High-efficiency embedded transmission grating

Abstract

In this study, we present high efficiency embedded gratings produced by atomic layer deposition (ALD). The chosen embedding material is a nanolaminate, which consists of alternating arranged titanium dioxide (TiO2) and alumina (Al2O3) layers, where the TiO2 layers are by a factor of 25 thicker than the Al2O3 layers. Consequently, the refractive index nearly equal to the refractive index of pure TiO2 layers. Titanium dioxide has one of the highest refractive index among dielectrics and no absorption at the operating wavelength. A pinhole free embedding of the grating is essential, since even tiny air pockets will reduce the efficiency of the diffraction optic. This has been successfully realized. However, the ALD coating produces indentations on the surface of the embedded grating. The method to remove the indentations in the excess layer on the embedded grating is discussed. The planarization is done by ion beam etching and the oxygen depletion of the top TiO2 component is fixed by thermal treatment in O2 atmosphere. Finally, we developed an embedded grating with transmission efficiency higher than 97.0 % at 1030 nm wavelength. The experimentally measured efficiency is in excellent agreement with the theoretical value obtained by rigorous coupled wave analysis. In contrast, a conventional, binary grating with the same period reaches only a maximum theoretical efficiency of 92.3 % at the same wavelength in Littrow-configuration.