Identification of competing ultrafast all-optical switching mechanisms in Si woodpile photonic crystals

We present a systematic study of ultrafast all-optical switching of Si photonic bandgap woodpile crystals using broadband tunable nondegenerate pump-probe spectroscopy. At pump-probe coincidence, we investigate the behavior of the differential reflectivity at the blue edge of the stop band for a wide range of pump and probe frequencies. Both dispersive and absorptive features are observed from the probe spectra at coincidence. As the pump frequency is tuned through half the electronic bandgap of Si, the magnitude of both these features increases. For the first time, to the best of our knowledge, we unambiguously identify this dispersive effect with the electronic Kerr effect in photonic crystals and attribute the absorptive features to nondegenerate two photon absorption. The dispersive and absorptive nonlinear coefficients are extracted and are found to agree well with the literature. Finally, we propose a nondegenerate figure of merit, which defines the quality of switching for all nondegenerate optical switching processes.

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