Abstract
The present invention relates to a metamaterial focal plane array for broad spectrum imaging. Electromagnetic energy in the form of light is absorbed in or on a metamaterial absorber and a subsequent hot carriers are collected either in a semiconductor space charge region (e.g. P-N junction), or in some other modern collection scheme. Following the accumulation of photogenerated charge (electrons or holes), the signal is then converted to a digital signal using conventional or slightly modified ROIC modules.
Claims
1. A metamaterial focal plane array (MMFPA) comprising: at least one metamaterial absorber (MMA); at least one charge-coupled device (CCD), wherein a first side of each CCD of the at least one CCD is coupled to a corresponding MMA of the at least one MMA; a substrate layer, wherein a second side of each CCD of the at least one CCD is coupled to a first side of the substrate layer; a ground layer coupled to a second side of the substrate layer.
2. The MMFPA of claim 1, wherein the MMA comprises a metal.
3. The MMFPA of claim 2, wherein the MMA comprises gold.
4. The MMFPA of claim 3, wherein the CCD comprises silicon.
5. The MMFPA of claim 4, wherein the substrate layer comprises silicon.
6. The MMFPA of claim 1, wherein the MMA comprises a dielectric material.
7. A metamaterial focal plane array (MMFPA) comprising: at least one metamaterial absorber (MMA); at least one charge-coupled device (CCD), wherein a first side of each CCD of the at least one CCD is coupled to a corresponding MMA of the at least one MMA; a substrate layer, wherein a second side of each CCD of the at least one CCD is coupled to a first side of the substrate layer; and a ground layer coupled to first side of the substrate layer.
8. The MMFPA of claim 7, wherein the MMA comprises a metal.
9. The MMFPA of claim 7, wherein the MMA comprises a dielectric material.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) The detailed description of the drawings particularly refers to the accompanying figures in which:
(2) FIG. 1 shows a cross-section of a metamaterial focal plane array.
(3) FIG. 2 shows a cross-section of a metamaterial imaging system.
(4) FIG. 3 shows a metal-semiconductor interface.
(5) FIG. 4 shows a cross-section of a metamaterial focal plane array.
(6) FIG. 5 shows a top-down view of a metamaterial focal plane array.
DETAILED DESCRIPTION OF THE DRAWINGS
(7) The embodiments of the invention described herein are not intended to be exhaustive or to limit the invention to precise forms disclosed. Rather, the embodiments selected for description have been chosen to enable one skilled in the art to practice the invention.
(8) FIG. 1 shows an exemplary metamaterial focal plane array 1. A metamaterial absorber (MMA) 3 is used to absorb light and provide a digital signal output through a read out integrated circuit (ROIC). MMA 3 is coupled to a charge-coupled device (CCD) 5, with the MMA 3 and CCD 5 embedded within substrate 7. For front end illumination, ground layer 9 is coupled to the bottom of substrate 7. In this process, electromagnetic energy in the form of light is absorbed in or on the metamaterial and the subsequent hot carriers are collected either in a semiconductor space charge region (e.g. P-N junction), or in some other modern collection scheme. Following the accumulation of photogenerated charge (electrons or holes), the signal is then converted to a digital signal using conventional or slightly modified ROIC modules. The process of getting the generated hot electrons from the surface of the metamaterial absorber is similar to that of a photoemissive detector. A MMA 3 is an array of sub-wavelength features, arranged either in the X-Y plane, Z-plane, or all three, to create an artificial permittivity. This permittivity is different than the bulk materials that individually make up the MMA. To have an MMA with higher absorption, the artificial permittivity should be as close as possible to the impedance (Z) of free space, i.e. Z=√(μ/ϵ). In this expression μ is the magnetic permeability (μ=μrμo) and ϵ is the electric permittivity (ϵ=ϵrϵo). The MMA can be made up of a metal based absorbers or dielectric metamaterial patterns. The metal based absorbers rely upon the plasmonic response of the electrons to create the matching |E| and |H| fields necessary for impedance matching. This type of design typically requires a ground plane for |H| matching otherwise the maximum absorption possible is 50% (for more information see Coupled Mode Theory). A ground plane to achieve light absorption above 50% is possible as long as charge collection isn't impeded. Dielectric based MMA are designed as Mie resonance dielectric confinement structures. MMA can be readily made to suit the short wave infrared (SWIR), mid-wave infrared (MWIR), and long wave infrared (LWIR) imaging bands through a complement of geometry and dielectric functions. For example, in the case of a metal based MMA using Au; the resonant absorption band can be tuned from SWIR to LWIR through a modification of geometry comparable to the wavelength of interest. This is due to low dispersion of the real and complex dielectric permittivity of Au in those bands.
(9) FIG. 2 shows a process from absorption through detection for a wavelength thermal detector. In a first step, light is directed towards a metamaterial focal plane array 1. In a second step, light absorbed by MMA 3 generates plasmons. In a third step, the plasmons decay into hot electrons 21. In a fourth step, hot electrons 21 transport from the MMA 3 to the CCD pixel 5. In a fifth step, the current generated by CCD pixel 5 is detected and measured.
(10) FIG. 3 shows a metal-semiconductor interface with a small Schottky barrier that is one possible scenario for the hot electrons to encounter. The absorption of light within the MMA creates plasmons which decay into hot electrons on the surface of the metal. This means that the boundary layers of metal/semiconductor and metal/dielectric with have an electron with an energy of at least k*T above the Fermi level, where k is Boltzmanns constant and T is the absolute temperature in units of Kelvin. For metal based MMA, the metal absorbing features have a higher efficiency if they have a minimal offset barrier between the metal and semiconductor (i.e. Ohmic contacts). It is possible for the device to function with a Schottky barrier between metal and semiconductor, but this design has a reduced internal quantum efficiency (IQE). The Schottky barrier design can be useful in creating a simple structure for charge separation in situations where a p-n or other junction isn't available. For designs where charge separation is accounted for by some other known means (e.g. p-n, p-in, nBn, pBp, etc.) then the maximum charge conversion efficiency (i.e. IQE) is dictated by the density of states of the metal with electrons having momentum matching in the semiconductor. The use of a dielectric MMA provides another avenue of light absorption. This approach provides a lower loss outside of the desired absorption band. This means it has a higher quality factor (Q-factor) than their metal counterparts. The trade-off with a dielectric MMA is the more difficult challenge of charge extraction due to the lower density of states and electrical mobility. To achieve MMA with dielectrics one must design a structure that when the confined, the |H| and |E| spectrally overlap. In this instance these dielectric Mie resonators will achieve 100% absorption. Work into dielectric MM and absorption outside of the imaging bands shows that lossy dielectrics improve overall absorption while also providing localized absorbing sites for potential charge extraction. This design has a charge creation that is mostly independent from the bandgap of the adjoining charge collection material. As such, the charge creation method can be selected based on independent design factors such as IQE, cost, pixel size, response speed, charge capacity, etc. For example, charge collection methods such as Schottky barrier, including 2D materials such as MoS2, WS2, n-type barrier n-type (nBn), photoconductor, photoemission, metal-insulator-metal (MIM), metal-semiconductor-metal (MSM), photodiode, avalanche photodiode (APD), and p-type barrier p-type (pBp) would work.
(11) FIG. 4 shows a cross-section view of a metamaterial focal plane array 1 backside illumination scheme.
(12) FIG. 5 shows a top-down view of a metamaterial focal plane array (as shown in FIG. 1).
(13) Although the invention has been described in detail with reference to certain preferred embodiments, variations and modifications exist within the spirit and scope of the invention as described and defined in the following claims.
