Terahertz generation from coherent optical phonons in a biased GaAs photoconductive emitter

摘要

Terahertz radiation, generated from a biased and asymmetrically excited low-temperature-grown GaAs photoconductive emitter, has been characterized with a 20-μm-thick ZnTe crystal using free-space electro-optic sampling. Pronounced coherent emission, originating from longitudinal optical phonon oscillations, has been observed, with a characteristic frequency of 8.7 THz and a decay time of 2.1 ps. In the frequency domain, the sharp spectral features from phonon oscillations superimpose on an ultrabroad (over 20 THz) response from the initial transient. Furthermore, a broad feature around 16 THz is observed, and interpreted as originating from the coherent plasmon-phonon coupled modes.