Microstructured surfaces for enhanced transmission

Binary microstructures are used to increase transmission of silicon (Si) surfaces in the infrared (IR) range. We consider hexagonal and square lattices with square and round pillars with the finite-difference frequency-domain (FDFD) method. In agreement with theory and previous publications, the height needed to improve transmission for all geometries is given by λ/4√n_Si where n_Si is the refractive index of Si. Optimal size of microstructures is found through rigorous simulations. Different effective medium theories (EMTs) are compared whether they can predict this optimal size. Zero-order EMT appears to be one of the best choices, another good choice is volume averaging of refractive index. Manufacture-related issues are also discussed: for example, choice of bigger period is preferential as it gives better stability of the transmission curve if variations of pillar diameter and height occur from batch to batch during fabrication.