Metallo-dielectric photonic crystals for infrared applications

Photonic crystals are structures with spatially periodic variations in optical properties. These periodic variations give rise to optical band structures (analogous to electronic band structure in solids) which can possess photonic band gaps: bands of frequency for which electromagnetic waves cannot propagate through the photonic crystal. These photonic band gaps correspond to bands of high reflection when the structure is used as an optical filter.; I consider metallo-dielectric photonic crystals (MDPCs) in which the variation in optical properties is produced by a metal structure embedded in dielectric. The large index contrast between metals and ordinary dielectrics gives rise to very broad photonic band gaps (and correspondingly wide and deep stop bands) in filters containing relatively few layers compared to all-dielectric photonic crystals, even when the metal is fabricated in thin, planar layers. These planar structures are similar to frequency selective surfaces (FSSs), but by closely spacing the metal layers in an MDPC, to exploit the three-dimensional photonic crystal structure, one may produce broad-band filters containing simple, easy-to-fabricate, low-Q metallic elements.; I have experimentally demonstrated the fabrication of a dual stop-band MDPC filter designed for the 3–5 and 8–12 μm infrared atmospheric windows. The filter performance, apparently limited only by avoidable dielectric losses, offers very strong rejection over broad bands, and over a range of incident angles. A finite-difference time-domain (FDTD) algorithm is used to simulate filter designs and accurately predicts the filter performance.; I also discuss the application of MDPC filters to spectral control for high-efficiency thermophotovoltaic (TPV) power generators. Simulations and experimental prototypes of MDPCs consisting of multiple inductive meshes indicate the possibility of constructing a low-loss filter with a broad high-transmission passband combined with a sharp transition to an extremely broad high-reflection stopband at long wavelengths. In order to rapidly evaluate these MDPC filter designs, I have developed a “hybrid” simulation technique, combining FDTD calculations with the matrix techniques of thin-film filter design, which appears to offer the possibility of efficiently and accurately optimizing inductive mesh filter designs. I present experimental results on microwave scale models and discuss their agreement with hybrid computations.