Slow dephasing and local field effects of exciton-biexciton transition in β-ZnP2 detected by femtosecond degenerate four-wave mixing

In femtosecond degenerate four-wave mixing (FWM) spectroscopy for the exciton and biexciton system in a semiconductor β-ZnP2, 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 β-ZnP2, which is made observable due to the narrow homogeneous linewidth of the exciton-biexciton transition.