Control of coherent phonons with excitonic quantum beats

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We have investigated dynamical interactions between excitonic quantum beats and coherent longitudinal optical (LO) phonons in various GaAs/AlAs multiple quantum wells (MQWs) grown by molecular beam epitaxy. The time-domain signals were detected using time-resolved reflection-type pump-probe spectroscopy with a femto-second pulse laser. The excitonic quantum-beat energy is controlled by the quantum-size effect with a change of layer thickness and a quantum-confined Stark effect with a change of applied electric field. It is found that the intensity of the coherent GaAs-like LO phonon is drastically enhanced under the condition that the quantum-beat energy is resonant with the LO-phonon energy. The systematic results of the layer-thickness dependence and the pump-energy dependence clearly demonstrates that the excitonic quantum beat and coherent LO phonon couples with each other via longitudinal polarization, which results in the formation of a coupled mode, and that the polarization coupling acts as a driving force for the enhancement of the coherent LO phonon. The finding of the coupling between the excitonic quantum beat and coherent LO phonon provides us a novel paradigm of control of the coherent LO phonon with the excitonic quantum beat, which leads to manipulation of coherent phonons. Furthermore, we have demonstrated for the first time that intense and monochromatic THz electromagnetic wave with a frequency of 8.8 THz is generated by the coherent GaAs-like LO phonon in the MQW. In addition, we have investigated the coherent LO phonons in CuI thin films grown by vacuum deposition. It is found that the intensity of the coherent LO phonon is markedly enhanced under an exciton resonant condition of the pump-pulse energy, which suggests that impulsive stimulated Raman scattering dominates the enhancement of the intensity in the CuI thin film.