THZ-WAVE EMISSION BY COHERENT OPTICAL PHONONS

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