Deterministic illumination diffractive-diffusers have nonperiodic short and mediumscale topography. Because of the deterministic locations of vertical sidewalls at the phase transition boundaries, over-coating diffractive diffusers with thin-film antireflection layers perturbs their function, resulting in performance deviations and nonuniformities. To mitigate these effects, we added antireflection random nanostructures on the surface of three different classes of fused-silica multiphase diffractive diffusers, using reactive-ion plasma etching. The diffusers were measured before and after the random nanostructures addition, using a polarized-laser scatterometer with a dynamic range of nine orders of magnitude. The bidirectional scatter distribution function was measured over the entire equatorial plane of incidence, to analyze the directionality of scattered light and the impact of the antireflective nanostructure presence on the optical performance of the diffusers. The overall reflectivity suppression was measured across the illumination patterns directions, as well as, across the entire 180-deg angle-sweep. The designed deterministic illumination patterns and their contrast were unaffected by the presence of the random antireflective structures, whereas Fresnel reflectivity was reduced by an order of magnitude on average.

Suppressing Fresnel reflections from dielectric boundaries using periodic and random antireflection structured surfaces (ARSSs) has been vigorously studied as an alternative to thin film coatings for high-power laser applications. A starting point in the design of ARSS profiles is effective medium theory (EMT), approximating the ARSS layer with a thin film of a specific effective permittivity, which has features with subwavelength transverse-scale dimensions, independent of their relative mutual positions or distributions. Using rigorous coupled-wave analysis, we studied the effects of various pseudo-random deterministic transverse feature distributions of ARSS on diffractive surfaces, analyzing the combined performance of the quarter-wave height nanoscale features, superimposed on a binary 50% duty cycle grating. Various distribution designs were investigated at 633 nm wavelength for TE and TM polarization states at normal incidence, comparable to EMT fill fractions for a fused silica substrate in air. The results show differences in performance between ARSS transverse feature distributions, exhibiting better overall performance for subwavelength and near-wavelength scaled unit cell periodicities with short auto-correlation lengths, in comparison to equivalent effective permittivity designs that have less complicated profiles. We conclude that structured layers of quarter-wavelength depth and specific feature distributions can outperform conventional periodic subwavelength gratings as antireflection treatments on diffractive optical components.

Random antireflection nano-structured surfaces (ARS) have been studied for their broadband antireflection (AR) properties and polarization insensitivity. ARS are designed and modeled using effective medium approximations (EMA) as thin layers of the desired effective permittivity through a global density average, independent of surface feature distributions. To study the AR efficiency of varying transverse feature distributions of ARS on optical surfaces, we methodically simulated and analyzed the performance of pseudo-random deterministic Dammann gratings, acting as a quarter-wave-thickness AR overcoating on a functional binary 50% duty cycle test grating, using rigorous coupled wave analysis. We chose a fused silica dielectric substrate, numerically simulated at normal incidence conditions for both polarizations at 633nm wavelength. The study parameters consisted of Dammann gratings of different orders for evanescent diffraction control, chosen to have effective permittivities comparable to predicted EMA requirements to match AR efficiency, varying periodic scales, and distinct surface distribution autocorrelation scales to control the structure factor. The goal is to elucidate the transition of evanescent coupling orders from the Dammann ARS to the functional test grating, without perturbing the original diffractive performance, while it enhances transmitted overall power efficiency. The simulated results exhibit variations in the performance of candidate designs, signifying the importance of surface feature distributions on the overall efficiency of ARS as an effective antireflection treatment for diffractive components. Not only subwavelength periodicity scales, but nearwavelength scales as well show high transmission efficiency without presence of parasitic orders from the base binary test grating, in contrast to EMA design guidelines.

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