Ultrafast time-resolved quantum beats in the polarization state of coherent emission from quantum wells.

Dual-channel spectral interferometry is used to measure the ellipticity, the orientation of the polarization ellipse, and the sense of rotation of the four-wave-mixing signal from a GaAs-AlGaAs multiple quantum well. Each parameter is observed to oscillate dramatically at the heavy-hole-light-hole quantum beat frequency. During each beat period (in the strong quantum beat regime), the ellipticity oscillates twice between linear and almost-circular polarization, the orientation of the polarization ellipse rotates through a complete 180 degrees , and the sense of rotation changes from left- to right-circular polarization.