Slow dephasing and local field effects of exciton–biexciton transition in image detected by femtosecond degenerate four-wave mixing

In femtosecond degenerate four-wave mixing (FWM) spectroscopy for the exciton and biexciton system in a semiconductor image, we have observed that the spectrally resolved FWM signals from the exciton–biexciton transition exhibit a specific time profile consisting of a growing up and decay for negative and positive delay between two excitation pulses, respectively. The decay profile indicates unexpectedly much slower dephasing of the exciton–biexciton transition compared to 1s exciton polariton. The time profile is interpreted as the manifestation of local field renormalization due to many-body Coulomb correlation among carriers in image, which is made observable due to the narrow homogeneous linewidth of the exciton–biexciton transition.