Development and application of a nonlinear optical characterization technique

We report a sensitive single beam experimental technique for measuring nonlinear refraction and nonlinear absorption in a wide variety of materials. We describe the experimental setup and present a comprehensive theoretical analysis including cases where nonlinear refraction and nonlinear absorption are simultaneously present. In these experiments, the transmittance of a sample through an aperture in the far field is measured as the sample is moved along the propagation path of a focused Gaussian beam. The transmittance curve (Z-scan) of a sample with negative nonlinear refraction has a maximum transmittance (peak) when the sample is before focus and minimum transmittance (valley) when the sample is after focus. For a sample with positive nonlinear refraction, the transmittance curve has a valley-peak signature. Thus, the sign of nonlinear refraction is readily given from a Z-scan experiment. The magnitude of the nonlinearity is given from a simple relation between the transmittance changes and the on-axis phase change. Employing this technique, a sensitivity of {dollar}lambda{dollar}/300 phase distortion is demonstrated in n{dollar}sb{lcub}rm 2{rcub}{dollar} measurements of a BaF{dollar}sb2{dollar} sample at 0.532 {dollar}mu{dollar}m. When nonlinear refraction is accompanied by nonlinear absorption, as in the case of two-photon absorbing semiconductors, we can separately measure the two effects by performing a second Z-scan with the aperture removed. In these semiconductors, we use this technique to separately measure nonlinear refraction due to bound electrons, two photon absorption (2PA) coefficients, and nonlinear refraction arising from the free carriers generated via 2PA.; The Z-scan is extended to measure nonlinear refraction in "thick" media. In this case the sample is considered as a stack of individual thin lenses. Using nanosecond and picosecond pulses at 10.6 {dollar}mu{dollar}m on a thick sample fo CS{dollar}sb2{dollar}, the thermal effect and reorientational Kerr effect of CS{dollar}sb2{dollar} are measured.