Ultra-thin sputter-deposited infrared rugate mirror for enhancing solar-to-thermal energy conversion

A dielectric mirror with high infrared reflection and high visible transmission, based on an easily fabricated stepped index rugate filter structure, is presented. Its fabrication involves sputtering depositions, using only two targets, to make five different material compositions. The ultra-wide reflection band is tunable in both position and width, adapting the thickness of the layers and eventually introducing chirped layers. When applied to evacuated solar thermal devices, efficiency improvements of up to 30% can be achieved, making this mirror an attractive solution for reducing radiative losses through the cold-side photon recycling mechanism.