Mid-wave and long-wave IR angular scatter of random anti-reflective nanostructured surfaces on ZnS, ZnSe, and GaAs (Conference Presentation)

Fresnel reflections at boundaries between layered media can be suppressed using anti-reflective randomly-nanostructured surfaces (rARSS). Previous studies have shown that rARSS can result in spectral broadband transmission enhancement and polarization insensitivity, in the specular direction, compared to unstructured optical windows. However, specular reflectance can be suppressed through scattering mechanisms, resulting in an angular redistribution of the reflected irradiance. In those cases, specular transmittance is adversely affected as well. We characterized ZnS, ZnSe, and GaAs windows with rARSS treatment on both surfaces, by measuring the specular transmittance and reflectance, as well as, the directional angular reflective scatter, in the mid-wave and long-wave infrared bands (2 - 12 μm). The incident light was directed off-normal incidence, and the angular reflectance distribution was measured over a ±30° cone, centered on the specular reflection angle. An accurate determination of the redistribution of the reflected energy was obtained, by comparing the scatter of the structured surfaces to the pre-processed, optically flat, substrate performance. Surface roughness was determined using a UV-confocal microscope and a scanning electron microscope. The rARSS feature dimensions were correlated to the overall optical performance. The results show reflective specular and scatter intensity suppression, along with specular transmission enhancement.