Two-dimensional Fourier transform spectroscopy, an extension of four-wave mixing (FWM), is able to resolve numerous aspects of many-body effects and higher-order Coulomb interactions that contribute to the ultrafast dynamics of quantum wells and bulk GaAs resonances. Coherent oscillations between heavy-hole and light-hole excitons - or quantum beats - can be unfolded from the exciton populations by Fourier transforming FWM data with respect to two time-axes. Excitation conditions such as pulse ordering, polarization, tuning and pulse energy can isolate Feynman pathways and highlight the coherent many-body correlations, including those from biexcitons. In addition, the bulk exciton and continuum states are studied more carefully for their dynamics.
Two-dimensional (2D) Fourier-transform optical spectroscopy is demonstrated on GaAs quantum wells. This technique represents a highly enhanced version of transient four-wave mixing (FWM). The 2D spectra are generated by measuring four-wave mixing signal fields as a function of the time after the third pulse and the time delay between the first two excitation pulses. Signal fields are measured, including phase information, by spectral interferometry. Active stabilization of the interferometer allows us to measure the phase of the emitted signal as a function of the phase between the first two excitation pulses. This enables the implementation of the Fourier transform analysis. Our spectra show the light-hole and heavy-hole exciton transitions on the main diagonal as well as coupling between those two levels as off-diagonal peaks.