Optical control of spin-polarized photocurrent in topological insulator thin films

Hiroaki Takeno,Kohji Mizoguchi,Shingo Saito

doi:10.1038/s41598-018-33716-0

Abstract

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.

Highlights

Topological insulators (TIs) are attractive materials in looking toward the generation of electronic devices because there exist Dirac electrons on their surface
In order to realize the precise photocurrent control with the polarization of excitation light, we have measured the terahertz (THz) waves radiated from topological insulators (TIs) polycrystalline thin films without electrodes, which gives us the information about the dynamics of photo-excited carriers, because the electric fields of the THz waves radiated from photocurrents due to photo-excited carriers are proportional to the time derivative of the photocurrents[11]
We demonstrate that the amplitudes of the THz waves radiated from the photocurrent flowing in the sample depend on the polarization of excitation pulses, which indicates the potential to precisely and intentionally control the flow directions of the photocurrents generated at the topological surface state by the polarization of excitation pulses

Summary

Results

The α-dependence of the s-polarized THz wave amplitude in Fig. 2e indicates that the magnitude of the photocurrents flowing in the x direction at the surface of the sample continuously changes with the ellipticity of the polarization of the excitation pulse, while background photocurrent due to the Seebeck effect is hardly observed. The α-dependence of the s-polarized THz wave amplitude is fitted with Eq (2) as indicated by the red curve, which is in good agreement with the experimental result This agreement illustrates that excitation pulses with various polarization ellipticities enable one to control the direction of the photocurrent generated at the surface of the Bi2Te3 thin film while suppressing bulk thermoelectric currents (C:L:D = 1:0.8:0.15). It is clear that the flow direction of the spin-polarized photocurrents can be controlled by the polarization of the excitation pulse alone

Discussion

Author Contributions

Additional Information