Influence of free carrier refraction due to linear absorption on Z-scan study of porous Si

We analyze the limitations imposed by free carrier refraction induced by the sample linear absorption in determination of nonlinear refractive index in porous silicon by the Z-scan technique. By simulation we show that for picosecond laser pulses there is a strong contribution to the Z-scan signal due to free carriers generated by linear absorption even in highly transmitting sample. By simulating various experimental conditions we show that for femtosecond pulses the contribution due to bound electronic nonlinearity is significant for samples with small two-photon absorption coefficient.