High-efficiency upward radiation in a slow-light grating beam scanner

Slow-light grating (SLG) is used as a solid-state optical beam scanner, but the efficiency of conventional SLGs has been constrained by unwanted downward radiation. In this study, we developed a high-efficiency SLG consisting of through-hole grating and surface grating, which selectively radiates upward. Via the optimization using the covariance matrix adaptation evolution strategy, we designed a structure showing a maximum upward emissivity of 95% as well as moderate radiation rates and beam divergence. Experimentally, the emissivity was enhanced by 2-4 dB and the roundtrip efficiency was improved by 5.4 dB, which is significant in applications to light detection and ranging.