Silicon-based photonic crystal slabs:: Two concepts

We compare theoretically two different concepts of vertical light confinement in two-dimensional (2-D) silicon photonic crystals. Light guidance obtained by variation of the refractive index in an SiO2/Si/SiO2 sandwich structure leads to a complete bandgap for all directions and polarizations with a gap-midgap ratio of about 8.5% and a bandgap for even modes only of about 27%. The complete bandgap is 50% smaller than for 2-D photonic crystals due to the lower confinement of light in the high-index material silicon and polarization mixing. Light guidance obtained by a vertical variation of the porosity, i.e., pore radius, leads under optimum conditions to a bandgap for even modes only, with a gap-midgap ratio of about 10%. The feasibility of such a structure is shown for macroporous silicon where the pore diameter can be varied with depth. In both cases, the optimum slab thickness can be approximated by classical waveguide optics, reducing the parameter space for optimization.