Glinka Yuri D, He Tingchao, Sun Xiao Wei
Guangdong University Key Lab for Advanced Quantum Dot Displays and Lighting, Shenzhen Key Laboratory for Advanced Quantum Dot Displays and Lighting, Department of Electrical and Electronic Engineering, Southern University of Science and Technology, Shenzhen, 518055, China.
Institute of Physics, National Academy of Sciences of Ukraine, Kyiv, 03028, Ukraine.
Sci Rep. 2022 Mar 18;12(1):4722. doi: 10.1038/s41598-022-08513-5.
Ultrafast carrier relaxation in the 2D topological insulator (TI) BiSe [gapped Dirac surface states (SS)] and how it inherits ultrafast relaxation in the 3D TI BiSe (gapless Dirac SS) remains a challenge for developing new optoelectronic devices based on these materials. Here ultrashort (~ 100 fs) pumping pulses of ~ 340 nm wavelength (~ 3.65 eV photon energy) were applied to study ultrafast electron relaxation in the 2D TI BiSe films with a thickness of 2 and 5 quintuple layers (~ 2 and ~ 5 nm, respectively) using transient absorption (TA) spectroscopy in the ultraviolet-visible spectral region (1.65-3.9 eV). The negative and positive contributions of TA spectra were attributed to absorption bleaching that mostly occurs in the bulk states and to the inverse bremsstrahlung type free carrier absorption in the gapped Dirac SS, respectively. Owing to this direct and selective access to the bulk and surface carrier dynamics, we were able to monitor coherent longitudinal optical (LO) phonon oscillations, which were successively launched in the bulk and surface states by the front of the relaxing electron population within the LO-phonon cascade emission. We have also recognized the coherent surface-to-bulk vibrational coupling that appears through the phase-dependent amplitude variations of coherent LO-phonon oscillations. This unique behavior manifests itself predominantly for the topologically trivial insulator phase of the 2D TI BiSe (2 nm thick film) in the photon energy range (~ 2.0-2.25 eV) where efficient energy exchange between the bulk and surface states occurs. We also found that the coherent surface-to-bulk vibrational coupling significantly weakens with increasing both the BiSe film thickness and pumping power.
二维拓扑绝缘体（TI）BiSe[带隙狄拉克表面态（SS）]中的超快载流子弛豫以及它如何继承三维TI BiSe（无隙狄拉克SS）中的超快弛豫，对于基于这些材料开发新型光电器件而言仍是一项挑战。在此，我们应用波长约为340 nm（光子能量约为3.65 eV）的超短（约100 fs）泵浦脉冲，利用紫外 - 可见光谱区域（1.65 - 3.9 eV）的瞬态吸收（TA）光谱，研究了厚度分别为2和5个五重层（分别约为2 nm和5 nm）的二维TI BiSe薄膜中的超快电子弛豫。TA光谱的负贡献和正贡献分别归因于主要发生在体态中的吸收漂白以及带隙狄拉克SS中的逆轫致辐射型自由载流子吸收。由于能够直接且选择性地获取体态和表面载流子动力学信息，我们得以监测相干纵向光学（LO）声子振荡，在LO声子级联发射过程中，弛豫电子群体的前沿在体态和表面态中相继激发了这种振荡。我们还识别出了通过相干LO声子振荡的相位相关振幅变化而出现的相干表面 - 体态振动耦合。这种独特行为主要在光子能量范围（约2.0 - 2.25 eV）内二维TI BiSe（2 nm厚薄膜）的拓扑平凡绝缘体相中表现出来，此时体态和表面态之间发生了有效的能量交换。我们还发现，随着BiSe薄膜厚度和泵浦功率的增加，相干表面 - 体态振动耦合显著减弱。
