Fig. 1. Schematic diagram of the polarization tunable terahertz emission from Bi2Te3. (a) Femtosecond laser pulses, horizontal linear polarization (HLP), vertical linear polarization (VLP), left-handed circular polarization (LCP), and right-handed circular polarization (RCP), are incident onto the topological insulator Bi2Te3 and produce polarization tunable terahertz waves. (b) Macroscopic helicity-dependent photocurrent and only unidirectional spin current can be generated. (c) Microscopic electronic transition under circularly polarized laser pulse illumination (Video 1, MP4, 15.5 MB [URL: https://doi.org/10.1117/1.AP.2.6.066003.1]).

Fig. 2. Linearly polarized terahertz emission and its shift current mechanism. (a) Schematic diagram of the experimental setup for linearly polarized terahertz emission. Linearly polarized laser pump passes through a half-wave plate of angle α and is incident onto the sample emitting linearly polarized terahertz wave. The incident angle of the pump laser, θ; the azimuthal angle of the topological insulator, φ. (b) Far-field polarization trajectories [Sx(t), Sy(t)] of the radiated terahertz waves obtained experimentally from 10-nm-thick Bi2Te3 when the pump laser polarization was nearly normal incidence while the sample azimuthal angle was varied. (c) The terahertz peak amplitude of Sy(t) as a function of the azimuthal angle exhibits a 120-deg period. (d) Pump fluence dependence of terahertz peak amplitude from Bi2Te3 with different thickness of 5, 8, and 10 nm. (e) Schematic of the shift current generated from the electron transfer along the Bi–Te bonds. (f) Parameters in the y axis extracted through the symmetry analysis of surface state using Eq. (1). (g) Ip denotes the laser intensity profile. Fitting the shape of the shift current (blue line) to the photocurrent in the material (green line).

Fig. 3. Generation of elliptically and circularly polarized terahertz beams. (a) Experimental layout for circularly-polarized terahertz wave generation. Linearly polarized laser pump passes through a quarter-wave plate of angle, α, producing elliptically or circularly polarized pump laser beams. The emitted terahertz wave polarization can be elliptical or circular. The incident angle of the pump laser, θ; the azimuthal angle of the topological insulator, φ. (b) Far-field detected elliptically polarized terahertz polarization trajectories when the pump laser polarization was fixed while rotating the sample azimuthal angles. (c) Production of circularly polarized terahertz waves when fixing the azimuthal angle while rotating the quarter-wave plate for the pump laser pulses. (d) Experimentally observed terahertz component Sx(t) polarity reversal depends on the pump laser helicity. (e) Helicity-independent component Syz(t) from Bi2Te3. δ+ and δ− mean left-handed and right-handed elliptically polarized terahertz, respectively.

Fig. 4. Macroscopic analysis of PGE. (a) The terahertz peak amplitude of Sx(t) and Syz(t), as a function of the quarter-wave plate angle, respectively. (b) and (c) The time-domain signals for the parameters C(t), L1(t), L2(t), and D(t) in the x axis and yz plane extracted using Eq. (3). (d) Spin-momentum-locked states selectively excited by spin-polarized pump laser form unidirectional spin currents. (e) and (f) The corresponding Fourier transformed spectra of the time-domain signals in (b) and (c).

Fig. 5. Arbitrary manipulation of various terahertz polarization states. Radiated terahertz chirality was consistent with that of the pump laser chirality. (a) LCP pump laser pulses can produce left-handed elliptically polarized terahertz waves and (b) right-handed elliptically polarized pump light can generate right-handed elliptically polarized terahertz beams. (c) Linearly polarized terahertz waves can be generated in Bi2Te3 pumped by linearly polarized pump laser beams.