Subwavelength Dielectric Grating-based Broadband Reflectors and Narrowband Transmission Filters.

Abstract

This dissertation includes investigations on infrared mirrors and filters for next generation thermal imaging and spectrometer-on-a-chip applications operating in the long-wavelength infrared (LWIR, 8-15 microns) spectral region. Using the concept of high index-contrast gratings, which have been studied in the near-infrared spectrum, we have demonstrated broadband reflectors and narrowband transmission filters using silicon/silicon-dioxide and silicon/air gratings. Using finite element method simulations, photolithography and Fourier transform infrared spectroscopy, we designed, fabricated, characterized, and analyzed structure response.

We provide the first demonstration of broadband reflectance in the LWIR using a high contrast grating, with the silicon/silicon-dioxide grating exhibiting reflectance greater than 70 percent between 13 and 16 microns. We subsequently developed a suspended silicon/air grating to eliminate the lossy silicon dioxide layer, which was limiting the reflectance of the silicon/silicon-dioxide system. LWIR broadband reflectors operating at normal and oblique incidence (15 degrees) with reflectances greater than 85 percent and 90 percent, respectively, are demonstrated.

These studies are followed with a group theoretical analysis showing strong plane wave coupling to guided modes supported by the grating is responsible for the broadband reflectance at normal incidence. We show that other modes are symmetry protected at normal incidence, preventing plane wave coupling. Off-normal incidence relaxes the selection rules and enables weak coupling to these previously protected modes. We use this analysis to predict narrowband transmission filtering capabilities by coupling to transverse magnetic and transverse electric mode sets supported by the grating.

Finally, we experimentally demonstrate and characterize narrowband transmission filtering capabilities based on the optimized broadband reflector and the group theoretical analysis. We use finite element modal analysis to determine the grating dispersion relations and show the transmission bands agree with the grating’s supported mode frequencies. Furthermore, the filtered wavelength is demonstrated to predictably change with the grating period, making it attractive for possible hyperspectral imaging applications.