Terahertz emission spectroscopy of coherent phonons in semiconductors

摘要

Abstract: We investigate infrared-active coherent phonons excited in Te, PbTe (un-doped, n-type and p-type), and CdTe by observing the terahertz (THz) emission from these samples. Coherent THz emission corresponding to the longitudinal- optical (LO)-phonon frequency has been observed for all the samples, while no significant THz emission is observed at the transverse-optical (TO)-phonon frequency even at high carrier densities ($GRT 10$+18$/ cm$+$MIN@3$/). The absence of THz emission at the TO-phonon frequency strongly contradicts the observation in the transient reflectivity measurements, where the signal oscillations at the TO-phonon frequencies arising from the LO-phonon-plasmon coupling (L_ mode) have been observed at high carrier densities. It is found from a model calculation that for a dipole oscillator on a sample surface the THz emission around the TO-phonon frequency is strongly suppressed due to the large dielectric constant near this frequency.!19