Intense coherent longitudinal optical phonons in CuI thin films under exciton-excitation conditions

We have investigated the dynamical properties of the coherent longitudinal optical (LO) phonon in CuI thin films grown on a NaCl substrate by vacuum deposition. The intense coherent LO phonon in the CuI thin film is observed under the exciton-excitation conditions. Moreover, the pump-energy dependence of the amplitude of the coherent LO phonon shows peaks at the heavy-hole and light-hole exciton energies. The enhancement of the coherent LO phonon under the exciton-resonance condition is much larger than that in an ordinary semiconductor quantum well system such as a GaAs/AlAs one. These facts demonstrate that the intense coherent LO phonon is generated under the exciton-excitation condition in a material with a strong exciton-phonon interaction such as CuI.