Ultrafast dynamics of photoexcited coherent phonon in Bi2Te3 thin films

Abstract

Nonequilibrium A 1g longitudinal optical phonon with a frequency of 1.84 THz in bismuth telluride ͑Bi 2 Te 3 ͒ is coherently excited by ultrafast pulses. Time-resolved reflectivity measurements show a distinct second harmonic vibration around 3.68 THz at room temperature caused by the nonlinearity of coherent phonon potentially related to the favorable crystal structure of Bi 2 Te 3. The scattering rate between A 1g coherent phonon and room temperature incoherent phonon is derived from the pump-fluence-dependent scattering rate of A 1g coherent phonon. It is also observed that energy coupling from photoexcited carriers to lattice through coherent phonon vibration is more efficient and faster at higher pump fluence.

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

“…Finally, to examine whether the second mode at ∼ 4 THz is the second harmonic of the first A 1g mode at 1.8 THz as suggested in ref [19], we re-fitted the digitally band pass filtered time domain data with ∆R R expressed in a second order approximation in Q as [19] …”

“…The signal contains both non-oscillatory and oscillatory components. The nonoscillatory background arising from carrier dynamics was removed by a digital band pass filter to extract the oscillatory part [19,20]. The oscillatory part of the transient normalized differential reflectivity data was analyzed using Eq.…”

“…The Gabor wavelet transform has been performed on time domain data to study the evolution of the coherent phonon modes. We note that earlier studies on Bi 2 Te 3 [19,20] were done only at room temperature and low pump fluences (with negligible chirping).…”

“…Bi 2 Te 3 is an important material both from the point of view of thermoelectric [16] as well as exotic physics of topological insulators [17,18]. There have been two earlier studies [19,20] of Bi 2 Te 3 using non-degenerate pump probe experiments at room temperature with 100 fs pulses where pump fluence was kept below 1 mJ/cm 2 . In ref [19], the two observed coherent phonon modes at 1.85 THz and 3.68 THz were assigned as A 1g and its second harmonic.…”

“…There have been two earlier studies [19,20] of Bi 2 Te 3 using non-degenerate pump probe experiments at room temperature with 100 fs pulses where pump fluence was kept below 1 mJ/cm 2 . In ref [19], the two observed coherent phonon modes at 1.85 THz and 3.68 THz were assigned as A 1g and its second harmonic. In a later study [20], the modes seen at 1.85 THz and 4.02 THz were assigned to two allowed A 1g modes, in agreement with conventional Raman measurements [21].…”