MEASUREMENT OF THE ULTRAFAST POLARIZATION DYNAMICS OF WEAK 4-WAVE-MIXING SIGNALS BY DUAL-CHANNEL FEMTOSECOND SPECTRAL INTERFEROMETRY

We demonstrate that a dual-beam version of spectral interferometry tog ether with a well-characterized reference pulse can be used to measure accurately the temporal dynamics of the polarization state of weak ul trafast four-wave mixing emission from semiconductor multiple quantum wells. We completely determine the electric field, including its vecto rial nature, by measuring the amplitude and the phase dynamics of two orthogonal components of the field. This technique directly measures q uantities in the spectral domain, and the temporal information is obta ined only after considerable processing. Nevertheless, we establish th e accuracy of the technique by comparing the spectral interferometric results with the corresponding data obtained by direct time-resolved e llipsometry. The data reveal that the four-wave mixing polarization st ate 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 const ant linear polarization state would be expected. (C) 1998 Optical Soci ety of America [S0740-3224(98)03403-1].