  We investigate the nonlinear response of GaAs-based photonic crystal cavities
at time scales which are much faster than the typical thermal relaxation rate
in photonic devices. We demonstrate a strong interplay between thermal and
carrier induced nonlinear effects. We have introduced a dynamical model
entailing two thermal relaxation constants which is in very good agreement with
experiments. These results will be very important for Photonic Crystal-based
nonlinear devices intended to deal with practical high repetition rate optical
signals.
