Influence of coherent optical phonon on ultrafast energy relaxation

Abstract

Ultrafast energy relaxation process in Bi2Te3 thin films is studied using a collinear two color pump-probe technique. The coherent optical phonon is enhanced and destroyed by changing the separation times of double pump pulses. The non-oscillatory component of the reflectivity trace after the second pump pulse shows a distinct difference with and without the presence of coherent optical phonons, thus providing a direct evidence of the effect of optical phonon on the hot carrier relaxation process. The deduced characteristic times are systematically smaller when coherent optical phonons are involved in the energy transfer process. Comparatively, the conventional relaxation process is relatively slow, which is explained by the screening effect of the incoherent optical phonon. This work suggests that the energy relaxation can be manipulated through the excitation of coherent optical phonons.

Carrier dynamics in femtosecond-laser-excited bismuth telluride

“…Multiple exponential decay functions have been widely used to fit the transient reflectivity traces, and the different carrier dynamics are proposed to feature different characteristic times [6][7][8][9][10][11][12][13][14]. These phenomenological models can fit the experimental data well, but are not sufficient to reveal the underlying mechanisms.…”

“…These phenomenological models can fit the experimental data well, but are not sufficient to reveal the underlying mechanisms. For instance, both positive and negative amplitudes are observed in the non-oscillatory part of the transient reflectivity signals in Bi 2 Se 3 [8], Bi [10] and Bi 2 Te 3 [13,14], and the electron-phonon interactions and trapping of electrons were speculated as the reasons of the positive and negative terms, respectively. The terms associated with lattice heating were sometimes discussed [10,13], but the effect of lattice heating on reflectivity is not fully understood.…”

Electron–phonon coupling in topological insulator Bi2Se3 thin films with different substrates

“…Thus, it is essential to explore the nonequilibrium properties of TIs under light excitation, such as phonon, carrier, as well as spin dynamics of both the SS and the bulk. The relaxation mechanism of photoexcited hot carriers has been intensively studied by various methods, including the optical pump optical probe (OPOP) [12][13][14][15][16][17][18][19] , optical pump terahertz probe (OPTP) [20][21][22][23] , time-resolved angle-resolved photoemission spectroscopy (Tr-ARPES) [24][25][26][27][28] , Raman spectroscopy [29,30] , second harmonic generation (SHG) [31][32][33] , as well as the time-resolved Kerr rotation (TRKR) [31,34] .…”

“…Despite the relevant studies and some convincing explanations for the relaxation path of electrons, there are some issues that remain obscure. Most of the transient reflection (TR) traces were studied by using a two-color pump-probe system, which can reveal the relaxation process of photo-excited carriers at a certain energy level [12][13][14][15][16][17] . However, there is a great need to study the relaxation process of electrons excited to different energy levels.…”

“…However, the amplitudes of the signals are quite different when measured at different probe wavelengths by the end of ΔT 1 . According to former researches [8,9,12] , the excited charge carriers would affect the reflection signal of Bi 2 Se 3 via two mechanisms, namely, the free-carrier absorption (weak due to indirect absorption) and the phase-space filling (dominant). From our pump-probe data acquired with different pump fluence at the probe energy of 2.07 eV [as shown in Fig.…”