High performance near-IR to longwave-IR broadband omnidirectional antireflection coating by differential evolution

Abstract

Broadband and omnidirectional antireflection coatings in the 3-5 μm and 3-12 μm infrared ranges are of crucial importance in maximizing quantum efficiency in MWIR and LWIR photodetectors, as well as in improving the efficiency of LWIR thermophotovoltaic devices. Conventional approaches to AR coatings on semiconductors, such as quarter-wavelength optical thickness layers, fail to provide consistently high transmission over mid-IR bandwidths and acute angles of incidence. Unconventional approaches such as metasurface and nanostructure AR coatings are difficult and expensive to fabricate on a large scale. We discuss ultra-broadband antireflection coating designed through an inverse design procedure driven by a differential evolution optimization algorithm. Our approach iteratively simulates the transmission characteristics of a multilayer structure using the Transfer-Matrix Method and optimizes layer parameters to maximize the average transmission within a given wavelength band and range of angles of incidence. We present AR coatings which exhibit 98% and 96% average transmission over the 3 μm - 5 μm and 3 μm - 12 μm ranges respectively, over angles of incidence between 0° and 70°. Compared to quarter optical wavelength antireflection coatings over the same ranges, our structures exhibit significantly higher broadband transmission – quarter-wavelength structures saturate near 80% transmission over the 3-5 μm or 3-12 μm ranges, with transmission significantly lowered at large angles. As such, our results rival the transmission performance of state-of-the-art nanostructure AR coatings even at large angles of incidence but are significantly easier to fabricate using standard e-beam evaporation and/or sputtering.