DETERMINATION OF 2-PHOTON-GENERATED FREE-CARRIER LIFETIME IN SEMICONDUCTORS BY A SINGLE-BEAM Z-SCAN TECHNIQUE

We report a method for determining photo-excited free-carrier recombin ation time in semiconductors. This method is based on the simulation o f 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 dur ation, the two-photon absorption coefficient, the nonlinear refractive index, and the refractive index change induced by an electron-hole pa ir 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 a ccurately extracted to be 7.0 +/- 1.0, 2.8 +/- 0.3, and 18 +/- 4 ns in semiconductors ZnSe, ZnO, and ZnS, respectively.