This article presents a detailed exploration of the optical characteristics of various one-dimensional photonic crystal structures designed for use as a means of improving the efficiency and power density of thermophotovoltaic ͑TPV͒ devices. The crystals being investigated have a ten-layer quarter-wave periodic structure, and are based on Si/ SiO 2 and Si/ SiON material systems. For TPV applications the crystals are designed to act as filters, transmitting photons with wavelengths below 1.78 m to a GaSb photodiode, while reflecting photons of longer wavelengths back to the source of thermal radiation. In the case of the Si/ SiO 2 structure, the Si and SiO 2 layers were designed to be 170 and 390 nm thick, respectively. This structure was fabricated using low-pressure chemical vapor deposition. Reflectance and transmittance measurements of the fabricated Si/ SiO 2 photonic crystals were taken from 0.8 to 3.3 m for both polarizations and for a range of incident angles. Measurement results were found to correlate well with simulation results for the ideal structure. Measurement results were used to predict the TPV system efficiency, power density and spectral efficiency using an ideal thermodynamic model of a TPV system.
Air trench structures for reduced-size bends in lowindex contrast waveguides are proposed. To minimize junction loss, the structures are designed to provide adiabatic mode shaping between low-and high-index contrast regions, which is achieved by the introduction of "cladding tapers." Drastic reduction in effective bend radius is predicted. We present two-dimensional (2-D) finite-difference time-domain/effective index method simulations of bends in representative silica index contrasts. We also argue that substrate loss, while present, can be controlled with such air trenches and reduced to arbitrarily low levels limited only by fabrication capabilities. The required trench depth, given an acceptable substrate loss, is calculated in three dimensions using an approximate equivalent current sheet method and also by a numerical solver for full-vector leaky modes. A simple, compact waveguide T-splitter using air trench bends is presented. Index Terms-Air trench bend (ATB), cladding taper, enhanced lateral mode confinement, low-index contrast, sharp bend loss, silica waveguide.
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