Light induced permanent modifications of the nonlinear optical properties of semiconductor doped glasses

The nonlinear refractive index, n₂, and the two photon absorption coefficient, β, of semiconductor doped glasses were experimentally investigated in the region of two photon absorption. We demonstrated that the magnitude and sign of the measured n₂ strongly depend on the previous exposure of the sample to the laser radiation. For fresh (not exposed) samples the nonlinearity is negative and large. After repeated measurements in the same spot, the magnitude of n₂ decreases, eventually becomes positive, and saturates to a value that approximates the value expected from bulk semiconductor theory.