Measurements of the nonlinear refractive index of freestanding porous silicon layers at different wavelengths

Measurements of the nonlinear refractive index n(2) of a high-porosity freestanding porous silicon sample are reported at different laser wavelengths by means of the z-scan technique. The results show a negative below-gap nonlinear refractive index, so that thermal contributions to the observed effects can be reasonably excluded. Values for n(2) of the order of 10(-9) esu are here reported. These results are compatible with the hypothesis of an enhancement of optical nonlinearities due to quantum confinement of carriers in nanocrystals present in the material.