Numerical and Optical Performance Characterization of Random Antireflective Surface Structures on Diffractive Optical Elements
Antireflective structured surfaces (ARSS) are periodic, or random, distributions of three-dimensional nano-features fabricated directly on optical quality substrates, for the suppression of surface reflectivity from the dielectric boundary. Within the spectral band of efficient antireflective operation, the structures are sub-wavelength scale in cross-sectional diameter, and near-wavelength scale in height. As incident light passes from superstrate to substrate, the ARSS induce a synthetic index of refraction with a surface-average optical dielectric density, effectively matching the electromagnetic impedance on the surface separating the media, thus reducing Fresnel reflectivity. Published studies often model random ARSS as stratified homogenous dielectric layers, globally averaging the transverse feature distributions to a single optical index value per layer, and ignoring their distributed profiles. In this work, the effects of pseudo-random deterministic transverse feature distributions within the ARSS and their antireflective properties were studied. Rigorous coupled wave analysis (RCWA) was used to compute the performance of periodic unit cell ARSS, superimposed on a binary-phase transmission grating as a function of the ARSS period, for TE and TM incident light polarization states, at normal angle of incidence. The results showed differences in performance between distinct ARSS distributions, despite their identical layer-averaged refractive index value. Sub-wavelength and near-wavelength scale unit-cell periodicities for ARSS with short autocorrelation lengths, show better overall anti-reflectivity performance, compared to less complicated feature distributions with comparable effective-permittivity layers. Numerical simulations for specific randomly distributed ARSS features correlated with anti-reflective performance efficiencies. In parallel, the fabrication process of random anti-reflective structures (rARSS) was optimized for fused silica optical flats, and then applied to deterministic phase-diffusing diffractive optical elements (DOE) to suppress Fresnel reflections. The goal of the effort was to examine the effects of rARSS application on existing optical components. Four commercially available DOE, a 2D spot array generator, a 1D spot array, a controlled-angle illumination diffuser, and a discrete-phase profile diffractive lens, were used to investigate the effects of rARSS on 3-dimensional segmented-phase topographies. Three diffractive diffusers were measured over the entire equatorial plane of incidence using a scatterometer, to determine changes from the original design illumination pattern due to the presence of rARSS beyond a simple transmission enhancement. The diffractive lens was measured using a power detector and a beam profiler to compare the focal spots before and after addition of rARSS in transmission and reflection. The tests verified significant reduction of Fresnel reflections by the rARSS on the surface of the DOE, without altering their original performance efficiency. Finally, the non-deterministic scatter effects due to inherent roughness of the rARSS on segmented phase profiles was characterized by comparing scatterometer measurements of optical flats and near-wavelength scale binary-phase gratings. It is shown that scatter effects because of rARSS presence on optical flats and binary-phase gratings were negligible, indicating rARSS can be applied as an effective anti-reflection treatment to pre-fabricated optical surfaces with complex topography without degrading their performance.