Nonresonant Kerr effect in microporous silicon: Nonbulk dispersive behavior of below band gap chi((3))(omega)

Although in recent years resonant optical nonlinearities in quantum confined silicon generated significant interest, no experimental work has been dedicated to the nonresonant regime, which is the range of interest for optical switching applications. In this article we report a systematic investigation on the different types of optical nonlinearities which can be activated in quantum-sized silicon. In particular, original measurements of nonresonant nonlinear refraction (Kerr effect) are reported at different wavelengths, spanning the infrared middle-gap range. The dispersive scaling rule and values of the nonlinear refractive index are clearly incompatible with those of three-dimensional semiconductors. Hence the quantum confined density of states plays a key role in determining the frequency dispersion of the nonresonant third-order susceptivity chi((3))(omega). Also, this suggests the need of further investigation of the influence of quantum-size effects (and related density of states modifications) on below-gap chi((3)).