Ultrafast tuning and probing of two-dimensional silicon photonic crystals

The linear and nonlinear optical properties of two-dimensional silicon photonic crystals are studied using ultrafast laser pulses. Third order optical nonlinearities were used to tune the long (1.6 mum) and short wavelength (1.3 mum) band-edges of a stop-gap. From the pump-probe reflectivity experiments, a 140 fs, 1.5 mum pump pulse was observed to induce optical tuning of the 1.3 mum band-edge via the Kerr effect whereas a 1.76 mum pulse induced tuning of the 1.6 mum band-edge via both Kerr and Drude effects with the latter related to two-photon induced generation of free carriers. The optical tuning response of the photonic crystal was further shown to be influenced by the spatial characteristics of the pump eigenmode. This was demonstrated using 1.5 mum pump pulses and monitored via time-resolved reflectivity measurements at the 1.9 mum band-edge. Electron-hole pairs generated by two-photon absorption of the pump pulses spatially modify the refractive index of the crystal. For a pump eigenmode producing an initial 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, surface recombination alone controls the time evolution of the probed mode. The phase characteristics and temporal shape of reflected pulses were shown to be strongly influenced by coherent Bragg scattering and incoherent, multiple scattering in the photonic crystal. The shape of Bragg reflected pulses differs insignificantly from the incident pulse as expected, however for pulses outside the stop-gap, multiple scattering distorts the specular pulse shape and the pulse width is broadened by as much as 46% depending on sample location. On the gap-edge, Bragg scattering dominates over multiple scattering and the reflected pulse shape is mostly influenced by the asymmetric reflectivity spectrum and the input pulse characteristics such as its bandwidth and chirp.