THz-wave emission by coherent optical phonons

Coherent optical phonons are excited in single-crystal Tellurium by above-band-gap excitation with femtosecond laser pulses. Coherent Raman-active modes are resolved in amplitude and phase by monitoring the time-resolved reflectivity changes in a pump-probe experiment. By detecting the far-infrared radiation emitted from the crystal, we observe for the first time coherent THz radiation from infrared-active lattice modes. This technique opens a new way for the spectroscopy of lattice dynamics. We discuss details of the generation and detection mechanism of the coherent lattice modes. We show that the driving force for the coherent infrared-active modes is a diffusive process, i.e. the ultrafast build-up of a photo-Dember field.