  432 ″ to an upper energy level (E 2 )  422 ″. Similarly, irradiation of the second quantum well  414  results in a finite absorption of radiation by the material of the second quantum well  414 . Again, the transition from E 1 ′ to E 2 ′ results in a transition of electrons from a lower energy level (E 1 ′)  434 ″ to an upper energy level (E 2 ′)  424 ″. These transitions are induced by optical-electric fields  270  that are perpendicular to the layers of the material. The perpendicular optical-electric field  270  corresponds to parallel light propagation. 
     When a plurality of quantum wells  412 ,  414  are stacked together but separated by relatively thick barriers, a multiple quantum well (MQW) is formed as an aggregate of the stacked, individual quantum wells  412 ,  414 . Typically, due to the relatively thick barriers, each of the quantum wells  412 ,  414  maintains its own absorption characteristics with very little effect on adjacent quantum wells  412 ,  414 . However, when barrier thicknesses are reduced, the electron wave functions in each well  412 ,  414  begin to spread into adjacent wells  412 ,  414 . The spreading of the electron wave functions into adjacent wells results in degenerate energy levels  420 ,  430  that are common to all of the quantum wells  412 ,  414 . In other words, if there are N quantum wells in a given structure, then there will be N slightly-separated energy levels that form an upper miniband  420  and N slightly-separated energy levels that form a lower miniband  430 . Optical transitions can initiate from any one of the N slightly-separated energy levels in the lower miniband  430  to any one of the N slightly-separated energy levels in upper miniband  420  with certain oscillator strength. 
     In a specific example, if two quantum wells  412 ,  414  are of different sizes (e.g., different well width, different well depth, or both), then each quantum well  412 ,  414  has a different quantized energy level (e.g., E 1  to E 2  for the first well, and E 1 ′ to E 2 ′ for the second well). The different quantized energy levels result in different absorption energies by each of the quantum wells  412 ,  414  when the quantum wells  412 ,  414  are far apart. When the sizes of the wells are sufficiently different, the two absorption energies may be separated from each other, resulting in two distinct absorption peaks. By joining the two quantum wells  412 ,  414  together using a relatively thin inter-well barrier  450  to form a binary basis unit  410 , the original energy levels change into degenerate common energy levels (e.g., e 1 , e 2 , e 3 , and e 4 )  420 ′,  430 ′ in the binary basis unit  410 . The four possible new transitions (e.g., e 1  to c 3 , c 1  to e 4 , e 2  to e 3 , and e 2  to e 4 ) create four absorption peaks. Since the separation between e 1  and e 2  is relatively small, the four peaks typically group into two widely separated pairs of peaks. The aggregation of additional binary basis units  410  results in a superlattice unit  310  having a greater number of degenerate energy levels. The increasing number of degenerate energy levels (as shown, for example, by  FIG. 4A ) results in a relatively continuous spectrum. Since the quantum wells having binary basis units  410  are further discussed in U.S. Pat. No. Re 34,649, further discussion of binary basis units  410  is omitted here. 
     As shown from  FIGS. 1A through 4C , since the wavelength selectivity is built into each QGIP element  110 , the operation of the QGIP spectrometer  100  is simple and reliable. Additionally, due to the inherent characteristics of the QGIP elements  110 , the QGIP spectrometer  100  is portable, small, and light. Furthermore, the QGIP spectrometer  100  need not be calibrated in the field because the wavelength selectivity is fabricated into each QGIP element  110 . In this regard, many deficiencies associated with conventional spectrometers are addressed by the QGIP spectrometer  100  as shown and described above. 
     While an individual QGIP spectrometer  100  may be used for performing spectroscopic measurements, several QGIP spectrometers  100  may be combined to create imaging systems. For example,  FIG. 5  shows a QGIP imager  500  formed by concatenating multiple QGIP spectrometers  100   a  . . .  100   n  (hereinafter simply referred to in the aggregate as “QGIP spectrometer(s)  100 ”). As shown in  FIG. 5 , the two-dimensional array of QGIP spectrometers  100  may yield spatial images as well as spectra for a particular scene. If many rows of QGIP spectrometers  100 , from 1 to n, are arranged as shown, and the entire array is moved from right to left at constant speed, a scene may be sequentially imaged at different wavelengths. Alternatively, the image of the scene may be scanned into each column of the array sequentially using moving mirrors (not shown) to detect the scene at different wavelengths. Under such conditions, hyperspectral imaging may be performed. 
     In other embodiments, an areal image may be obtained by adding repeating columns of QGIP spectrometers  100 . By reading signals from different rows and columns of the corresponding spectrometer elements, an image of a particular wavelength may be formed. This arrangement eliminates the need for a scanning mirror. 
     It can be appreciated from the example of  FIG. 5  that the compact and robust nature of the QGIP spectrometer  100  permits use of the QGIP spectrometer  100  as a building block for many types of imaging systems. While an imaging system is specifically illustrated in  FIG. 5 , the QGIP spectrometer  100  may be a component part of other, more complex systems employing spectroscopic measurements. 
       FIGS. 6 through 8  show method steps associated with spectroscopic processes  600 ,  700 ,  800 . As shown in  FIG. 6 , one embodiment of the spectroscopic process  600  may be seen as a two-step process that begins with detecting ( 620 ) energy at a plurality of QGIP elements. The detected ( 620 ) energy is defined by the wavelength of the energy. Upon detecting ( 620 ) the energy, the process  600  continues by generating ( 630 ) photocurrents that are proportional to the detected ( 620 ) energy. In other embodiments, the spectroscopic process may continue as shown in  FIG. 7 . For those embodiments, the process may be seen as further including the steps of amplifying ( 720 ) the generated photocurrents and generating ( 730 ) a spectrum, which is indicative of the detected energy defined by the plurality of wavelengths. 
     In some embodiments, the step of detecting ( 620 ) energy may further be defined as shown in  FIG. 8  to include the steps of receiving ( 820 ) incident light through a substrate layer of each of the QGIP elements. The received ( 820 ) light is then guided ( 830 ) through the QGIP elements. Thereafter, the light is scattered ( 840 ) by being reflected back through the QGIP elements. Preferably, the scattered incident light resonates through the QGIP element at a predefined frequency that preferably corresponds to an integer multiple of a half wavelength. The scattered ( 840 ) light is then absorbed by the QGIP element. 
     Embodiments of processes adapted for imaging may also include the step of arranging the plurality of QGIP elements into a matrix of elements. The matrix of elements may then be used to perform hyperspectral imaging. 
     While the process of  FIG. 6  may be carried out by the structures as described in  FIGS. 1 through 5 , any arrangement of QGIP elements  110  may be used to carry out the process of  FIG. 6 . 
     As shown in the embodiments of  FIGS. 1 through 6 , many deficiencies associated with conventional spectrometers are addressed by the QGIP spectrometer  100 . 
     Although exemplary embodiments have been shown and described, a number of changes, modifications, or alterations to the invention as described may be made. For example, while a binary basis is shown as a component element of the superlattice, it should be appreciated that a single quantum well could be used as the basis for the superlattice. Similarly, a ternary basis, a quaternary basis, or any other basis may be used as the elemental basis for the superlattice. Additionally, while specific dimensions have been provided to better illustrate example embodiments of a QGIP spectrometer, many of these dimensions may be altered without adversely effecting the invention. Likewise, while the QGIP spectrometer of  FIG. 1B  is shown with ten QGIP elements, the QGIP spectrometer may have any number of QGIP elements so long as at least two QGIP elements have different wavelength absorption characteristics. All such changes, modifications, and alterations should therefore be seen as being within the scope of the disclosure.