A Suspended Metamaterial Mirror for Hyperspectral Shortwave Infrared Fabry-Perot Filters

This paper reports the development of a broadband highly reflective and structurally flat large-area 2-D subwavelength grating (SWG) reflector, based on a tensioned single-layer silicon metamaterial membrane. Rigorous coupled-wave analysis is adopted to design the metamaterial reflector, resulting in a wavelength range of 600 nm (1.9-2.5 <inline-formula> <tex-math notation="LaTeX">$\mu $</tex-math> </inline-formula>m) with <inline-formula> <tex-math notation="LaTeX">$&gt;$</tex-math> </inline-formula>99% reflectivity, which represents the largest reported fractional bandwidth <inline-formula> <tex-math notation="LaTeX">$\Delta \lambda $</tex-math> </inline-formula>/<inline-formula> <tex-math notation="LaTeX">$\lambda_{\mathrm{c}}$</tex-math> </inline-formula> of 27%. Effects of design parameter deviations on the SWG reflector reflectivity are studied, showing that the reflector design has good fabrication tolerances. A freestanding 2-mm dimension 2-D SWG reflector has been fabricated with high precision, and optical measurements indicate polarization-independent average reflectivity of 99% over the wavelength range of 1.91-2.49 <inline-formula> <tex-math notation="LaTeX">$\mu $</tex-math> </inline-formula>m, which agrees well with the modeled result. This paper also, for the first time, examines the surface flatness characteristics of a suspended single-layer 2-D SWG reflector. With a tensile stress of 10 <inline-formula> <tex-math notation="LaTeX">$\pm$</tex-math> </inline-formula> 5 MPa in the silicon grating membrane, the freestanding reflector achieves nanometer-scale surface flatness, which provides a significant advantage over multilayer DBRs requiring stress-balancing. Furthermore, a hyperspectral Fabry-Perot shortwave infrared filter based on a top suspended 2-D SWG reflector and a bottom 4-pair Si/SiO<inline-formula> <tex-math notation="LaTeX">$_{2}$</tex-math> </inline-formula> distributed Bragg reflector (DBR) has been experimentally demonstrated for the first time. The filter is measured to have a peak transmission of 80% with a narrow full-width at half-maximum (FWHM) of about 4 nm. This spectral resolution is one order of magnitude higher than that of other reported MEMS/DBR-based Fabry-Perot filters and is well-suited to highly demanding hyperspectral shortwave infrared imaging applications.2022-0060