Abstract
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.
Highlights
Fresnel reflection occurs when light is incident on boundaries separating dielectric regions with different optical refractive indices
The optical substrates were coated with a thin, hard dielectric film at an empirically determined thickness, which is sufficient to mask the substrate from the reactive-ion plasma etching radicals for the duration of the fabrication etch process
Downloaded From: https://www.spiedigitallibrary.org/journals/Optical-Engineering on 04 Jan 2022 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use schedule and the surface energy between the metal and the dielectric, whereas the island height is a function of the deposited metal-film thickness
Summary
Fresnel reflection occurs when light is incident on boundaries separating dielectric regions with different optical refractive indices. Antireflective (AR) treatments, such as thin-film coatings, suppress reflections by destructive interference along the propagating directional axis. Recent studies have demonstrated that random antireflective structured surfaces (rARSS) can be used as an alternative to thin film coatings, reducing Fresnel reflections in the visible and the infrared (IR).[1,2,3,4,5,6,7,8] Broadband response, high transmission across wide ranges of angle of incidence (AOI), and polarization insensitivity have been reported.[9,10] In general, the AR effects correlate with the density and depth of the nanostructures, simulating an effective medium gradient-index boundary profile, which reduces Fresnel reflectivity. In cases where surface-induced scatter is evident, the on-axis spectral reflection and transmission can be reduced simultaneously. In those cases, the sum of the detected
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