Gradient-index metasurface multilayer for quasioptical coupling of infrared detectors

Periodic surface grating structures made of machinable infrared-transparent dielectrics can offer broadband reflection and transmission in THz waves. We present geometrically varying periodic metasurface made by micromachining and integrated above a gradient-index dielectric multilayer to achieve a near-perfect longpass filter. We utilized techniques in two-dimensional and three-dimensional rigorous, coupled-wave analysis for understanding quasioptical electromagnetic wave coupling within photonic gratings. The low-index pyramidal metasurface shape factors show at-period interference to allow near-perfect transmission. A purely infrared dielectric and polymeric low-index material is compared to seek trade-offs in infrared absorption. We also establish relationships between minimum deposited layer thickness and longpass filter performance. This micromachined metamaterial focal plane filter-on-chip can perform combined antireflection and Bragg band-pass filtering at the near- to far-infrared wavelengths. Its capability can be used for infrared/THz focal plane thermal imaging for space-based applications, understanding the Earth's surface and atmosphere, and observing cryogenic features of far bodies and exoplanets. (C) 2021 Society of Photo-Optical Instrumentation Engineers (SPIE)