Optical characterization of random anti-reflecting subwavelength surface structures on binary gratings

Random anti-reflecting subwavelength surface structures on optical components have been proven to enhance transmission by reducing Fresnel reflection losses. These structures have a broadband anti-reflective effect, high angle-of-incidence tolerance, and polarization insensitivity, which makes their performance comparable to more complex multilayered anti-reflective coatings. Conformal anti-reflective multilayered thin-film coatings on grating structures have coverage issues, especially in deep grooves of gratings, thereby impacting the diffractive characteristics of the gratings. We fabricated the random anti-reflective structures on preexisting fused silica binary gratings, using a reactive ion etching process, and compared the prior and postimplementation optical properties of the gratings. We observed that fabrication of random anti-reflective subwavelength surface structures on existing binary gratings retained the diffractive performance of the original gratings, such as the angle of diffraction and intensity balance of the orders, while reducing Fresnel reflection. Transmission and reflection data of the non-evanescent diffracted orders were collected at normal and Bragg incidence, for both incident polarizations, S (TE) and P (TM). The measurements were taken using a selectable multi-wavelength He-Ne laser working at 543nm, 594nm, 604nm, 612nm and 633nm. The reflected intensity of the η 0 and η +1 orders at 1st Bragg angle of incidence were suppressed to 0.5% -1.0% for the rARSS fused silica grating, from the original 3% - 8% for the planar fused silica gratings, at all test wavelengths. Our results indicate that modification of existing binary gratings with added rARSS is possible without adverse effects on the grating performance.