Direct femtosecond laser writing of inverted array for broadband antireflection in the far-infrared

Abstract

The surface optical loss in optical windows is primarily attributed to Fresnel reflection at the interface between air and substrate of high refractive index materials such as zinc sulfide (ZnS). In this study, the concave antireflective subwavelength structures (ASS) on ZnS have been numerically and experimentally investigated to obtain high transmittance over a wide bandwidth in the far-infrared range from 7 μm to 14 μm. The mechanism for transmittance enhancement is explored by localized field selective enhancement in structures based on Wood-Rayleigh law, also being confirmed by the light energy flowing in Poynting vector distribution. The geometric parameters including shapes, period, and depth of as-designed array have been specifically optimized by finite-difference time-domain (FDTD) method and effective medium theory (EMT). Hereby, three-dimensional direct femtosecond laser writing (3D fs-DLW) is employed to fabricate the ASS in inverted conical and pyramidal array with the largest transmittance reaching to 85.2% in 9 μm wavelength and robust mechanical characteristics (hardness >2.39 GPa), expecting to be of great potential applications in infrared optical windows.