International audienceKerr and four-wave mixing spectroscopy is shown to be a powerful technique to quantify the strong enhancement of the third-order optical nonlinear susceptibilities at the band edge of photonic crystals. Local field factors of about 5 are demonstrated for crossed Kerr effect and a narrow resonance peak observed for the conjugate reflectivity. Moreover, a reduction of the effective nonlinear susceptibility of the four-wave mixing process with increasing pump intensities is measured, which is due to different Kerr-induced blueshifts of the band edge for forward and backward pump beams and signal and conjugate beams. This observation definitely demonstrates the need for considering all the nonlinear processes for the optimization of nonlinear photonic crystals for a given application in optical signal processin

We present a modeling of a degenerate four-wave-mixing nonlinear process in one-dimensional photonic crystals. The model is based on the nonlinear extension of the transfer-matrix description of propagation in the structure. The influence of light localization, near the band edge of the structure, on the enhancement of the phase-conjugate reflectivity is studied. The phase-conjugate reflectivity is shown to increase as the eighth power of the number of layers with an additional large dependence on the index contrast of the structure. In both cases the enhancement is accompanied by a strong reduction of the resonance width, which may lead to some limitation of the enhancement when ultrashort pulses are used. A strong influence of the losses on the nonlinear efficiency of the structure is also predicted with a great importance of scattering losses at the multiple interfaces of the structure.

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 demonstrate 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.2