Bidirectional scattering distribution function of random antireflective nano-roughened surfaces

Randomly nano-roughened interfaces between optical media can result in polarization-insensitive, angle-of-incidence independent, broadband anti-reflective (AR) performance. The on-axis AR effect correlates with the nano-roughness longitudinal scale (depth), whereas feature transverse dimension distributions (average feature diameters) cause selective scatter. Subwavelength, random AR structured surfaces (rARSS) can forward scatter light in the transmission direction, suppress specular Fresnel reflectivity, and enhance overall axial transmission throughput. In cases where the random nanoroughness average value approaches that of the incident radiation wavelength, the induced scatter can be detrimental to imaging applications due to uniformly distributing light from the specular into wider angles, reducing image contrast and broadening (blurring) the axial image spot. Using a polarized-laser scatterometer, we measured the bidirectional scattering distribution function (BSDF) of nano-roughened fused silica (FS) windows and compared them to an optical-quality FS flat and two different flat mirrors. The fabricated rARSS windows were optimized to have a broadband, anti-reflective specular response (R 400 nm). The BSDF was measured at 633-nm-wavelength, over the equatorial plane of the unit sphere, and at selected incidence angles below Brewster’s value. Granulometry of the roughened surfaces is correlated with the scattering data, for assessment of roughness-induced scattering effect. Using the BSDF data, single-surface and double-surface rARSS-treated windows were evaluated to determine any surface-to-surface scatter cascading effects.