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 single crystal of bismuth telluride (Bi 2 Te 3 ) as a function of sample temperature (3K to 296K) 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 A 1g modes at 1.85 THz and 3.6THz at 296K which blue shift to 1.9 THz and 4.02 THz, respectively at 3K. At high photoexcited carrier density of ∼ 1.7 × 10 21 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.

Polarization dependence of coherent phonon generation and detection in the three-dimensional topological insulatorBi2Te3

“…We note that a similar 3.6 THz mode has been previously observed in a number of time-domain studies on Bi 2 Te 3 [12,15,16]. For instance, in Refs.…”

“…In later studies [13,14], the modes at 1.85 THz and 4.02 THz were assigned to two allowed A 1g modes, in agreement with spontaneous Raman measurements [9,10]. The detailed time-domain study [15] in which both the temperature and power dependence of coherent phonons were measured confirmed that the higher-frequency coherent phonon mode, which appears with 3.6 THz frequency at room temperature and increases to 4.0 THz at helium temperature, is an A This high-frequency mode acquires two orders of magnitude higher positive chirp at the lowest temperatures and high pump fluence as compared to the A (I) 1g phonon. Our previous study [16] revealed that depending on the pump polarization with respect to the crystal orientation one can see either only two fully symmetric coherent phonons at 1.8 and 4.0 THz, or, in addition, the mode at 3.6 THz assigned tentatively to a doubly degenerate E (II) g phonon.…”

“…For instance, in Refs. [12,15] this mode appeared at high fluence excitation and, moreover, its frequency was downshifted by increasing the excitation power. Such behavior reminds us of the fully symmetric mode of Te, which is known to decrease in frequency due to electronic softening [27,28].…”

“…2(b) by solid lines. The frequency domain is a better choice for identifying the chirp as it is difficult to see the chirp in the time domain, especially for the high-frequency mode, which is short lived [15]. From the FT spectra one can see that both fully symmetric phonons are negatively chirped as the low-frequency slope of the lines is steeper (Fano parameter is positive).…”

“…Each oscillator provided 50 fs pulses with wavelength of 818 nm for the pump and of 830 nm for the probe. The crystal was excited with 50 mW average pump power focused to a spot with a diameter of 60 μm (typical pump fluence 1.6 μJ/cm 2 , much less than the damage threshold of 4.5 mJ/cm 2 [15]). The changes in reflection intensity were sampled with 200 MS/s (megasamples per second) on a detector with a bandwidth of 125 MHz.…”