Femtosecond Sagnac interferometer for the measurement of third-order nonlinear optical susceptibilities

A novel interferometer, a Sagnac interferometer (SI), is presented for the measurement of the difference phase and amplitude spectra induced by photoexcitation with a femtosecond time resolution. The SI has a remarkable advantage of high stability owing to the common-path configuration. In order to separate the phase and amplitude changes, the optical path difference is scanned between the probe and reference pulses making the best use of polarization. This improved polarization-division Sagnac interferometer (PSI) provides a nearly sinusoidal fringe. To demonstrate PSI we examined the nonlinear phase and amplitude changes in CS2 and a GaAs/AlGaAs quantum well (QW) structure. The sepctral feature of the nonlinear dispersion relation in QW can be explained by the blue-shift of the excitonic resonance. The time dependence of these changes is observed to be determined by the exciton lifetime. These experimental results are consistently explained from neutralization of the built-in potential inside the QW sample.