Quantum beats between heavy-hole and light-hole excitons in CuI thin films

We have investigated the dynamical properties of coherent exciton states in CuI thin films grown by vacuum deposition using a transmission-type pump–probe technique at 10 K. The absorption spectrum exhibits the energy splitting of the heavy-hole (HH) and light-hole (LH) excitons caused by thermal strains, where the splitting energy is 14.0 meV. In the case of the simultaneous excitation of the HH and LH excitons, the time-domain signal clearly shows the damped oscillation with a frequency of 3.3 THz corresponding to the splitting energy, while such an oscillation signal is not observed for the excitation of the HH or LH exciton only. These results conclusively demonstrate that the observed oscillation signal is due to the quantum beat between the HH and LH excitons. We discuss the pump energy and pump-power dependence of the quantum beat signal from the aspect of the relaxation dynamics of excitons.