Electromagnetic limit of the optical gain for long-wave IR diffractive optics

Focal plane collection optics consisting of arrays of microlenses may be us ed to reduce the physical size of an infrared detector without reduction of its apparent optical size and quantum efficiency. This results in reductio n of the thermal generation rate in the detector, allowing improvement of p erformance of infrared devices operating at elevated temperatures. In this paper, we report on calculations of the optical gain achievable with the us e of diffractive microlenses for long-wave (8-12 mu m) infrared (LWIR) dete ctors. Fast diffractive lenses are required for this application, and binar y optics is the most popular technology of their fabrication. A large fract ion of the binary lens surface relief is built of staircase annular structu res whose width is of wavelength-scale. Therefore, the electromagnetic theo ry of gratings has been applied in this paper to calculate the diffraction efficiency of the Fresnel zones for the multi-phase-level lenses. It is sho wn that electromagnetic effects limit the speed of the LWIR diffractive len ses.