Origin of coherent phonons in Bi2Te3 excited by ultrafast laser pulses

Femtosecond laser pulses are used to excite coherent optical phonons in single crystal Bi2Te3 thin films. Oscillations from lowand high-frequency A1g phonon modes are observed. A perturbation model based on molecular dynamics reveals various possibilities of phonon generation due to complex interactions among different phonon modes. In order to elucidate the process of phonon generation, measurements on thin films with thicknesses below the optical absorption depth are carried out, showing that a gradient force is necessary to excite the observed A1g phonon modes, which provides a refined picture of displacive excitation of coherent phonon.