Enhancement of terahertz radiation from coherent optical phonons via impulsive interference of excitons in GaAs/AlAs multiple quantum wells

We have investigated the terahertz (THz) radiation from the coherent GaAs-like longitudinal optical (LO) phonons at room temperature in GaAs/AlAs multiple quantum wells utilizing an optical gating method with an ultrashort pulse laser. It has been found that the intensity of the THz radiation with a long dephasing time is markedly enhanced under the condition that the energy spacing between the fundamental heavy-hole and light-hole excitons is almost equal to the LO-phonon energy. This fact demonstrates that the impulsive excitonic interference, which produces no beating oscillation at room temperature owing to a very fast dephasing time, acts as a driving force for the coherent LO phonon emitting the intense THz radiation.