Coherent two-exciton dynamics measured using two-quantum rephasing two-dimensional electronic spectroscopy

We use fifth-order two-dimensional electronic spectroscopy to measure coherent four-particle dynamics in a semiconductor nanostructure. By using optical polarization control in two-quantum measurements enabled by the COLBERT spectrometer, we separate coherent signals due to bound biexcitons and unbound two-exciton correlations. The rephasing nature of the measurement allows us to separate homogeneous from inhomogeneous contributions to the two-quantum line shapes. We find that, unlike the bound biexciton state, the energy of the unbound pair and its homogeneous linewidth depend on the laser fluence. Simulations using an extended phenomenological model help determine the primary interaction mechanism that leads to the formation of the unbound exciton pair; the model also indicates that seventh-order interactions contribute to the measured spectra under high pulse fluences.