Surface-engineered coatings for multispectral infrared mirrors

This paper summarizes the research and development of multispectral infrared mirror designs based on the surface patterning of coating materials. Three-dimensional structures can be etched into the coating materials to create infrared devices capable of meeting a wide variety of different multispectral performance requirements. The design methodology developed in this work is based on optimizing the patterning of a singly periodic three-dimensional structure (grating) that is etched directly into a coating material. Each of these designs consists of a unit cell that possesses a single 2-D strip with various heights, thicknesses, and sidewall profiles. The goal of this work is to develop mirror designs that exhibit high reflectance properties (R>85%) in the spectral regions of interest. A robust genetic algorithm (GA) optimization technique is employed to modify the height, thickness, and sidewall profile of the grating structure in order to meet these design goals. In addition, the optimizations take into account the fabrication constraints needed to guarantee that the resulting geometries can be reproduced experimentally. A prototype device based on an amorphous silicon (a-Si) grating with an optimized sidewall profile backed by a quartz substrate was fabricated.