Measurement of the ultrafast polarization dynamics of weak four-wave mixing signals by dual-channel femtosecond spectral interferometry

We demonstrate that a dual-beam version of spectral interferometry together with a well-characterized reference pulse can be used to measure accurately the temporal dynamics of the polarization state of weak ultrafast four-wave mixing emission from semiconductor multiple quantum wells. We completely determine the electric field, including its vectorial nature, by measuring the amplitude and the phase dynamics of two orthogonal components of the field. This technique directly measures quantities in the spectral domain, and the temporal information is obtained only after considerable processing. Nevertheless, we establish the accuracy of the technique by comparing the spectral interferometric results with the corresponding data obtained by direct time-resolved ellipsometry. The data reveal that the four-wave mixing polarization state changes its ellipticity and its orientation dramatically during a single emission. The latter attribute indicates the usefulness of this technique for studying manybody effects, because without them a constant linear polarization state would be expected. (C) 1998 Optical Society of America [S0740-3224(98)03403-1].