Influence of eigenmode characteristics on optical tuning of a two-dimensional silicon photonic crystal

Ultrafast pump-probe measurements on a two-dimensional (2-D) silicon photonic crystal are used to show the influence of a pump eigenmode’s spatial characteristics on the crystal’s optical tuning properties. Electron-hole pairs are generated by two-photon absorption of 130 fs, 1.5 μm pulses via different crystal eigenmodes and spatially modify the crystal’s refractive index via Drude effects; characteristics of an eigenmode near 1.9 μm are monitored via time-resolved reflectivity. For a pump eigenmode producing an inhomogeneous carrier distribution, diffusion is responsible for an initially fast (10 ps time scale) component of the recovery of the probe reflectivity with surface recombination accounting for a slower response (700 ps time scale) after the carriers are nearly uniformly distributed within the silicon backbone. When carriers are initially generated homogeneously in the silicon, surface recombination alone controls the time evolution of the probed mode. A simple expression is developed for the mode-dependent two photon absorption in photonic crystals and a simple perturbation technique is used to treat modifications to photonic crystal eigenmodes by small refractive index changes.