THz electromagnetic emission by coherent infrared-active phonons

Coherent phonons are excited in single-crystal tellurium by above-band-gap excitation with femtosecond laser pulses. Coherent infrared-active lattice vibrations give rise to the emission of electromagnetic waves at the phonon frequency. The excitation mechanism is based on the ultrafast buildup of a photo-Dember field and the coupling of a polar lattice mode to this field. The dynamics of the Dember field is obtained from a numerical simulation of the electron-hole plasma dynamics close to the crystals surface. This effect itself gives rise to a broadband emission of THz radiation from highly absorbing semiconductor surfaces. Details of the generation and the outcoupling of the radiation are discussed. {} 1996 The American Physical Society.