Radiation of Terahertz Electromagnetic Waves from Coherent Longitudinal Optical Phonons in Multiple Quantum Wells

The radiation of terahertz electromagnetic waves from coherent longitudinal optical (LO) phonons in multiple quantum wells (MQWs) under illumination of a femtosecond pulse laser is reviewed from the viewpoint of the enhancement mechanism at room temperature, which is essential in applications to optoelectronics in a terahertz region. Two enhancement mechanisms in MQWs are focused on. One is that the impulsive quantum interference between the heavy-hole (HH) and light-hole (LH) excitons acts as a driving force to the coherent GaAs-like LO phonon in GaAs/AlAs MQWs under a condition that the splitting energy between the HH and LH excitons resonates with the LO-phonon energy. The other is that a piezoelectric field due to the lattice-mismatch strain along a [11n] direction produces an initial polarization of the GaAs-like LO phonon via an electric susceptibility, which results in considerable enhancement of the intensity of the terahertz radiation from the coherent LO phonon, in (11n)-oriented GaAs/In_0.1Al_0.9As strained MQWs.