Determination of two-photon-generated free-carrier lifetime in semiconductors by a single-beam Z-scan technique

We report a method for determining photo-excited free-carrier recombination time in semiconductors. This method is based on the simulation of single-beam Z scans with laser pulses of two different widths: 25 ps and 7 ns (FWHM). By conducting Z scans with laser pulses of 25-ps duration, the two-photon absorption coefficient, the nonlinear refractive index, and the refractive index change induced by an electron-hole pair excited by two-photon absorption are first determined unambiguously. While using these nonlinear parameters in Z scans obtained with 7-ns laser pulses, the lifetimes of two-photon-excited free carriers are accurately extracted to be 7.0 +/- 1.0, 2.8 +/- 0.3, and 18 +/- 4 ns in semiconductors ZnSe, ZnO, and ZnS, respectively.