Investigation of directional THz spin currents in topological surface states

拓扑绝缘体（TI）表现出拓扑保护的自旋极化表面态，其为自旋电子器件提供了很高的应用潜力，优选地在达到难以捉摸的太赫兹（THz）窗口的频率下工作。在我们的项目中，我们已经研究了表面/体电子，自旋和声子之间的耦合，通过超快激发和探测模型3D TI，如Bi 2Se 3。我们主要使用适当偏振的光学激光脉冲（~1.5 eV光子能量）来选择性地激发表面电子。为了探测由此产生的自旋，电荷和输运动力学，我们已经开发了新的测量方案，包括磁光自旋和太赫兹（THz）电导率和电流探针。我们能够揭示光学自旋极化电子超快弛豫的重要特征以及新型超快表面光电流。虽然第一个资助期专注于光学激发和单一材料样品，但我们希望在第二个资助期转向（i）更定制和选择性的激发和（ii）混合TI结构。目标（i）通过使用THz至中红外光谱范围内的较低泵浦光子能量（~1至400 meV）来实现。在3D模型TI中，如Bi 2Se 3，这些能量非常适合直接和选择性地驱动狄拉克态之间的带间跃迁。由于这些表面态位于TI体带隙中，我们预计将有更大的自旋极化寿命，表面电流振幅和独家洞察带内表面态动力学。自旋、电荷和输运的动力学由第一个供资期开发的探测器监测。实验将伴随着从头算电子结构和运输理论。通过可调谐和稳定的载波包络相位的激光和太赫兹脉冲，实现了对非常早期的超快自旋动力学的额外控制和洞察。关于目标（ii），我们将研究铁磁体/TI混合结构的超快动力学的激发态。我们将揭示拓扑表面状态对铁磁序动力学的影响（例如通过自旋转移矩和光学自旋矩）和反向效应，从铁磁体到TI表面和体的超快自旋注入。利用我们新开发的自旋、电导率和输运探针，我们将在真实的时间内研究这些过程的动力学。我们希望获得新的见解的铁磁体上的TI动力学的影响，以及TI自旋霍尔角的估计。重要的是，第一个应用程序将出现：在相邻的铁磁体中产生的自旋扭矩由TI自旋电流和自旋电荷转换在TI表面/体状态，从而直接导致新的和有效的宽带太赫兹辐射的发射器。

Topological insulators (TIs) exhibit topologically protected spin-polarized surface states that offer a high application potential for spin electronic devices, preferably working at frequencies reaching the elusive terahertz (THz) window. In our project, we have studied the coupling between surface/bulk electrons, spins and phonons by ultrafast excitation and probing of model 3D TIs such as Bi2Se3. We have predominantly used optical laser pulses (~1.5 eV photon energy) of suitable polarization to selectively excite surface electrons. To probe the resulting spin, charge and transport dynamics, we have developed novel measurement schemes including magneto-optic spin and terahertz (THz) conductivity and current probes. We were able to reveal important features of the ultrafast relaxation of optically spin-polarized electrons as well as novel types of ultrafast surface photocurrents.While the first funding period was focused on optical excitation and single-material samples, we want move to (i) substantially more tailored and selective excitation and (ii) hybrid TI structures in the second period. Goal (i) is achieved by using lower pump photon energies (~1 to 400 meV) in the THz to mid-infrared spectral range. In 3D model TIs such as Bi2Se3, these energies are perfectly suited to directly and selectively drive interband transitions between Dirac states. As these surface states are located in the TI bulk band gap, we expect substantially larger spin-polarization lifetimes, surface current amplitudes and exclusive insights into the intraband surface state dynamics. The dynamics of spins, charges and transport are monitored by the probes developed in the first funding period. Experiments will be accompanied by ab initio electronic-structure and transport theory. Additional control of and insight into the very early ultrafast spin dynamics is achieved by laser and THz pulses of tunable and stabilized carrier-envelope phase. Concerning goal (ii), we will study ferromagnet/TI hybrid structures in terms of the ultrafast dynamics of their excited states. We will reveal the influence of the topological surface state on dynamics of the ferromagnetic order (e.g via spin-transfer torque and optical spin torque) and the reverse effect, the ultrafast spin injection from the ferromagnet into the TI surface and bulk. Using our newly developed spin, conductivity and transport probes, we will study the dynamics of these processes in real time. We expect to gain new insights into the impact of the ferromagnet onto the TI dynamics as well as an estimate of the TI spin Hall angle. Importantly, first applications will emerge: the generation of spin torque in the adjacent ferromagnet induced by TI spin currents and spin-to-charge conversion in the TI surface/bulk states, thereby directly leading to new and efficient emitters of broadband THz radiation.