Abstract
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 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 vibrational modes, the quasiequilibrium nuclear coordinates are displaced with no change in lattice symmetry, giving rise to a coherent vibration of symmetry about the displaced quasiequilibrium coordinates. One important prediction of the DECP mechanism is the excitation of only modes with symmetry. Furthermore, the oscillations in the reflectivity R are excited with a cos(t) dependence, where t=0 is the time of arrival of the pump pulse peak, and is the vibrational frequency of the mode. These predictions agree well with our observations in Bi, Sb, Te, and . The fit of the experimental ΔR(t)/R(0) data to the theory is excellent.
- Received 1 July 1991
DOI:https://doi.org/10.1103/PhysRevB.45.768
©1992 American Physical Society