Patent Publication Number: US-2020295075-A1

Title: Slot Antennas for Graphene Mid-IR Imaging Arrays as well an Approach for CMOS Implementation Thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims priority, under 35 U.S.C. § 119(e), to U.S. Application No. 62/792,548, filed Jan. 15, 2019, which is incorporated herein by reference in its entirety. 
    
    
     GOVERNMENT SUPPORT STATEMENT 
     This invention was made with Government support under Grant Nos. W911NF-17-1-0435 and W911NF-18-2-0048 awarded by the Army Research Office (ARO). The Government has certain rights in the invention. 
    
    
     BACKGROUND 
     Mid-infrared imaging has a wide range of applications, such as night vision, surveillance, remote chemical sensing, and medical diagnostics. However, no available mid-IR imagers simultaneously offer high sensitivity, high bandwidth and small size, with cooled detectors offering excellent performance at the cost of power, size and system complexity, while uncooled bolometers struggle with slow response times and low detectivity. Graphene is a promising alternative mid-IR sensitive optoelectronic material due to its broadband absorption, strong electrical response and wide process compatibility. However, its low absorption for normally incident light poses a challenge in designing high efficiency devices. Here, we propose to couple graphene with slot antennas, compact resonators which capture specific wavelengths of light and significantly enhance the free space to graphene light coupling efficiency. Additionally, since the antennas&#39; footprints are much smaller than their absorption cross sections, multiple antennas with different resonant frequencies can be placed in close proximity, allowing broadband and spectrally selective photodetection. 
     It is easy to take spectrally resolved imaging for granted in daily life. Our own eyes are spectral imagers, and this functionality allows to infer not just geometry, but also to some extent the composition of what we see. The same can be said not just for visible light, but also for the infrared and terahertz sections of the electromagnetic spectrum in which chemicals feature characteristic absorption resonances. In particular, infrared spectrally resolved imaging has been used for such varied applications as gas emission monitoring, ecological monitoring, agriculture and food quality control, automatic waste sorting, biological research, and oceanography while terahertz spectrally resolved imaging has been used for drug and other chemical detection as well as diagnostic analysis of human tissue. The variety of such applications has driven the development of the various spectral resolved imaging technologies, covering various wavelength ranges, which are commercially available today. 
     Spectrally resolved imaging technologies can be roughly sorted into the categories of scanning and snapshot spectral imaging. The ultimate goal of spectral imaging is to measure a spectral “data cube,” which represents light intensity as a function of two spatial dimensions and one spectral dimension. Scanning spectral imagers achieve this by sequentially measuring different portions of the data cube over time, and then combining the data from multiple exposures to form the full data cube. 
     One example of a scanning spectral imager is the push-broom scanner, in which light enters the device through a slit, passes through a prism or a grating that separates the light by wavelength and then images the resulting beam onto a focal plane array. This yields an array of spectra along one spatial dimension. The second spatial dimension is attained by physically scanning the entire device (or the imaged scene relative to the device), hence the name push-broom. Push-broom scanners are suitable for use on satellites or airplanes, where the imager is naturally moving, or for inspecting objects on a conveyor belt, where the scene is naturally moving. Another spectral imaging technique is to image a scene through a tunable filter, such as an etalon, a liquid crystal filter, or a Michelson-Morley interferometer, which essentially implements an imaging Fourier Transform Infrared (FTIR) spectrometer. While scanning spectral imagers have found their niches, they need either moving parts (which increases system complexity and failure rate) or linear relative motion between the camera and sensor, which imposes a limit on their application range. 
     Snapshot spectral imagers capture a data cube with a single exposure. Most snapshot spectral imagers operate either with absorptive spectral filters, dichroic filters, or dispersive optics to map the elements of a 3D data cube representing the input image onto one or a small number of focal plane arrays. A CMOS color camera may be considered as a filtering snapshot spectral imager in that each pixel is filtered (e.g., with a Bayer filter) to detect only red, green or blue light. Such systems incur a loss of throughput of 1/N, where N is the number of spectral channels. To compensate for this loss, they typically have proportionally larger apertures to collect more light. 
     In contrast, another category of spectral imagers uses optical systems containing dispersive elements or dichroic filters to break up an incoming light beam into multiple spectral channels or spatial cut-sections, each of which is imaged separately. The separate images are used to reconstruct the original scene. Although these spectral imagers don&#39;t suffer from the same light loss as filtering spectral imagers, splitting an optical beam into N different spectral channels invariably requires expanding the etendue of the optical system N-fold. Achieving this etendue typically involves reducing the input acceptance angle, as is the case with lenslet-based systems, or making the total focal plane array area considerably larger than the input aperture, as is the case with multispectral imagers based on dichroic beam splitters. 
     SUMMARY 
     Slot antenna elements and arrays of slot antenna elements with graphene patches over the slots can be used to detect mid-infrared light or Terahertz radiation. These devices can be made using CMOS fabrication techniques and integrated with CMOS electronics that read out the absorbed light as electrical signals. They feature faster response times than bolometers and add spectral selectivity while maintaining detectivities similar to those of more conventional room-temperature sensing elements. 
     An array of slot antennas can be part of an imaging system that detects infrared light at a first wavelength. Each slot antenna in the array of slot antennas includes a conductive material defining a three-dimensional slot resonant at the first wavelength; a graphene patch suspended over the three-dimensional slot in electrical communication with the conductive material; and a pair of electrical contacts in electrical communication with the graphene patch. The three-dimensional slot has a width, a length, and a depth, each of which can be less than the first wavelength. At least a portion of the conductive material may be formed as a mesh of conductive traces patterned in a complementary metal-oxide-semiconductor (CMOS) process. In operation, the conductive material couples the infrared light at the first wavelength into the graphene patch, and the electrical contacts sense a thermovoltage caused by absorption of the infrared light by the graphene patch. 
     The graphene patch may be offset with respect to the three-dimensional slot to introduce asymmetry into a Fermi level profile of the graphene patch. Alternatively, or in addition, there may be an electrolyte, a transparent conductor, or a dopant, in electrical communication with the graphene patch, to shift a Fermi level of the graphene patch. 
     Each slot antenna may be a first slot antenna and the array of slot antennas may be an array of first slot antennas, in which case the imaging system may also include an array of second slot antennas interleaved with the array of first slot antennas. This array of second slot antennas detects infrared light at a second wavelength different that the first wavelength. Each second slot antenna in the array of second slot antennas may include a conductive material defining a three-dimensional slot resonant at the second wavelength; a graphene patch suspended over the three-dimensional slot in electrical communication with the conductive material; and a pair of electrical contacts in electrical communication with the graphene patch. In operation, the conductive material couples infrared light at the second wavelength into the graphene patch, and the electrical contacts sense a thermovoltage caused by absorption of the infrared light at the second wavelength by the graphene patch. The arrays of first and second slot antennas can have pitches less than the first wavelength and less than the second wavelength. 
     An infrared imaging system with slot antennas can be made by forming an array of slot antennas, disposing a graphene patch over a three-dimensional slot in each slot antenna in the array of slot antennas, and coupling a pair of electrical contacts in electrical communication with each graphene patch. Forming the array of slot antennas may include patterning conductive vias to define the three-dimensional slots in a complementary metal-oxide-semiconductor (CMOS) back-end-of-line (BEOL) process. It can also include forming three-dimensional slots having different dimensions. The slot antennas can be formed by etching at least one three-dimensional slot in a conductive material or by etching at least one three-dimensional slot in a dielectric material and depositing a conductive material on the surface of the three-dimensional slot(s). And the electrical contacts can be coupled to the graphene patch by patterning metal via lithography to make electrical contact with the graphene patch. 
     A multi-spectral imaging system can sense mid-infrared light with a two-dimensional array of unit cells. The unit cell pitch is less than about 8 μm, with each unit cell in the array of unit cells comprising a plurality of different slot antennas. Each slot antenna in the plurality of different slot antennas is resonant at a different mid-infrared wavelength and has a graphene patch, disposed over a three-dimensional slot defined by a conductive material, to absorb light at that mid-infrared wavelength. Each slot antenna may have a resonance width of less than about 1 μm and an absorption cross section of about 4 μm 2  to about 40 μm 2 , with the mid-infrared light in a wavelength range of about 6 μm to about 12 μm 
     All combinations of the foregoing concepts and additional concepts discussed in greater detail below (provided such concepts are not mutually inconsistent) are part of the inventive subject matter disclosed herein. In particular, all combinations of claimed subject matter appearing at the end of this disclosure are part of the inventive subject matter disclosed herein. The terminology used herein that also may appear in any disclosure incorporated by reference should be accorded a meaning most consistent with the particular concepts disclosed herein. 
    
    
     
       BRIEF DESCRIPTIONS OF THE DRAWINGS 
       The skilled artisan will understand that the drawings primarily are for illustrative purposes and are not intended to limit the scope of the inventive subject matter described herein. The drawings are not necessarily to scale; in some instances, various aspects of the inventive subject matter disclosed herein may be shown exaggerated or enlarged in the drawings to facilitate an understanding of different features. In the drawings, like reference characters generally refer to like features (e.g., functionally similar and/or structurally similar elements). 
         FIGS. 1A and 1B  show orthographic and top views, respectively, of an individual slot antenna coupled to a graphene patch. 
         FIGS. 1C and 1D  shows cross-sectional and orthographic views, respectively, of a slot antenna with a graphene patch over a narrowed entrance slit to a three-dimensional cavity. 
         FIGS. 1E and 1F  show antennas with graphene patches and nonzero magnetic polarizabilities. 
         FIG. 1G  shows a slot antenna with a slit that electrical isolates the two sides of the cavity. 
         FIG. 1H  shows a cross section of slot antenna coupled to a HgCdTe patch. 
         FIG. 1I  shows a cross section of slot antenna coupled to a VO x  patch, e.g., for use as a heat sensing element in a bolometer. 
         FIGS. 2A-2D  show “flat” antenna shapes for coupling incoming light to graphene patches. 
         FIG. 3  shows a monochromatic array of slot antennas coupled to a graphene patch. 
         FIG. 4  shows a multispectral array of slot antennas coupled to a graphene patch. 
         FIGS. 5A-5D  show different views of a 6-channel multispectral array of slot antennas coupled to a graphene patch. 
         FIG. 5E  is a plot of the simulated absorption spectra for a 6-antenna unit cell like the one shown in  FIGS. 5A-5D . 
         FIG. 6  shows an 8-channel multispectral array of slot antennas coupled to graphene patches. 
         FIG. 7  shows a multispectral array of slot antennas coupled to graphene patches and arranged to detect light of different polarizations. 
         FIG. 8  shows planar antennas with a refractive index contrast to suppress coupling in one direction. 
         FIG. 9A  shows a planar antenna between high- and low-index layers with a Schottky diode between its conductive elements for detecting Terahertz radiation. 
         FIG. 9B  shows a slot antenna between high- and low-index layers with a Schottky diode between its conductive elements for detecting Terahertz radiation. 
         FIGS. 10A-10F  show different approaches for creating an asymmetric Fermi level profile in graphene used to produce a photovoltage. 
         FIG. 11  shows a complementary metal-oxide-semiconductor (CMOS) back-end-of-line (BEOL)-compatible slot antenna with spaces between the vias. 
         FIG. 12  illustrates a modified CMOS BEOL antenna fabrication process. 
         FIG. 13  illustrates a process for contacting graphene to a slot antenna. 
         FIG. 14  illustrates an alternative process for making a slot antenna suitable for coupling to a graphene patch. 
     
    
    
     DETAILED DESCRIPTION 
     Using an array of differently-tuned slot antennas coupled to respective graphene patches can allow spectrally sensitive and efficient photodetection in the mid-infrared (IR) region of the electromagnetic spectrum thanks (1) the coupling efficiency between a slot antenna to a 2-dimensional (2D) material and (2) the small footprint of the slot antenna with respect to its absorption cross-section. Unlike other spectrally sensitive detectors, this array does not rely on color filter arrays which achieve spectral selectivity by filtering out and therefore wasting light. Instead, it uses the spectral selectivity and relatively large absorption cross section of each slot antenna to detect mid-IR light efficiently. 
     An inventive multispectral antenna array can be used for a wide range of mid- and thermal-IR imaging applications. (It can also be scaled to work at other wavelength ranges, including the near-IR and visible wavelength ranges.) The most intuitive of these imaging applications may be thermal imaging, encompassing night vision and surveillance, since objects near room temperature emit blackbody radiation around 10 μm with a brightness which varies as the fourth power of temperature. The standard technology for small, low-power or portable such systems is VO x  or Si bolometry, a mature technology which is sensitive enough to discern temperature differences of less than 0.1 K. 
     An inventive multispectral antenna array offers at least three advantages over a conventional bolometer. First, unlike a conventional bolometer, an inventive multispectral antenna array can distinguish different wavelengths. This ability to spectrally distinguish light could be used to identify the temperature and composition information of objects in view. Or, in cases where the user can engineer the objects in view, a spectral marker could be incorporated onto objects whose temperature is of especial concern. Being able to see not just the total blackbody brightness but the spectrum as well allows accurate temperature measurement independent of changes in emissivity. Second, since the photothermoelectric effect is extremely fast, a multispectral antenna array has enough bandwidth to be used for artificial intelligence applications, such as robotics, where fast refresh rates and low latency are critical for real-time decision making. High bandwidth also allows chopping to eliminate flicker noise. Third, an inventive multispectral imaging array doesn&#39;t need the sophisticated and expensive vacuum packaging required by a bolometer. 
     Other applications for include remote chemical sensing, which utilize the absorption peaks of various gases (typically in the mid-IR) to “see” plumes of gas against a background of blackbody radiation, and medical diagnostics. For instance, a mid-IR multispectral antenna array could detect melanomas based on their signature in the mid-IR wavelength range. The possibility of producing mid-IR cameras with spectral resolution at low costs opens up the potential market of home health scanners. 
     Inventive antenna elements can be characterized by a number of different parameters, including:
         Solid angle of emission Ω G : The spherical integral of an antenna or antenna array&#39;s radiation intensity divided by the maximum intensity among all radiation directions. For a point dipole, Ω G =8π/3.   Absorption cross section A A : Power absorbed by an antenna/load system divided by incoming light intensity, for light incident in the direction and polarization with the highest absorption. For slot antennas, this direction is normal to the substrate, and polarized perpendicular to the long side of the slot. (For the slot antenna element  100  in  FIGS. 1A and 1B , the load is the graphene patch  120 .)   Active cross section A L : Power absorbed by the load in an antenna/load system divided by incoming light intensity, for light incident in the direction and polarization with the highest absorption. In the absence of ohmic loss in the antenna element, A L =A A .   Geometric cross section (also known as effective aperture) A G : A L  for an antenna design assuming no ohmic loss (thus =A A ), in the case where the load is conjugate impedance matched to the antenna; that is, for which the load impedance is chosen to maximize such cross-section. A G =λ 2 /Ω G , where λ is the wavelength of the incident radiation. More generally, the geometric cross section represents the antenna&#39;s absorption cross section without any ohmic loss and a load impedance selected to give conjugate impedance matching. The geometric cross section captures only those properties of the antenna which arise due to the antenna&#39;s shape, which is intricately related to the antenna&#39;s radiation pattern.   Antenna receiving efficiency η L : the ratio of the active cross section to the geometric cross section, η L =A L /A G . An antenna which meets the criteria in the definition of A G  has η L =1. Realistic antennas have ohmic loss and imperfect impedance matching and thus η L &lt;1.       

     Note the following subtlety in these terms: The antenna impedance, and hence conjugate matched load impedance, is wavelength-dependent, but in the definition for A G , the load is assumed to be conjugate matched for each wavelength. Hence A G  is independent of the load, and as such it can be related to the load-independent solid angle of emission, Ω G . In contrast, the absorption cross section A A , active cross section A L , and antenna receiving efficiency η L  are defined with fixed loads, so they feature resonances as a function of wavelength. The wavelength of an antenna element&#39;s first-order resonance is called its resonant wavelength, λ r , and is the largest wavelength that maximizes the absorption cross section A A . This active cross section A L  and antenna receiving efficiency η L  also depend on wavelength and are also at or near their maxima at the resonant wavelength, λ r . 
     Three-Dimensional Antenna Elements with 2D Material Patches 
       FIGS. 1A and 1B  show orthographic and top views, respectively, of a slot antenna element  100  that can detect mid-IR light (e.g., light with a wavelength range of 6-12 μm, including ranges of 6-10 μm or 7-12 μm). The slot antenna element  100  is formed of a conductive material  110 , such as metal or a heavily doped semiconductor, that defines a three-dimensional (3D) slot or cavity  130  fabricated on a semiconductor or dielectric substrate (e.g., a silicon substrate; not shown). 
     The conductive material  110  extends over at least a portion of the cavity&#39;s surface and has a thickness equal to about 3-4 times its skin depth at the slot antenna element&#39;s operating wavelength to prevent light leakage. This cavity  130  has a width w, a length l, and a depth d, each of which can be varied to adjust the spectral sensitivity of the slot antenna element  100 . For example, the cavity may be about 400 nm wide, about 3 μm to about 8 μm long, and about 1.5 μm to about 3 μm deep for an operating wavelength of about 6 μm to about 10 μm. Similarly, the shapes of the cavity  130  and conductive material  110  may be selected based on the antenna element&#39;s desired receptivity pattern. If desired, the cavity  130  may be partially or completely filled with a dielectric material  140  that is transparent at the slot antenna element&#39;s operating wavelength. 
     The cavity&#39;s width and height define an aperture  132  that is surrounded by an apron  116  of conductive material  110 . The aperture  132  in  FIGS. 1A and 1D  is rectangular, but other aperture shapes are also possible, including square, polygonal, circular, elliptical, crosses, and irregular shapes. A patch  120  of monolayer or bilayer graphene, HgCdTe, vanadium oxide (VO x ), black phosphorus (e.g., for sensing light at wavelengths near 3 μm), transition metal chalcogenides (e.g., for sensing near-infrared or visible light), colloidal quantum dots, thin semiconductor quantum well structure, or another 2D material on a transferred dielectric  1020  is suspended across the aperture  132  and over the cavity  130 . More generally, the patch  120  should include material that is sensitive in the desired wavelength range where the absorptive region is localized to the near field of the antenna  100 . The patch  120  in  FIGS. 1A and 1B  is rectangular and has a width that is greater than the width of the cavity  130  and aperture  132  and less than the width of the apron  116 . The graphene patch&#39;s length is less than the length of the cavity  130  and aperture  132 . Electrical contacts  1002  contact the patch  120  close to the optical absorption region for measuring changes in the thermovoltage across the patch  120  caused by absorption of incident light. 
     In operation, the slot antenna element  100  couples mid-infrared light into the graphene patch  120 . At mid-infrared wavelengths, the graphene patch  120  can be modeled as an infinitely thin sheet with a sheet resistance of at least 16.4 kΩ/□. This high resistance makes conjugate matching difficult, and antenna design in this case benefits from care. 
     Simulations of the slot antenna element  100  in  FIGS. 1A and 1B  show an antenna receiving efficiency at the resonant wavelength η Lr  up to 0.35 at λ r =9.2 μm for slots with w=400 nm, l=6.5 μm, and d=2.25 μm. Other simulations show an antenna receiving efficiency of η Lr =0.48 at λ r =6 μm for slots with w=240 nm and d=1.5 μm using slot antennas like those in  FIGS. 1C and 1D  with the perforations shown in  FIG. 11  (discussed below). Generally, simulations show that slot antennas with resonant wavelengths from 4-8 μm can have very high efficiencies of 0.45-0.55 for input apertures between 100-200 nm wide and a few 100 nm long that expand out to internal cavities with widths of 400-600 nm, lengths greater than 4 μm (e.g., up to 5.25 μm long), and depths of 1-2 μm, with shallower cavities favoring shorter resonant wavelengths. 
     For gold slot antennas loaded with graphene patches operating in the mid-IR, the absorption cross section is on the order of 10 μm 2 , although this scales with λ 2  and may vary over even an octave. The ratio of the slot antenna element&#39;s active cross section to its absorption cross section A L /A A  (that is, the ratio of absorbed light which is absorbed in the graphene) can be in the range of 0.4-0.6, which indicates the slot geometry does not incur additional ohmic loss compared to planar dipole antennas. This demonstrates that good antenna efficiency can be achieved using a slot antenna design, and without pushing lithographic limits. 
     The slot antenna&#39;s “built-in” ground plane is another fundamental advantage. As described below, an array of closely spaced, lossless, conjugate-matched slot antenna elements can fully absorb a properly polarized normally incident beam of light. The same cannot be said for an array of planar antennas because they lack a ground plane. In this latter case, a reflector can be buried a distance of λ/4 (a so-called “Salisbury screen”), but due to the wavelength specificity this is not a broadband solution. 
     Put differently, in a broadband antenna metasurface detector with planar antennas, the planar antennas emit light both up and down. By electromagnetic reciprocity, the planar antennas cannot absorb 100% of a beam of light coming in from one direction. One way around this problem is to have a much higher refractive index in the desired incident light direction (e.g., down), as shown in  FIG. 8  (described below). Another way to increase absorption is to put a ground plane about a quarter wavelength below the antennas. Based on the theory of dielectric thin films, this causes the metasurface to “see” an open circuit below it, so that it only interacts with the space in above it. Because the slot antenna has a built-in ground plane, its radiation pattern is unidirectional as a result of its geometry, so reciprocity does not impede it ability to absorb 100% of a beam incident in one direction. 
       FIGS. 1C-1I  show variations of the basic slot antenna design.  FIGS. 1C and 1D  shows cross-sectional and orthographic views, respectively, of a slot antenna  100   c  with a cavity  130   c  having a narrowed entrance slit/aperture  132   c . (This type of slot antenna is also called a cavity antenna.) The narrowed entrance slit  118   c  concentrates electric fields near the graphene patch  120 , while the wider cavity  130   c  below the entrance slit  112   c  reduces ohmic loss in the metal or other conductive material  110   c  that defines the cavity  130   c  and entrance slit  118   c.    
       FIGS. 1E and 1F  show slot antennas  100   e  and  100   f , respectively, with non-zero magnetic polarizabilities. The slot antenna  100   e  in  FIG. 1E  has conductive material  110   e  formed roughly in a “C” shape, with a graphene patch  120   e  suspended across the gap in the “C” from the two arms of the “C.” Similarly, the slot antenna  100   f  in  FIG. 1F  has conductive material  110   f  formed in mirrored “C” shapes, with gaps between the “C” shapes. One “C” shape is connected to a source electrode  114   f , and the other is connected to drain electrode  116   f . A graphene patch  120   f  bridges the top gap. When these slot antennas  100   e  and  100   f  are suspended in a dielectric, they can be engineered to absorb an entire incident plane wave with no radiation in the −z direction. 
       FIG. 1G  shows an orthographic view of a slot antenna  100   g  with a slit that electrically isolates conductive material  110   g  on opposite sides of the cavity  130   g . This slot antenna  100   g  supports a resonance, which may be shifted due to the capacitance across the slit. Electrical isolation between the two sides of the antenna  100   g  allows the graphene patch  120   g  to contact source and drain electrodes  114   g ,  116   g  or gating electrodes that are separated from the graphene patch  120   g  by a thin layer of dielectric material (not shown). 
       FIG. 1H  shows a cross section of a slot antenna  100   h  with an epitaxial HgCdTe patch  120   h  instead of a graphene patch for sensing mid-IR radiation. The HgCdTe patch  120   h  is bonded to the slot antenna&#39;s conductive material  110   h  with one or more metal bonds  150   h . These metal bonds  150   h  extend through a passivation layer  126   h  on the underside of the HgCdTe patch  120   h . A thin region  122   h  of the HgCdTe patch  120   h  bordering the metal bonds  150   h  is n-doped and the rest of the HgCdTe patch  120   h  is p-doped to form a pn junction. And a transparent substrate  128   h  on the top of the HgCdTe patch  120   h  transmits mid-IR radiation and protects the HgCdTe patch  120   h . The HgCdTe patch  120   h  can be replaced with an InSb, InGaAs, or Si patch for detecting light at other wavelengths, including near-IR wavelengths. 
       FIG. 1I  shows a cross section of a slot antenna  100   i  with a thermal bolometer element, such as a VO x  or amorphous silicon (a-Si) patch  120   i , suspended over the cavity  130   i . A pair of conductive contacts  150   i  and  152   i  support the VO x  or a-Si patch  120   i  above the cavity  130   i . One contact  150   i  connects the VO x  or a-Si patch  120   i  to the conductive material  110   i  that forms the antenna cavity  130   i , and the other contact  152   i  connects the VO x  or a-Si patch  120   i  to another metal via  114   i . This metal via  114   i  and the conductive material  110   i  serve as electrodes that measure changes in the resistance of the VO x  or a-Si patch  120   i  caused by absorption of incident infrared radiation. 
       FIGS. 2A-2D  show different views of “flat” antennas  200   a - 200   c  (collectively, antennas  200 ) that couple light to patches of graphene or other 2D materials:  FIG. 2A  shows a top view of a dipole antenna  200   a  with a graphene patch  220   a  bridging a gap between two rectangular metal segments  210   a ;  FIG. 2B  shows top view of a bowtie antenna  200   b  with a graphene patch  220   b  bridging a gap between two trapezoidal metal segments  210   b ;  FIG. 2C  shows a synapse antenna  200   c  with a graphene patch  220   c  in a gap between two T-shaped metal segments  210   c ; and  FIG. 2D  shows a side view of all the antennas  200  with the metal  210  and graphene on a transparent substrate  230  (omitted from  FIGS. 2A-2C  for clarity). 
     In each antenna  200 , the metal  210  is patterned in a thin layer (e.g., roughly 50 nm to 100 nm tall) on the transparent substrate  230 . Adopting reasonable fabrication resolution constraints (metal spacing&gt;100 nm) gives an antenna receiving efficiency at the resonant wavelength η Lr  of about 0.12 at a resonant wavelength λ r =6.8 μm assuming intrinsically doped graphene  220  with no thermal Pauli blocking (that is to say, the ideal R □ =16.4 kΩ/□ case). A higher antenna receiving efficiency η Lr  favors narrower, shorter (in the vertical direction) antennas  200  with narrow gaps between the two arms. Of the designs shown in  FIGS. 2A-2C , the synapse antenna  200   c  has the lowest lateral aspect ratio of the graphene patch  220   c . This lower lateral aspect ratio means that the graphene patch  220   c  presents a lower impedance to the antenna  200   c , improving the antenna receiving efficiency at the resonant wavelength η Lr . 
     Monochromatic Antenna Arrays 
       FIG. 3  shows a monochromatic antenna array  300  composed of slot antenna elements  100  arrayed on a square lattice. The slot antennas are monolithically integrated with CMOS readout circuitry, so the array is formed on the silicon substrate out of metal in back-end-of-the-line CMOS fabrication steps. Alternatively, the substrate can be a dielectric and the logic (circuitry) can be implemented in graphene or carbon nanotube transistors. The device can also be fabricated “upside down”, starting with a mid-IR transparent substrate followed by the sensitive element (e.g., graphene), then the slots or some other antenna structure, then flip-bonded to another chip with the digital logic. 
     The slot antenna elements  100  are identical—they have slots  130  ( FIGS. 1A and 1B ) with identical dimensions—so their resonance wavelengths are the same. The antenna pitch on the square lattice is equal to or less than the resonance wavelength, e.g., half the resonance wavelength. Each slot antenna element  100  has its own graphene patch  120  and can be conjugate-matched to a corresponding antenna, which may have an impedance of 500-1000 ohms on resonance. 
     Under certain conditions, a 2D lattice of antennas like the monochromatic antenna array  300  in  FIG. 3  can fully absorb an incoming beam of light. This can be intuited in the context of reciprocity by imagining that each slot antenna radiates light instead of receiving light. Imagine that each slot is not terminated in a short (a cavity), but rather fed with electromagnetic waves from the inside in such a way that each antenna&#39;s theoretical “source” deep down inside the structure sees a conjugate-matched load. This is the opposite of actual operation, where the antennas are absorbing light, but yields insight about the antenna&#39;s receptivity pattern from its emission (diffraction) pattern. 
     More specifically, consider a 2D lattice of lossless antennas fed with impedance transformers to conjugate-match the antennas with 50-ohm feeds (here, the impedance of the feeds is arbitrary and can be matched to the antennas&#39; radiation impedance). The antennas in the array are fed with a signal of amplitude exp(i   xy · ), where    xy  is the desired lateral wavevector component of the radiated beam and   is the antenna position. The amplitude is the amplitude of the field emitted from each antenna as a function of its position. If |   xy + |&gt;2π/λ for all reciprocal lattice vectors   of the antenna array except for  =0, then the antenna radiation has no higher-order diffraction modes—that is, the radiated light forms a plane wave. Since an array of slot antennas emits in one half-plane, feeding an array of slot antennas with a signal of amplitude exp(i   xy · ) forms a single uniform beam. By Lorentz reciprocity, if such a beam is redirected back at this same antenna array, the light should be funneled losslessly and without reflection into the 50-ohm feeds. Therefore, an antenna lattice&#39;s reciprocal lattice basis vectors    a  and    b  define a set of incoming light directions    xy  ∈ K which experience perfect absorption. 
     For a square lattice of antennas with spacing λ, K includes the point    xy =0. For a square lattice with spacing λ/2, e.g., as shown in  FIG. 3 , K includes all    xy  with |   xy |≤2π/λ; that is, all possible plane waves of wavelength λ. This illustrates that if slot antennas are small enough to accommodate close spacing, are lossless, and have conjugate-matched loads, they can absorb an entire incoming light beam, neglecting coupling between adjacent antennas. Planar antennas cannot achieve such perfect absorption without a back-reflector because they also radiate into the substrate when used as emitters. In practice, the loads may not be perfectly matched to the slot antennas and there may be ohmic loss. In this case, the proportion of beam energy absorbed in the slot antenna array&#39;s loads is the antenna receiving efficiency η L . The remaining energy is reflected or converted to heat. 
     Polychromatic/Multispectral Antenna Arrays 
     Depending on its footprint, there may considerable empty space in a monochromatic antenna array with K covering the desired acceptance angle. The remaining space can be used to superimpose additional monochromatic antenna arrays with different resonant wavelengths. The resulting superimposed monochromatic antenna arrays from a polychromatic or multispectral antenna array. Care should be taken for the arrangement of the antennas in both space and frequency-space. The antennas should not be placed tightly enough to interfere with their near-field current profiles, nor should the antennas&#39; resonances overlap considerably (e.g., the overlap point for neighboring antennas should less than or equal to half the resonance amplitude). Antennas with partially overlapping absorption spectra may be spatially separated by about one wave-radian (λ/2π), as this represents the spatial extent of the near-field. Antennas without overlapping absorption spectra can be packed more tightly that one wave-radian without shifting the antennas&#39; resonances. 
     If the antennas&#39; resonances overlap too much, light absorption at a given wavelength is no longer dominated by one or two monochromatic sub-arrays. Similarly, if the antenna pitch is too small, the received light spreads out among multiple sub-arrays, wasting physical space that could otherwise be used to extend the antenna array&#39;s total wavelengths range. 
       FIG. 4  shows a four-channel multispectral antenna array  400  composed of square unit cells  402 , each of which includes four slot antenna elements  100 - 1 ,  100 - 2 ,  100 - 3 , and  100 - 4  (collectively, slot antennas elements  100 ) like those shown in  FIGS. 1A and 1B . The antenna elements  100  have different slot dimensions and hence different resonance wavelengths. In this case, the resonance wavelengths span a band in the mid-IR region of the electromagnetic spectrum (e.g., 6-12 μm). 
     The square unit cells  402  are arranged on square lattice with a pitch of about λ r,max /2, where λ r,max  is the resonance wavelength of the antenna element  100  with the longest resonance wavelength. (Practically, the pitch may deviate from λ r,max /2, e.g., due to fabrication imperfections or space constraints, with any deviation having little to no effect on antenna performance depending on the acceptance angle of the overall optical system.) From a different perspective, the multispectral antenna array  400  can be thought of as four interleaved monochromatic antenna arrays (i.e., one for each type of antenna element  100 - 1  through  100 - 4 ), each of which is on square lattice with a pitch of λ r,max /2, where the square lattices are offset from each by λ r,max /4 in the x, y, or x and y directions. 
       FIGS. 5A-5E  illustrate a six-channel multispectral antenna array  500  with six different types of slot antenna elements  100 - 1  through  100 - 6  arranged on a rectangular lattice.  FIGS. 5A-5D  show different views of the multispectral antenna array  500 , which has a unit cell  502  that repeats in the place of the array  500 . The slots  130  in the slot antenna elements  100  have the same width (about 0.4 μm) but different lengths, with slot antenna element  100 - 1  having the shortest slot length (about 3.4 μm) and slot antenna element  100 - 6  having the longest slot length (about 7.4 μm). The antenna pitch is about 7 μm in the widthwise direction and about 12.666 μm in the lengthwise dimensions. 
     The slot lengths set the slot antenna elements&#39; resonance wavelengths, so the slot antenna element  100 - 6  with the longest slot has the longest resonance wavelengths as shown in  FIG. 5E , which is a plot of the resonances for each antenna element  100 - 1  through  100 - 6  and for the entire array  500 . The resonances have roughly equal full-width half-maxima (FWHM) (about 1 μm each) and together span a wavelength range of about 6 μm to about 10 μm, with adjacent resonances separated by about 0.8 μm. The slot antenna elements  100  are arranged so that slot antenna elements with adjacent resonances are not next to each other in the unit cell  502 . In  FIG. 5A , the slot antenna elements are arranged linearly in the following repeating pattern:  100 - 1 ,  100 - 5 ,  100 - 3 ,  100 - 6 ,  100 - 2 ,  100 - 4 ,  100 - 1 ,  100 - 5 ,  100 - 3 ,  100 - 6 ,  100 - 2 ,  100 - 4 , and so on. Spatially separating antenna elements  100  with adjacent resonances increases the probability that incoming light will be absorbed by the antenna element most sensitive at that wavelength. 
     Other multispectral antenna arrays are also possible. For example, a multispectral antenna array may have more slot antenna elements, e.g., as in  FIG. 6 , which shows one unit cell  602  of a multispectral antenna array with eight different slot antenna elements, each of which has a different slot length and a different resonance wavelength. (The slot antenna elements may have different depths and widths as well.) A multispectral antenna array may have different arrangements of antenna elements, including sparse arrays and non-rectilinear arrays, as well as different types of antenna elements, including any of those shown in  FIG. 1C-1I or 2A-2D . A multispectral antenna array can also have slot antenna elements with different loads (e.g., different combinations of graphene, HgCdTe, or a-Si patches) for sensing light at different wavelengths. 
       FIG. 7  shows a multispectral antenna array  700  with rotated slot antenna elements  100 - 1  through  100 - 4  to sense light projected into orthogonal (e.g., vertical and horizontal) polarizations. In this multispectral antenna array  700 , each unit cell  702  includes four slot antenna elements  100 - 1  through  100 - 4  aligned with the long axes of the slots parallel to each other. Each unit cell  702  is rotated 90° with respect to its neighboring unit cells  702 . Since the slots&#39; orientations determines the slot antenna elements&#39; polarization sensitivity, arranging slots to be perpendicular to each other by rotating half the unit cells  702  enables the array  700  to sense both vertically and horizontally polarized light. 
     The multispectral antenna array  700  is sensitive to light in every polarization state, including unpolarized light. Its sensitivity depends on how tightly the antenna elements  100  are packed together. If the unit cells  702  are arrayed at a pitch of half a wavelength, then rotating the antenna elements  100  with respect to each other shouldn&#39;t affect the array&#39;s sensitivity. In practice, fabrication and loss constraints may limit how closely the antennas are spaced. 
     Planar Antenna Arrays 
       FIG. 8  shows an imager  800  with an array  810  of planar antennas  200  like those shown in  FIGS. 2A-2D . The planar antennas  200  may be tuned to the same resonance wavelength for sensing monochromatic radiation or to different resonance wavelengths for sensing polychromatic radiation. They may be oriented in one direction or in different directions depending on the desired polarization sensitivity. 
     In any case, the planar antenna array  810  is formed on a low-index substrate  820  and covered with or encapsulated by a higher-index material  830 , such as germanium. If desired, the high-index material  830  may be curved or patterned to form a solid immersion lens (SIL) that focuses incident mid-IR light on the planar antenna array  810 . The index contrast between the low-index substrate  820  and the high-index SIL  830  reduces bidirectional coupling (i.e., coupling in the ±z directions) that affects many planar antennas by suppressing the interaction between the antennas  200  and the far field in the −z direction. 
     The imager  800  may also include a λ/4 back-reflector  840  to enhance absorption in the antennas  200 . The electrical contacts to the antennas  200  are channeled through the low-index dielectric  820  and back-reflector  840 . 
     Terahertz Antenna Arrays 
       FIGS. 9A and 9B  illustrate Terahertz antenna arrays like those described above.  FIG. 9A  shows a THz imager  900  similar to the mid-IR imager  800  shown in  FIG. 8 . The THz imager  900  includes a 2D array  910  of THz antenna elements  912  integrated with a low-index plastic substrate  920  under a high-index SIL  930 . Instead of graphene as the sensing element, each antenna element  912  includes a Schottky diode for detecting radiation at frequencies near 1.5 THz. 
       FIG. 9B  shows a THz version  950  of the slot antenna with a slit shown in  FIG. 1G . The THz version has dimensions on the order of 100 μm to 200 μm and has an aperture  966  formed in a metal layer  962 . A detecting element  964 , such as a Schottky diode, tunnel diode, or graphene FET or HEMT, is suspended above the aperture  966  from the metal layer  962 . For graphene FET or HEMT, the antenna is wired to the source and gate and a drain terminal is added, with the drain current representing the detected power. A cage of metal vias  968  in a plastic substrate  970  under the metal layer  962  defines a cavity that contains the resonant radiation and isolates the two sides of the metal plate  962  from each other. The cavity may be filled with air or plastic  980 , which may also extend over the metal plate  962 , detecting element  964 , and aperture  966 . 
     In the Terahertz range, the larger length scales may lead to fabrication paradigms more akin to printed circuit board (PCB) manufacturing than to CMOS manufacturing. Typical transparent materials in the Terahertz include certain polymers, such as TPX plastic, as well as silicon and sapphire. For the &lt;1.5 THz range, Schottky diodes are a viable detection device as shown in  FIGS. 9A and 9B . 
     Reading Out Absorbed Light from a Graphene as an Electrical Signal 
     As mentioned briefly above, the conductive material in each slot antenna couples light at the slot&#39;s resonant wavelength into the graphene patch suspended above the slot. One approach for reading out light absorbed in the graphene patch is via the photothermoelectric effect. This involves engineering a Fermi level E F  step in the graphene patch such that the Seebeck coefficient differs between the two sides of the step. Absorbed light heats the electron gas in the graphene patch, which then produces a thermovoltage due to the Seebeck coefficient difference. This effect is particularly strong in graphene patch due to graphene&#39; s extremely low electronic heat capacity and its long electron-phonon scattering times. 
     There are at least three ways to manipulate the Fermi level E F  of a graphene patch in a slot antenna element. First, Electrons can be pulled into or pushed out of graphene using a nearby electrostatic gate, similar to in a MOSFET. In fact, such graphene devices are commonly referred to as “FETs” regardless of whether the device is intended as an electrical switch or not. Second, an ionic electrolyte may be deposited onto graphene, which sets up an electric double layer on the graphene surface which may be charged or discharged using a remote anode. Third, chemical dopants can be introduced onto a graphene surface, providing electrons or holes. 
     The first two approaches are used in graphene device demonstrations, perhaps because they allow active tuning of E F . The first approach allows fast E F  modulation, while the second approach is usually limited to modulation rates on the order of Hertz. The electrolyte is encapsulated to prevent deterioration in air. However, it is convenient to implement, as it does not require fabrication of additional conductor and dielectric layers. 
       FIGS. 10A-10F  show cross sections of slot antenna elements with E F  step “doped” into the graphene patches using one or more of the three methods above.  FIG. 10A  shows a first approach  1000   a  for doping the graphene patch using the antenna structure itself. This permits a working device with fewer fabrication steps. If a voltage is applied between the slot  130  and the graphene patch  120  through a dielectric  1010  and metal contacts  1002 , the regions of the graphene patch  120  directly above metal or doped semiconductor will experience the strongest Fermi level E F  tuning effect. In  FIG. 10A , the graphene patch  120  and a supporting dielectric  1010 , such as hexagonal boron nitride (hBN), are suspended over the slot  130 . Here, the patterned graphene/hBN stack is prepared on another wafer and transferred onto the slot  130 , with the graphene patch  120  offset laterally with respect to the slot  130  in order to produce the asymmetric Fermi level profile for a net photovoltage. The graphene patch  120  may or may not also be encapsulated by more hBN for stability (not illustrated). 
     In the slot antenna  1000   b  in  FIG. 10B , the slot  130  is filled with a relatively mid-IR-transparent dielectric  1012 , and the graphene patch  120  is transferred on top of the dielectric  1012  and offset laterally with respect to the slot  130 . This case is more suitable when the slot antenna wafer has topography which precludes transferring a whole graphene/dielectric stack, as techniques for conformal graphene transfer exist and may be utilized here. Again, the graphene patch  120  is offset with respect to the slot  130  in order to produce the asymmetric E F  profile for a net photovoltage. 
     The slot antennas  1000   a  and  1000   b  in  FIGS. 10A and 10B  feature two nonidealities. First, the graphene patch&#39;s Fermi level E F  is only tunable on one side of each device. This can be remedied by using an electrolyte  1020  to dope the whole graphene patch  120 , as in the slot antenna  1000   c  in  FIG. 10C  for the suspended stack case and in the slot antenna  1000   d  in  FIG. 10D  for the filled-slot case. However, the electrolyte  1020  should be encapsulated to prevent deterioration and may absorb mid-IR light in some wavelength bands, so using an electrolyte  1020  is not without disadvantages. Second, the electric field of the antenna mode is not centered on the Fermi level E F  step. As a result, the graphene patch&#39;s electronic temperature at the E F  step is not as high as if the region of absorbed light were centered, and thus the photovoltage is lower. An alternative view of the problem is that the absorbed light region is close to the left-hand ohmic contact  1002 , so the resulting electronic thermal energy is lost more readily by diffusion to the contact. These problems are addressed in slot antennas  1000   e  and  1000   f  of  FIGS. 10E and 10F , respectively, where a very thin transparent conducting layer  1030  is placed underneath one or both sides of the graphene patch  120  as a gate. In  FIG. 10F , there is a back gate on only one side of the graphene patch  120  in case the narrow gap between back gates in  FIG. 10E  poses trouble for lithography, in which case an electrolyte  1020  may be used to restore two-sided control over the Fermi level E F . 
     Fabricating Slot Antennas in CMOS Back-End-of-the-Line (BEOL) Wiring 
       FIG. 11  illustrates a slot antenna made in the copper wiring layers of a CMOS process. Care is taken to prevent oxidization of exposed copper and that the interlayer dielectric (which typically has strong absorption bands in the thermal IR) is removed from inside the slot. Since CMOS back-end-of-line (BEOL) wiring processes include metal patterning layers alternating with via layers, which could include vias of a fixed size, the conductive material that defines the slot takes the form of a mesh. If the perforations or holes in the mesh have cutoff wavelengths below the slot antenna&#39;s operating wavelength, they do not leak light, so the slot antenna may support a resonant mode. 
     One advantage of the design in  FIG. 11  is leveraging existing fabrication technology. However, there are few downsides. First, the highest metal layers in a CMOS process typically use masks with a coarser resolution. To achieve a finer resolution, the slot may be buried in lower metal layers to achieve the desired slot width (in the range of 100 nm to 200 nm). Realistically speaking, the first few BEOL layers may be removed from the image-sensing region of each die, introducing considerable topography. Second, since the vias act as a current bottleneck for the antenna mode, there is more loss than for a slot defined by a mesh than for a slot defined by an unbroken or continuous conductive layer. 
     Modified CMOS BEOL Fabrication 
       FIG. 12  illustrates a slot antenna  1200  made using a modified version of a standard BEOL copper metallization process. The slot antenna  1200  is next to a copper electrical interconnect  1220  in an antenna layer  1210  above a normal CMOS layer  1212 . A passivation layer  1222  separates the antenna layer  1210  from the normal CMOS layer  1212 , which includes patterned copper  1226  in a low-K dielectric  1224 . 
     A standard BEOL copper metallization process is a dual damascene process where the via and metal patterns are etched in sequence ( 1202 ), followed by conformal deposition of a thin diffusion barrier ( 1208 ), copper plating ( 1204 ), and chemical-mechanical polishing (CMP) ( 1206 ). This standard process is modified to fabricate an unbroken conductive layer around a slot within this framework. The slot is a long, deep rectangular “via” with a slit down the middle for the electromagnetic mode to occupy. Due to the high aspect ratio, achieving a good sidewall angle for the dielectric etch and cavity-less plating of the copper can be difficult. Strict design rules for the antenna layout may be used to ensure process reliability. After the plating and CMP, the dielectric inside the metal slot is etched away. 
     Electrical Interfaces between Graphene Devices and CMOS Interconnects 
       FIG. 13  illustrates a process for suspending a patch  1320  of 2D material, such as graphene, HgCdTe, VO x , or a-Si, above the slot of the slot antenna  1200  of  FIG. 12 . In practice, the patch  1320  may be covered or encapsulated in a dielectric  1322 , e.g., a Van der Waals solid such as hBN, to safeguard it from contamination. Taking note of this, the process  1300  shown in  FIG. 13  produces an edge contact to the graphene layer in the form of a metal plug  1340 . Here, a layer of photoresist  1310  is used to mask the graphene contact plug etch which penetrates through the dielectric  1322 , graphene patch  1320 , and copper passivation layer. The same resist layer  1310  is then used as a plating mold for the graphene contact metal  1340 . The remaining metal and resist can then be etched away. 
     Subtractive Fabrication 
     Various approaches for fabricating the slots fall into the category of “subtractive fabrication” where the slots are either etched directly into a layer of conductive material or into an easy-to-etch substrate after which the conductor is conformally deposited (e.g., via atomic layer deposition (ALD)). In the former approach, the material should be amenable to high-aspect-ratio etching; in the latter approach, it should be amenable to ALD and have a skin depth in the low 10&#39;s of nm range to prevent light from leaking into the substrate. In both cases, the materials should be suitably low-loss for maximum efficiency. Suitable conductors include metals, heavily doped semiconductors, silicides, and conductive oxides and nitrides. For example, platinum can be the conductor, grown via ALD on slots etched with reactive ion etching (RIE) in silicon, but platinum has a skin depth of 60 nm and is expensive to deposit using ALD. 
       FIG. 14  shows an alternative process  1400  for making a slot antenna starting from a silicon substrate with a thermal oxide layer ( 1402 ). In the first step ( 1404 ), SU-8 photoresist is spin-coated onto the thermal oxide layer and lithographically patterned to form a pillar or protrusion roughly the size and shape of the cavity in the slot antenna ( 1406 ). This pillar is conformally coated with titanium and gold using electron-beam deposition ( 1408 ) to yield thin films of titanium and gold over the pillar ( 1410 ). More gold is then electroplated onto the pillar ( 1412 ) to form a thicker gold layer over the pillar and the substrate ( 1414 ). The gold layer is bonded to a quartz substrate ( 1416 ) using SU-8 photoresist as an adhesive ( 1416 ). The entire assembly is then flipped over ( 1418 ), and most of the silicon is removed to leave the “pillar” facing up on the gold film and quartz substrate ( 1420 ). Any remaining silicon and the pillar are etched away ( 1422 ) to leave a slot antenna formed of gold surrounding a slot or cavity ( 1424 ). 
     Conclusion 
     While various inventive embodiments have been described and illustrated herein, those of ordinary skill in the art will readily envision a variety of other means and/or structures for performing the function and/or obtaining the results and/or one or more of the advantages described herein, and each of such variations and/or modifications is deemed to be within the scope of the inventive embodiments described herein. More generally, those skilled in the art will readily appreciate that all parameters, dimensions, materials, and configurations described herein are meant to be exemplary and that the actual parameters, dimensions, materials, and/or configurations will depend upon the specific application or applications for which the inventive teachings is/are used. Those skilled in the art will recognize or be able to ascertain, using no more than routine experimentation, many equivalents to the specific inventive embodiments described herein. The foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, inventive embodiments may be practiced otherwise than as specifically described and claimed. Inventive embodiments of the present disclosure are directed to each individual feature, system, article, material, kit, and/or method described herein. In addition, any combination of two or more such features, systems, articles, materials, kits, and/or methods, if such features, systems, articles, materials, kits, and/or methods are not mutually inconsistent, is included within the inventive scope of the present disclosure. 
     Also, various inventive concepts may be embodied as one or more methods, of which an example has been provided. The acts performed as part of the method may be ordered in any suitable way. Accordingly, embodiments may be constructed in which acts are performed in an order different than illustrated, which may include performing some acts simultaneously, even though shown as sequential acts in illustrative embodiments. 
     All definitions, as defined and used herein, should be understood to control over dictionary definitions, definitions in documents incorporated by reference, and/or ordinary meanings of the defined terms. 
     The indefinite articles “a” and “an,” as used herein in the specification and in the claims, unless clearly indicated to the contrary, should be understood to mean “at least one.” 
     The phrase “and/or,” as used herein in the specification and in the claims, should be understood to mean “either or both” of the elements so conjoined, i.e., elements that are conjunctively present in some cases and disjunctively present in other cases. Multiple elements listed with “and/or” should be construed in the same fashion, i.e., “one or more” of the elements so conjoined. Other elements may optionally be present other than the elements specifically identified by the “and/or” clause, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, a reference to “A and/or B”, when used in conjunction with open-ended language such as “comprising” can refer, in one embodiment, to A only (optionally including elements other than B); in another embodiment, to B only (optionally including elements other than A); in yet another embodiment, to both A and B (optionally including other elements); etc. 
     As used herein in the specification and in the claims, “or” should be understood to have the same meaning as “and/or” as defined above. For example, when separating items in a list, “or” or “and/or” shall be interpreted as being inclusive, i.e., the inclusion of at least one, but also including more than one, of a number or list of elements, and, optionally, additional unlisted items. Only terms clearly indicated to the contrary, such as “only one of” or “exactly one of,” or, when used in the claims, “consisting of,” will refer to the inclusion of exactly one element of a number or list of elements. In general, the term “or” as used herein shall only be interpreted as indicating exclusive alternatives (i.e., “one or the other but not both”) when preceded by terms of exclusivity, such as “either,” “one of,” “only one of,” or “exactly one of.” “Consisting essentially of,” when used in the claims, shall have its ordinary meaning as used in the field of patent law. 
     As used herein in the specification and in the claims, the phrase “at least one,” in reference to a list of one or more elements, should be understood to mean at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and every element specifically listed within the list of elements and not excluding any combinations of elements in the list of elements. This definition also allows that elements may optionally be present other than the elements specifically identified within the list of elements to which the phrase “at least one” refers, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, “at least one of A and B” (or, equivalently, “at least one of A or B,” or, equivalently “at least one of A and/or B”) can refer, in one embodiment, to at least one, optionally including more than one, A, with no B present (and optionally including elements other than B); in another embodiment, to at least one, optionally including more than one, B, with no A present (and optionally including elements other than A); in yet another embodiment, to at least one, optionally including more than one, A, and at least one, optionally including more than one, B (and optionally including other elements); etc. 
     In the claims, as well as in the specification above, all transitional phrases such as “comprising,” “including,” “carrying,” “having,” “containing,” “involving,” “holding,” “composed of,” and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases “consisting of” and “consisting essentially of” shall be closed or semi-closed transitional phrases, respectively, as set forth in the United States Patent Office Manual of Patent Examining Procedures, Section 2111.03.