Ultrafast carrier and phonon dynamics in Bi$_{2}$Se$_{3}$ crystals

Ultrafast time-resolved differential reflectivity of Bi$_{2}$Se$_{3}$ crystals is studied using optical pump-probe spectroscopy. Three distinct relaxation processes are found to contribute to the initial transient reflectivity changes: a sub-ps and a few-ps electron-phonon relaxation process due to different phonon modes and a defect-induced charge trapping process. After the crystal is exposed to air, the relative strength of these processes is altered and becomes strongly dependent on the excitation photon energy. We argue that the observed behavior is likely due to the presence of Se vacancies. Further, weaker charge trapping process and vanishing air doping effect are observed in magnetically doped samples, supportive of our argument. Part of this work is published on Appl. Phys. Lett. 97, 182102(2010).