Success of Optical Control of Spin-Polarized Photocurrent in Topological Insulator!
LastUpDate： October 19, 2018
The research group which includes Hiroaki Takeno, Graduate student, Prof. Kohji Mizoguchi, Graduate school of Science, Osaka Prefecture University, and Dr. Shingo Saito, Advanced ICT Research Institute, National Institute of Information and Communications Technology has successfully generated spin-polarized(*1) photocurrents(*2) on the surface of topological insulator (TI(*3)) thin films by light irradiation alone, without any need to external fields such as electronic field and magnetic field. Moreover, this group demonstrates that the flow direction of spin-polarized photocurrents on the surface of TI thin films can be controlled by manipulating the polarization of the irradiation light(*4) (see figure).
The main point of research
- Spin-polarized photocurrents are successfully generated on the surface of topological insulator (TI) thin films by light irradiation without electronic field or magnetic field.
- We demonstrate that the flow direction of spin-polarized photocurrents can be precisely and selectively controlled by manipulating the polarization of the optical pulses.
- These results will open the way as innovative methods to control spin-polarized electrons in optoelectronic and spintronic TI devices.
The realization of spintronic devices which utilize the characteristic of spin is expected for the development of electronic devices. Topological insulators (TIs) are attractive materials in looking toward the next generation of spintronic devices. We succeeded to fabricate the TI thin films oriented with crystal axis. Usually external fields such as electronic field and magnetic field are necessary to generate electric currents in materials. In this study, we reveal that the spin-polarized photocurrents at the surface layer of TI polycrystalline thin films can be generated by the irradiation of the polarized light alone, which is characterized by performing time-domain terahertz (THz) wave(*5) measurement that is a non-contact method to observe THz waves radiated from photocurrents in materials. Moreover, we demonstrate that the net flow directions of spin-polarized photocurrents can be precisely and selectively controlled by manipulating the polarization of the irradiation light.
This study was published in online journal of Scientific Reports on October 18th, 2018.
The title: “Optical control of spin-polarized photocurrent in topological insulator thin films“
*1 Spin polarization
Spin which is the intrinsic angular momentum of elementary particle (frequently described as the rotation of electrons) has magnetic properties. Spin polarization is the degree to which the spin is aligned with a given direction.
Photocurrent is the electric current composed of excited electrons by absorbing light. When the spins of the photocurrent are polarized with a given direction, this photocurrent is referred to as spin-polarized photocurrent.
*3 Topological insulator
Topological insulator (TI), which is characterized by mathematical topology, shows the property of insulating material inside crystals and the metallic conducting property at the surface. The electron on the surface is spin polarized, and the spin polarization has the direction perpendicular to the wave vector of electrons. In 2016, Dr. D. J. Thouless (Washington University, USA), Dr. F. D. M. Haldane (Princeton University, USA), and Dr. J. M. Kosterlitz (Brown University, USA) have received Noble Prize in physics for theoretical discoveries of topological phase transitions and topological phases of matter.
*4 Polarization of Light
Light which is electromagnetic radiation wave has the property of transverse waves, and propagates in space with oscillating electric field and magnetic field. The polarization of light in general is the oscillating direction of the electric field. When the electric field linearly oscillates in the plane perpendicular to the direction of the propagation, it is referred to as linearly polarized light. When the oscillation of the electric field circularly (elliptically) rotates, it is referred to as circularly (elliptical) polarized light. There are two kinds of circularly (elliptical) polarized light: right circular (elliptical) polarization in which the electric field rotates in a clockwise, and left circular (elliptical) polarization in which the electric field rotates in a counter-clockwise.
*5 Terahertz wave
Terahertz (THz) wave is electromagnetic radiation wave in the frequency range from 0.1 THz to 10 THz. The radiation of electromagnetic waves is related to the temporal change of magnitude and direction of currents. Using this property, the information of currents can be obtained by performing time-domain THz wave measurement that is a non-contact method.
Graduate School of Science, Osaka Prefecture University
Prof. Kohji Mizoguchi