Suspended Si/air high contrast subwavelength gratings for long-wavelength infrared reflectors

We report broadband reflectance in the long-wavelength infrared (LWIR, 8-12 mu m) utilizing suspended-Si, high-index-contrast subwavelength gratings (HCGs). Iterative design optimization using finite element analysis software has been performed accounting for silicon's wavelength-dependent index of refraction and extinction coefficient. Grating arrays were fabricated using commercial silicon-on-insulator (SOI) substrates, photolithography and reactive ion etching; subsequent selective wet etching of SiO2 was used to provide suspended Si/air gratings. Fourier transform infrared (FTIR) spectroscopy demonstrates broadband, polarization-dependent reflectance between 8.5 and 12 mu m, which agrees with the simulated response.