Terahertz radiation from coherent phonons excited in semiconductors

Terahertz radiation emitted by coherent phonons in Te, PbTe, and CdTe has been investigated by using an ultrafast photoconductive sampling detector. Pronounced coherent radiation originating from the longitudinal optical (LO) phonon oscillations of infrared-active modes was observed for all samples, irrespective of the different crystal structures. In addition, spectral dips at the transverse optical (TO) phonon frequencies, which could not be explained by absorption in the emitting volume, were observed for all samples. The model calculations indicate that the emission rate of the radiation into the air to that into the dielectric (semiconductor) side is scaled by ${\text{1/{1+(n}}_d^2{\mathrm{+\kappa }}_d^2{\mathrm{)n}}_d^3\}$ ${\mathrm{(n}}_d$ and ${\kappa }_d$ are the real and imaginary part of the complex refractive index, respectively). Thus, the enhanced emission of radiation by the coherent LO phonons and the spectral dips at the TO phonon frequencies can be explained by the respective increase and reduction of the emission efficiency of the radiation to the air due to the small and large value of the dielectric constant ${\mathrm{|\epsilon }}_d{\mathrm{(\omega )|=n}}_d^2{\mathrm{+\kappa }}_d^2$ near the LO and TO phonon frequencies, respectively.