We demonstrate that spectral interferometry can be used together with a well-characterized reference pulse to directly determine the spectral amplitude and spectral phase of coherent four-wave-mixing (FWM) emission from multiple quantum wells. The temporal amplitude and phase are then readily obtained by an inverse Fourier transformation. We illustrate that it is necessary to accurately determine the spectral phase, as well as the spectral amplitude, in order to correctly determine the temporal behavior. We subsequently use this technique to measure discontinuities in the temporal phase. Instabilities in the phase occur and phase jumps of $\sim \pi$ are produced when both the real and imaginary parts of the FWM field are zero. These jumps occur at the minima in the quantum beats, and this condition requires the excitation of approximately equal numbers of heavy- and light-hole excitons.

• Received 5 May 1997

DOI:https://doi.org/10.1103/PhysRevB.56.9738