Takeno Hiroaki, Saito Shingo, Mizoguchi Kohji
Department of Physical Science, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka, 599-8531, Japan.
National Institute of Information and Communications Technology, 4-2-1 Nukui-Kitamachi, Koganei, Tokyo, 184-8795, Japan.
Sci Rep. 2018 Oct 18;8(1):15392. doi: 10.1038/s41598-018-33716-0.
Dirac electrons in topological insulators (TIs) provide one possible avenue to achieve control of photocurrents and spin currents without the need to apply external fields by utilizing characteristic spin-momentum locking. However, for TI crystals with electrodes it is actually difficult to characterize the net flow of spin-polarized photocurrents because of the coexistence of surface carriers and bulk carriers generated by optical excitations. We demonstrate here that the net flow directions of spin-polarized photocurrents in TI polycrystalline thin films without electrodes can be precisely and intentionally controlled by the polarization of the excitation pulse alone, which is characterized by performing time-domain terahertz (THz) wave measurements and time-resolved magneto-optical Kerr rotation measurements that are non-contact methods. We show that the amplitudes of s-polarized THz waves radiated from photocurrents under right- and left-circularly polarized excitations are inverted relative to one another. Moreover, we observe the inversion of time-resolved magneto-optical Kerr rotation signals between the two excitations. Our results will open the way as innovative methods to control spin-polarized electrons in optoelectronic and spintronic TI devices without the need to apply external fields.
拓扑绝缘体（TIs）中的狄拉克电子提供了一种可能的途径，即通过利用其独特的自旋-动量锁定特性，无需施加外部场就能实现对光电流和自旋电流的控制。然而，对于带有电极的TI晶体，由于光激发产生的表面载流子和体载流子共存，实际上很难表征自旋极化光电流的净流动。我们在此证明，在没有电极的TI多晶薄膜中，自旋极化光电流的净流动方向可以仅通过激发脉冲的极化来精确且有意地控制，这是通过执行时域太赫兹（THz）波测量和时间分辨磁光克尔旋转测量来表征的，这些都是非接触方法。我们表明，在右旋和左旋圆偏振激发下，光电流辐射出的s偏振THz波的振幅彼此相反。此外，我们观察到两种激发之间时间分辨磁光克尔旋转信号的反转。我们的结果将为无需施加外部场就能控制光电子和自旋电子TI器件中自旋极化电子的创新方法开辟道路。
