Abstract
The characteristics of terahertz (THz) electromagnetic waves emitted from coherent GaAs-like longitudinal optical (LO) phonons in GaAs/AlAs multiple quantum wells were investigated. It was found that the amplitude of the THz wave from the coherent LO phonon is increased linearly with increasing number of periods of the quantum well. This result demonstrates that the THz wave originates from the superposition of the oscillatory polarization induced by the coherent GaAs-like LO phonon in each quantum-well layer. The amplitude of the THz wave is proportional to the excitation-power density below about 0.4 μJ∕cm2, and its radiation power reaches about 1 μW at the excitation-power density of 2 μJ∕cm2. Moreover, the amplitude of the THz wave shows a sinusoidal change with the polarization angle of the pump pulse. These results indicate that the coherent LO phonon emitting the THz wave is generated through the combination of the following two mechanisms: impulsive stimulated Raman scattering and instantaneous screening of the surface electric field.
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