High-performance surface-engineered gradient refractive index (GRIN) coatings

Abstract

New missions and technical systems require lightweight, high performance, wide-field of view (W-FOV) infrared (IR) imaging systems. Traditionally, multilayer antireflective (AR) coatings are utilized with optical components to facilitate these high performance demands. However, fundamental limits in these multilayer AR coatings currently prevent the extremely high broadband transmission (<95%) and W-FOV (<100°) requirements of next generation IR imaging systems from being realized. Furthermore, there is restricted availability of suitable thin film IR materials with high index contract used in these AR coatings, preventing tuning and broad application of the technology. By contrast, surface-engineered gradient reflective index (GRIN) films afford a substrate and application independent means of generating and tuning transmissive and W-FOV properties in optical components. Herein, we present efforts toward designing devices with highly AR properties from GRIN surfaces. GRIN surfaces are generated through lithographic patterning of optical surfaces and dry etching processes to generate dense arrays of air holes. The density of these air holes offer a mean to tune the index of refraction of the optical surface, providing highly AR properties in a tunable optical range. State-of-the-art laser writing technology enables us to achieve features of 500 nm and below with high throughput (<1.25 min write time per 1 cm2 patterned). Control of depth etch through standard etching processes (Al2O3 hard mask, deep etch using Bosch-process type or other dry etch) allows for fully tunable GRIN films.