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

Abstract

Terahertz radiation, generated from a biased and asymmetrically excited low-temperature-grown GaAs photoconductive emitter, has been characterized with a $20\text{\ensuremath{-}}\ensuremath{\mu}\mathrm{m}$-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 $8.7\phantom{\rule{0.3em}{0ex}}\mathrm{THz}$ and a decay time of $2.1\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$. In the frequency domain, the sharp spectral features from phonon oscillations superimpose on an ultrabroad (over $20\phantom{\rule{0.3em}{0ex}}\mathrm{THz}$) response from the initial transient. Furthermore, a broad feature around $16\phantom{\rule{0.3em}{0ex}}\mathrm{THz}$ is observed, and interpreted as originating from the coherent plasmon-phonon coupled modes.