Nonlinear optical refraction of free-standing porous silicon layers

Measurements of the nonlinear refractive index of free-standing porous silicon samples by means of the Z-scan technique are reported. A sensitive enhancement of the optical nonlinearity is found with respect to bulk silicon. The results are in agreement with a simple theoretical model which is also presented and discussed, that attributes the enhancement to quantum confinement of carriers. The negative sign of nonlinear refractive index suggests that optical Stark effect gives the dominating contribution to the nonlinearity. It is also found that the nonlinearity is mainly refractive, which is very promising in order to use porous silicon for nonlinear optical applications such as power limiting or optical switching.