Three Photon Absorption and the Nonlinear Index of Refraction in GaAs

We demonstrate and quantitatively measure the previously unknown nonlinear absorption and refraction in GaAs in the spectral range from 1200 to 2630 nm using the Z-Scan Technique. We measured the transmission of the femtosecond pulses tightly focused on a 350-$\mu$m-thick GaAs wafer as a function of incident pump intensity by scanning the sample position with respect to the focal plane. The intensity dependent two-photon and three photon absorption coefficients are obtained by analyzing the data. The nonlinear index of refraction, n$^{2}$, is acquired by the closed-aperture z-scan in which a small aperture in the far field is inserted and the self-lensing was measured by the change in on-axis transmittance. Scaling rules based on a quasi-dimensional analysis confirms the experimental results.