Diffraction efficiency performance of random anti-reflecting subwavelength surface structures on prefabricated fused...

Random anti-reflecting subwavelength surface structures have been reported to enhance transmission of optical windows and lenses. Specifically, for fused silica substrates, 99.9% specular transmission has been verified by various groups. Diffractive optical elements, such as gratings, also experience net Fresnel losses on both their planar and structured surfaces. We investigated the performance of prefabricated 50% duty-cycle, binary, fused silica linear gratings, with a period of 1.6 μm, before and after application of random anti-reflecting subwavelength surface structures, in order to reduce their initial Fresnel reflectivity. We compared the diffraction order directions and their efficiencies at three test wavelengths: 594, 612, and 633 nm, for both TE(s) and TM(p) incident light polarization states, under three different mountings: normal, first Bragg, and second Bragg incidence. We report transmission enhancement of the sum of all propagating grating orders for all cases tested by factors between 2% and 10%, with reduction of the respective reflected orders by similar ratios. Transmission enhancement of the -2 diffraction order at Bragg incidence suggests that the random etch has different rates between the raised and lowered linear grating topography.