  We present a systematic study of ultrafast all-optical switching of Si
photonic band gap woodpile crystals using broadband tunable nondegenerate
pump-probe spectroscopy. At pump-probe coincidence, we investigate the behavior
the differential reflectivity at the blue edge of the stopband 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 we unambiguously identify this dispersive effect
with the electronic Kerr effect in photonic crystals, and attribute the the
absorptive features to nondegenerate two photon absorption. The dispersive and
absorptive nonlinear coefficients are extracted, and are found to agree well
with literature. Finally, we propose a nondegenerate figure of merit (NFOM),
which defines the quality of switching for all nondegenerate optical switching
processes.
