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1. An infrared photodetector having a predetermined wavelength detection range comprising: a contact layer through which infrared light enters;an absorption region positioned such that light admitted through the contact layer passes into the absorption region;a diffractive region operatively associated with the absorption region comprising a plurality of diffractive elements operating to diffract light into the absorption region; the configuration of the diffractive region and diffractive elements being determined by computer simulation to determine an o...
1. An infrared photodetector having a predetermined wavelength detection range comprising: a contact layer through which infrared light enters;an absorption region positioned such that light admitted through the contact layer passes into the absorption region;a diffractive region operatively associated with the absorption region comprising a plurality of diffractive elements operating to diffract light into the absorption region; the configuration of the diffractive region and diffractive elements being determined by computer simulation to determine an optimal diffractive region and absorption region configuration based upon a predetermined wavelength detection range, the diffractive region operating to diffract light entering through the contact layer such that phases of diffracted waves from locations within the photodetector including waves reflected by sidewalls, and waves reflected by the diffractive elements form a constructive interference pattern inside the absorption region. 2. The photodetector of claim 1 wherein the photodetector is a quantum well infrared photodetector and wherein optical absorption at a particular location is linearly proportional to light intensity at that location, and wherein light is detected in the wavelength range from 3 to 15 microns and wherein the quantum efficiency (η) is determined by η=1P0∫VⅆI(r→),=1P0∫VαI(r→)ⅆ3r,=αAcɛ02E02∫Vncɛ02Ez(r→)2ⅆ3r,=nαAE02∫VEz(r→)2ⅆ3r, where P0 is the optical power incident normally on a detector area A, V is the detector active volume, I is the optical intensity associated with Ez, α is the absorption coefficient, r is the spatial coordinate, n is the material reflective index, ε0 is permittivity of free space, c is the speed of light, E0 is the electric field in free space, EZ is the electric polarization perpendicular to the layers and wherein the classical quantum efficiency η for a corrugated-QWIP is determined by η=tsηint=ts1p[t+ⅇ-αp2α(1-ⅇ2αt)] where ts is the substrate transmission coefficient, p is the pixel pitch, α is the absorption coefficient, t is the detector material thickness. 3. The infrared photodetector of claim 1 produced by the method comprising selecting a material composition for the photodetector; determining a configuration of at least one pixel in the array of pixels using a computer simulation, each pixel comprising an active region and a diffractive region, and a contact layer through which light enters, the computer simulation operating to process different configurations of the pixel to determine an optimal configuration for a predetermined wavelength or wavelength range occurring when waves reflected by the diffractive element form a constructive interference pattern inside the active region to thereby increase the quantum efficiency of the photodetector. 4. The infrared photodetector of claim 1 wherein the configuration of the diffractive elements comprise a plurality of reflective walls, the configuration of plurality of the reflective walls being determined using computer simulation to determine optimal diffraction occurring when light entering the contact layer is diffracted such that the phase of the diffracted wave at one location and the phases of waves returning back to the same location, by reflection and diffraction, from locations within the photodetector form a constructive interference pattern inside the absorption region to thereby achieve resonance. 5. The infrared photodetector of claim 1 having a configuration geometry determined by three-dimensional computer simulation and wherein diffractive elements comprise geometrical objects selected by computer; properties of geometrical objects defining physical relations such that phase of the diffracted wave at one location and the phases of waves returning back to the same location by reflection and diffraction from other locations within the photodetector including sidewalls form a constructive interference pattern inside the absorption region to thereby achieve resonance. 6. The infrared photodetector of claim 1 wherein the plurality of diffractive-elements within the diffractive region comprise a plurality of reflective walls and wherein the absorption region and the plurality of diffractive elements are formed of at least one material and wherein the configuration of the diffraction region and reflective walls is determined by three-dimensional computer simulation that operates to simulate the light absorption within the photodetector; the computer simulation comprising the steps of: inputting the composition of the at least one material; inputting the configuration of the absorption region and the diffractive region;calculating the electromagnetic field distributions using the three-dimensional computer simulation;calculating the eigen functions and energies of the at least one material;using three-dimensional computer simulation, changing the size and shape of the diffractive region such that a different set of eigen modes are created and the excitation of these eigen modes, and their superpositions if they are degenerate, by the incident light determine the detector quantum efficiency spectrum which is used to calculate the quantum efficiency of the detector configuration geometry; whereby the configuration geometry is selected based on the three-dimensional computer simulation. 7. The infrared photodetector of claim 1 wherein the absorption region comprises an active layer comprising a quantum well material having a thickness in the range of 0.1 μm-12 μm and wherein the absorption region is sensitive to light only when the light is propagated parallel to the growth plane of the active layer, and where light entering the photodetector is normal to the active layer. 8. The infrared detector of claim 1 wherein the infrared detector comprises a plurality of pixels electrically connected to a readout circuit and wherein the readout circuit is connected to a display for displaying an infrared image. 9. The photodetector of claim 1 wherein the physical geometry is designed on a computer comprising: (a) selecting a material based upon absorption characteristics of the material; calculating Eigen functions & Eigen energies of material structure to obtain an absorption coefficient of material;(b) designing a physical geometry of a photodetector on a computer using a computer program; the photodetector comprising components of different materials that are treated as subdomains;(c) inputting input parameters of materials for each subdomain into the computer program;(d) defining material properties of subdomains and physical relations between boundaries of the subdomains;(e) determining a node density for numerical computation;(f) defining an electromagnetic wave incident condition;(g) defining numerical solver parameters;(h) based upon an electromagnetic field distribution, computing the quantum efficiency for the photodetector;(i) repeating steps (a) through (h) until an optimal design and quantum efficiency is determined. 10. The photodetector of claim 9 wherein the designing the physical geometry on a computer comprises using a computer aided design program to construct a configuration, and wherein the step of defining the density of the nodes comprises determining the maximum element size and other factors that control the density of a mesh; and wherein for a shorter optical wavelength, a higher frequency of the spatial variations will occur, and the denser the mesh should be, and wherein if the mesh is too coarse, a self consistent solution can never be reached or the solution is not accurate enough and if the mesh is too dense, too many computations will be required; and wherein the optimum density is determined by trial and error. 11. The photodetector of claim 9 wherein the step of defining numerical solver parameters comprises using wavelength as a parameter in a predetermined range and defining the maximum number of iterations and the maximum accuracy tolerance and wherein the step of computing the quantum efficiency for the photodetector comprises integrating the electromagnetic field according to the formula η=nαAE02∫VEz(r→)2ⅆ3r where n is the refractive index of the detector material, α is the absorption coefficient of the detector material, A is the detector area, E0 is the incident electric field of light in free space, V is the detector volume, Ez is the electric field vertical to the material layers, r is the spatial coordinate. 12. The photodetector of claim 1 wherein the photodetector is a quantum well infrared photodetector and wherein a computer simulation is utilized to determine an optimal configuration for optimal quantum efficiency and wherein the optical absorption at a particular location is linearly proportional to the light intensity at that location, and wherein light is detected in the wavelength range from 3 to 15 microns and wherein the calculating of the optimum quantum efficiency (η) is determined by η=1P0∫VⅆI(r→),=1P0∫VαI(r→)ⅆ3r,=αAcɛ02E02∫Vncɛ02Ez(r→)2ⅆ3r,=nαAE02∫VEz(r→)2ⅆ3r, where P0 is the optical power incident normally on a detector area A, V is the detector active volume, I is the optical intensity associated with Ez, α is the absorption coefficient, r is the spatial coordinate, n is the material reflective index, ε0 is permittivity of free space, c is the speed of light, E0 is the electric field in free space, EZ is the electric polarization perpendicular to the layers. 13. The photodetector of claim 1 further comprising a composite film layer beneath the contact layer comprising a plurality of infrared transparent materials of different refractive indices which increase the number of reflective surfaces so as to effect resonant wavelengths. 14. The photodetector of claim 1 wherein the computer simulation operates to simulate a matching of the phase within the photodetector and comprises: calculating electromagnetic field distributions using a three-dimensional electromagnetic field simulation;calculating eigen functions and energies of at least one material to determine a material absorption coefficient;using computer simulation, changing a cavity's size and shape such that a different set of cavity eigen modes are created and excitation of these eigen modes, and their superpositions if they are degenerate, by incident light determine a detector quantum efficiency spectrum which is used to calculate a quantum efficiency of a detector configuration geometry;selecting the configuration geometry based on the computer simulation. 15. The photodetector of claim 14 wherein the selecting of at least one material from which to construct the photodetector comprises selecting an amount of doping, a material and doping selected being dependent upon a wavelength of the light to be detected, the method further comprising calculating an eigen function and eigen energies of each of the at least one material, and iteratively simulating the quantum efficiencies obtained for different configurations and diffraction elements using the calculated eigen functions and eigen energies of the at least one material structure and wherein the eigen functions in material layer n may be expressed using the equation Ψ=Aneiknz+Bne−iknz where the eigen functions An and Bn of layer n are related to the eigen functions An+1 and Bn+1 of the next layer by: [AnBn]=12[(1+γn,n+1)ⅇⅈ(kn+1-kn)dn,n+1(1-γn,n+1)ⅇ-ⅈ(kn+1+kn)dn,n+1(1-γn,n+1)ⅇⅈ(kn+1+kn)dn,n+1(1+γn,n+1)ⅇ-ⅈ(kn,n+1-kn)dn,n+1][An+1Bn+1]=12Mn,n+1[An+1Bn+1]whereγn,n+1=mn*kn+1mn+1*kn and wherein at an eigen energy E, the electron transmission coefficient TG(E) through all layers has a local maximum, where TG(E)=1A1(E)2vp(E)v1(E)=22p-2a11(E)2m1*(E)mp*(E)kp(E)k1(E) and wherein after identifying all the eigen energies, the corresponding eigen functions An and Bn can be obtained. 16. The photodetector of claim 15 further comprising determining the absorption coefficient of material, including determining an oscillator strength fn fn=2ℏm*ω〈Ψn∂∂zΨ1〉2,n=2,3… wherein due to layer thickness fluctuation in actual material, the eigen energies have finite energy distributions, which leads to finite energy distribution ρ for each optical transition ρn(λ)=12πσexp[-12σ2(hcλ-En+E1)2] such that the absorption coefficient α can be determined using the equation α(λ)=∑nNDWLπⅇ2ℏ2ɛhɛ0m*cfnρn(λ)where ND is doping density, W is well width, L is length of a quantum well period, e is electric charge, is the Plank's constant, εh is relative permittivity in z direction, ε0 is permittivity of free space, m* is electron effective mass, c is the speed of light in vacuum, fn is oscillator strength for the optical transition from a ground state to the nth excited state, and ρn is the linewidth of an nth optical transition. 17. The photodetector of claim 16 wherein the step of designing the physical geometry of the detector comprises: inputting computer parameters for the material to be used based upon the application requirements;defining material properties of all geometrical objects in the configuration geometry;defining physical relations among all geometrical boundaries; and wherein the step of calculating the electromagnetic field distributions using a computer comprises: defining density of nodes for numerical computation;defining an electromagnetic wave incident condition; anddefining a numerical solver; and wherein the step of determining a quantum efficiency spectrum at the desired wavelength or wavelength range comprises using the electromagnetic wave distribution to compute the quantum efficiency. 18. The photodetector of claim 1: wherein the computer simulation operates to simulate matching of the phase within the photodetector and comprises calculating eigen values and energies of at least one material to determine corresponding eigen functions;calculating an absorption coefficient of the at least one material based upon the eigen functions;calculating oscillator strength based upon the eigen functions and eigen energies,obtaining a finite energy distribution for each optical transition;determining an absorption coefficient from the oscillator strength and finite energy distribution;designing the physical geometry of the detector including the diffractive element using computer aided design to create a computer model, the computer model comprising geometrical subdomains;inputting parameters in the computer model for at least one material including the relative permittivity εr, which is related to absorption coefficient α, the conductivity σ and the relative permeability μr of each geometrical subdomain;defining other geometrical objects and the material properties of the other geometrical objects, including the conductivity of the electrical contacts;defining physical relations among geometrical boundaries;determining density of electromagnetic nodes for numerical computation, comprising defining the maximum element size and factors that control the density of the mesh to determine optimum density;defining the electromagnetic incident wave condition at the photodetector/air interface;defining the numerical solver comprising defining the accuracy desired and the number of iterations;based upon the scattered light in the direction of incidence, computing the quantum efficiency.