Optimized two-layer random motheye structures for SiO₂ windows

Abstract

We computationally investigate the near-field transmission efficiency of two-layer motheye structures on SiO₂ windows. In these structures, a random motheye layer is imposed on top of a periodic motheye structure that consists of truncated pyramids. We first validate our simulation by comparing simulation results using a single layer of random pillars to experimental results. To maximize the transmission efficiency of our two-layer structures over the wavelength range of 0.4 to 2 µm, we used the previously optimized one-layer periodic pyramidal motheye structures as the bottom layer and we varied the statistical properties of the random pillars on the upper layer, which include the mean and span of their diameters and the mean and span of the pillar heights. We determine that the transmission generally increases as the range of the statistical parameters increases. It is theoretically possible to achieve an average transmission efficiency of 99.8% over the wavelength range from 0.4 to 2 µm by adding a random motheye layer over the periodic truncated pyramid structure, thereby increasing the average transmission efficiency by 0.3% over the same wavelength range and reducing the reflection by more than a factor of two. The large reduction in reflections over a broad bandwidth can be important in optical systems that rely on minimal reflections.