Temperature-dependent chirped coherent phonon dynamics in Bi ₂ Te ₃ using high-intensity femtosecond laser pulses

Abstract

Degenerate pump-probe reflectivity experiments have been performed on a\nsingle crystal of bismuth telluride (Bi$_2$Te$_3$) as a function of sample\ntemperature (3K to 296K) and pump intensity using $\\sim$ 50 femtosecond laser\npulses with central photon energy of 1.57 eV. The time resolved reflectivity\ndata show two coherently generated totally symmetric A$_{1g}$ modes at 1.85 THz\nand 3.6 THz at 296K which blue shift to 1.9 THz and 4.02 THz, respectively at\n3K. At high photoexcited carrier density of $\\sim$ 1.7 $\\times$\n10$^{21}$cm$^{-3}$, the phonon mode at 4.02 THz is two orders of magnitude\nhigher positively chirped (i.e the phonon time period decreases with increasing\ndelay time between the pump and the probe pulses) than the lower frequency mode\nat 1.9 THz. The chirp parameter, $\\beta$ is shown to be inversely varying with\ntemperature. The time evolution of these modes is studied using continuous\nwavelet transform of the time-resolved reflectivity data.\n