Robustness of monolithic high-contrast grating focusing mirrors to random variations in geometrical parameters

One of the critical challenges in designing focusing mirrors based on monolithic high-contrast gratings (MHCGs) is the precise selection of geometrical parameters for each individual stripe, to ensure that the reflected light acquires the required phase shift for focusing at the focal point. However, achieving such precision in practice is hindered by fabrication effects, such as the proximity effect and etching lag, which can significantly alter the final structure. In this article, we numerically investigate the impact of random disruptions in stripe width, non-uniform etching depth caused by variations in air trench widths, and missing grating stripes on the reflectivity and light intensity at the focal point. The analysis is conducted for GaAs gratings designed for 980 nm and TM polarization. Our results show that variations in geometrical parameters degrade both reflectivity and light intensity at the focal point, with light intensity being more severely affected. Despite these variations, the MHCG focusing mirrors exhibit a high degree of resilience, maintaining robust optical performance disruptions that mimic realistic fabrication imperfections.