Large refractive index changes have been measured at the band edge frequency of 1D photonic crystals. Results concerning both thin and thick samples of high and low refractive index contrast respectively are presented. The very large value of the refractive index changes obtained at moderate pump powers thanks to the strong enhancement of the local intensity inside the photonic crystal open the way to very small volume devices for optical signal processing. However, time-resolved experiments

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... onstrate the photo-generation of high free carrier densities through two-or even three-photon absorptions which are shown to be also strongly enhanced at the band edge of the photonic crystal. This drawback may the most probably be circumvented by using lower pump intensities in photonic crystals showing narrower resonances.