THEORY FOR DISPLACIVE EXCITATION OF COHERENT PHONONS

We report femtosecond time-resolved pump-probe reflection experiments in semimetals and semiconductors that show large-amplitude oscillations with periods characteristic of lattice vibrations. Only A1 modes are detected, although modes with other symmetries are observed with comparable intensity in Raman scattering. We present a theory of the excitation process in this class of materials, which we refer to as displacive excitation of coherent phonons (DECP). In DECP, after excitation by a pump pulse, the electronically excited system rapidly comes to quasiequilibrium in a time short compared to nuclear response times. In materials with A1 vibrational modes, the quasiequilibrium nuclear A1 coordinates are displaced with no change in lattice symmetry, giving rise to a coherent vibration of A1 symmetry about the displaced quasiequilibrium coordinates. One important prediction of the DECP mechanism is the excitation of only modes with A1 symmetry. Furthermore, the oscillations in the reflectivity R are excited with a cos(omega(0)t) dependence, where t = 0 is the time of arrival of the pump pulse peak, and omega(0) is the vibrational frequency of the A1 mode. These predictions agree well with our observations in Bi, Sb, Te, and Ti2O3. The fit of the experimental DELTA-R(t)/R(0) data to the theory is excellent.