Temperature-dependent chirped coherent phonon dynamics in Bi2Te3 using high-intensity femtosecond laser pulses
Center for Ultrafast Laser Applications (CULA) and Department of Physics, Indian Institute of Science Bangalore - 560 012, India
Accepted: 2 November 2010
Degenerate pump-probe reflectivity experiments have been performed on a single crystal of bismuth telluride (Bi2Te3) as a function of sample temperature (3 K to 296 K) and pump intensity using ∼50 femtosecond laser pulses with central photon energy of 1.57 eV. The time-resolved reflectivity data show two coherently generated totally symmetric A1g modes at 1.85 THz and 3.6 THz at 296 K which blue-shift to 1.9 THz and 4.02 THz, respectively, at 3 K. At high photoexcited carrier density of ∼1.7×1021 cm− 3, the phonon mode at 4.02 THz is two orders of magnitude higher positively chirped (i.e the phonon time period decreases with increasing delay time between the pump and the probe pulses) than the lower-frequency mode at 1.9 THz. The chirp parameter, β is shown to be inversely varying with temperature. The time evolution of these modes is studied using continuous-wavelet transform of the time-resolved reflectivity data.
PACS: 78.47.J- – Ultrafast spectroscopy (<1 psec) / 78.47.jg – Time resolved reflection spectroscopy / 68.55.ag – Semiconductors
© EPLA, 2010