  We present pump-probe measurements of an all-optical photonic crystal switch
based on a nanocavity, resolving fast coherent temporal dynamics. The
measurements demonstrate the importance of coherent effects typically neglected
when considering nanocavity dynamics. In particular, we report the observation
of an idler pulse. The measurements are in good agreement with a theoretical
model that allows us to ascribe the observation to oscillations of the free
carrier population in the nanocavity. The effect opens perspectives for the
realization of new all-optical photonic crystal switches with unprecedented
switching contrast.
