  Nonlinear propagation experiments in GaAs photonic crystal waveguides (PCW)
were performed, which exhibit a large enhancement of third order
nonlinearities, due to light propagation in a slow mode regime, such as
two-photon absorption (TPA), optical Kerr effect and refractive index changes
due to TPA generated free-carriers. A theoretical model has been established
that shows very good quantitative agreement with experimental data and
demonstrates the important role that group velocity plays. These observations
give a strong insight into the use of PCWs for optical switching devices.
