Antireflective nanostructured surfaces (ARSS) enhance optical transmission through suppression of Fresnel reflection at boundaries between layered media. Previous studies show that random ARSS (rARSS) exhibit broadband enhancement and polarization insensitivity in transmission when applied to flat optical windows. Zinc selenide windows with rARSS treatment were fully characterized (transmittance, reflectance, and angular scatter) in the midwave and long-wave infrared range (2 to 12 μm). Four morphologically different, random nanoroughness, antireflective surfaces were tested at: normal incidence transmission, at 15 deg angle of incidence, and 15 deg to 45 deg angle of reflection. The angular reflectance distribution resembles a diffuse dipole radiator due to the finite elongated beam cross section at the incidence surface. Scattering diagrams with main and side lobes are presented. Partially integrated scatter values were obtained, allowing the comparison of random antireflective boundary performance to optically flat surfaces. Comparing axial transmission and specular reflection with the scattered performance, an accurate determination of the redistribution of the incident energy is obtained. Measurements of the rARSS feature topology were determined from autocorrelation of the scanning electron microscope images of the nanoroughened substrates, to assess the structured surfaces’ feature scales. The results show differences in scattered intensity over the wavelength bands of interest, correlating with surface random feature populations.
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.
Anti-reflective nanostructured surfaces (ARSS) enhance optical transmission through suppression of Fresnel reflection at boundaries between layered media. Previous studies show random ARSS (rARSS) exhibit broadband enhancement in transmission and polarization insensitivity compared to typical optical windows. ZnSe samples with rARSS treatment were characterized (transmittance, reflectance, and angular scatter) in the mid-wave and long-wave infrared (2 - 12 μm) using a spectrophotometer. Five different random nano-roughness antireflective surfaces were tested at: normal incidence transmission and 15° angle of incidence -15 to 45° angle of reflection. The angular reflectance distribution resembles a diffuse dipole radiator, due to a finite elongated beam cross-section at the incident surface. Scattering diagrams with main and side lobes are presented. Comparing specular transmission and reflection with the scattered performance, an accurate determination of the redistribution of incident energy is obtained. Measurements of rms surface roughness using a confocal microscope is presented alongside the scattering data, for assessment of structured surface feature size effects. The results show differences in the scattered intensity, over the wavelength bands of interest, depending on the random topology of the surface. The partial-integrated scatter values were obtained, allowing the comparison of random anti-reflective surfaces to optically flat surfaces.
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