  We present ultrafast all-optical switching measurements of Si woodpile
photonic band gap crystals. The crystals are spatially homogeneously excited,
and probed by measuring reflectivity over an octave in frequency (including the
telecom range) as a function of time. After 300 fs, the complete stop band has
shifted to higher frequencies as a result of optically excited free carriers.
The switched state relaxes quickly with a time constant of 18 ps. We present a
quantitative analysis of switched spectra with theory for finite photonic
crystals. The induced changes in refractive index are well described by a Drude
model with a carrier relaxation time of 10 fs. We briefly discuss possible
applications of high-repetition rate switching of photonic crystal cavities.
