A Subwavelength-Grating-Mirror-Based MEMS Tunable Fabry-Perot Filter for Hyperspectral Infrared Imaging

This paper reports on the first demonstration of a MEMS tunable hyperspectral Fabry-Perot filter based on a suspended tensile-strained single-layer dielectric two-dimensional subwavelength grating (2-D SWG) mirror. Rigorous coupled-wave analysis is used to design the 2-D SWG mirror, and the fab-ricated mirror demonstrates a very high reflectivity of 98.7-99.3% over a broad wavelength range of 500 nm (2.0-2.5 mu m), which represents an unprecedented fractional bandwidth A1/1c of 23%. The surface profiles of the actuated 2-D SWG mirror are examined for the first time. The movable SWG mirror shows nanometer-scale surface flatness across an 800-mu m dimension area both at static state and during actuation. Moreover, a proof-of-concept MEMS tunable hyperspectral shortwave infrared Fabry-Perot filter based on a top suspended 2-D SWG mirror and a bottom 3-pair Si/SiO2 DBR has been fabricated. With an actuation voltage of 15 V, a large wavelength tuning range of 210 nm from 2.51 mu m to 2.3 mu m has been achieved. In addition, over the entire actuation range the tunable filter exhibits peak transmission higher than 70% with an extremely narrow full-width at half-maximum of 6 nm. This spectral resolution is one order of magnitude higher than that of the previously reported DBR-based tunable Fabry-Perot filters and fulfills the optical requirement of 81/1c < 1% for hyperspectral infrared imaging applications. Overall, the presented 2-D SWG mirror based tunable filter shows a high figure-of-merit (tuning range / linewidth) of 35, which is a more than 40% increase compared to those of DBR-based tunable filters.