Glinka Yuri D, He Tingchao, Sun Xiao Wei
Guangdong University Key Laboratory 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, People's Republic of China.
Institute of Physics, National Academy of Sciences of Ukraine, Kyiv 03028, Ukraine.
J Phys Condens Matter. 2022 Sep 19;34(46). doi: 10.1088/1361-648X/ac90a7.
It is often taken for granted that in pump-probe experiments on the topological insulator (TI) BiSeusing IR pumping with a commercial Ti:sapphire laser [∼800 nm (1.55 eV photon energy)], the electrons are excited in the one-photon absorption regime, even when pumped with absorbed fluences in the mJ cmrange. Here, using UV-Vis transient absorption (TA) spectroscopy, we show that even at low-power Infrared (IR) pumping with absorbed fluences in theJ cmrange, the TA spectra of the TI BiSeextend across a part of the UV and the entire visible region. This observation suggests unambiguously that the two-photon pumping regime accompanies the usual one-photon pumping regime even at low laser powers applied. We attribute the high efficiency of two-photon pumping to the giant nonlinearity of Dirac fermions in the Dirac surface states (SS). On the contrary, one-photon pumping is associated with the excitation of bound valence electrons in the bulk into the conduction band. Two mechanisms of absorption bleaching were also revealed since they manifest themselves in different spectral regions of probing and cause the appearance of three different relaxation dynamics. These two mechanisms were attributed to the filling of the phase-space in the Dirac SS and bulk states, followed by the corresponding Pauli blocking.
人们通常认为，在用商用钛宝石激光器[~800 nm（光子能量为1.55 eV）]进行红外泵浦的拓扑绝缘体（TI）BiSe的泵浦-探测实验中，即使在以mJ/cm范围内的吸收通量进行泵浦时，电子也是在单光子吸收 regime 下被激发的。在这里，我们使用紫外-可见瞬态吸收（TA）光谱表明，即使在以J/cm范围内的吸收通量进行低功率红外（IR）泵浦时，TI BiSe的TA光谱也会延伸到紫外区域的一部分和整个可见光区域。这一观察结果明确表明，即使在施加低激光功率时，双光子泵浦 regime 也伴随着通常的单光子泵浦 regime。我们将双光子泵浦的高效率归因于狄拉克表面态（SS）中狄拉克费米子的巨大非线性。相反，单光子泵浦与体内束缚价电子激发到导带有关。还揭示了两种吸收漂白机制，因为它们在不同的探测光谱区域表现出来，并导致出现三种不同的弛豫动力学。这两种机制归因于狄拉克SS和体内态中相空间的填充，随后是相应的泡利阻塞。
