Ultrafast energy relaxation process in Bi2Te3 thin films is studied using a collinear two color pump-probe technique. The coherent optical phonon is enhanced and destroyed by changing the separation times of double pump pulses. The non-oscillatory component of the reflectivity trace after the second pump pulse shows a distinct difference with and without the presence of coherent optical phonons, thus providing a direct evidence of the effect of optical phonon on the hot carrier relaxation process. The deduced characteristic times are systematically smaller when coherent optical phonons are involved in the energy transfer process. Comparatively, the conventional relaxation process is relatively slow, which is explained by the screening effect of the incoherent optical phonon. This work suggests that the energy relaxation can be manipulated through the excitation of coherent optical phonons.

The carrier dynamics of Bi 2 Te 3 is studied using the femtosecond pump-probe technique. Three distinct processes, including free carrier absorption, band filling and electron-hole recombination, are found to contribute to the reflectivity changes. The two temperature model is used to describe the intraband energy relaxation process of carriers, and the Drude contribution well explains the intensity dependence of the peak values of the non-oscillatory component in the reflectivity signal. The combined effects of free carrier absorption and band filling result in a reflection minimum at about 2 ps after the laser excitation. The non-zero background signal increases linearly with the pump fluence, which is attributed to the electron-hole recombination. Finally, our results provide an illustration of investigating the carrier dynamics in semiconductors from the ultrafast reflectivity spectra.