Optical scattering measurements of random anti-reflection subwavelength surface structures on binary gratings

Abstract

Random anti-reflection sub-wavelength structures (rARSS) on optical component surfaces have been shown to enhance transmission by reducing Fresnel reflection. These structures have a broadband anti-reflection effect, and insensitivity to high angle of incidence and polarization. We have fabricated random anti-reflection surface structures on commercially available linear gratings, using reactive ion etching. An overall enhancement was measured in the specular transmission direction, with suppression of the specular reflection to less than 1%, from initial values between 3 to 8% under 1st Bragg angle of incidence, without effecting the diffractive performance of the grating. We studied the effect of these structures on a 1.595 μm period and 1.166 μm period gratings, with and without rARSS, using scattering measurements for polarized light, at normal, 1st Bragg and 2nd Bragg incidence. The directional transmission distribution function (BTDF) was measured over 180 degrees (full field), to analyze the directionality of the scattered light and the impact of the rARSS on the optical performance of the gratings. Correlations of the surface roughness with the individual diffraction order scattering profile and overall BTDF were investigated.