Terahertz probe of nonequilibrium carrier dynamics and ultrafast photocurrents in the topological insulator Sb2Te3

Abstract

In this study, we report the ultrafast excitation and dynamics of the carrier and photocurrent in a 3D typical topological insulator Sb2Te3. We utilize time-resolved optical pump-THz probe spectroscopy to explore the nonequilibrium carrier dynamics of Sb2Te3. The electron system undergoes an ultrafast relaxation and releases through intra-band and inter-band scattering. Additionally, THz emission spectroscopy is employed to investigate the different ultrafast photocurrents in Sb2Te3 through tuning the polarization of excitation pulses and rotating the sample's azimuthal angle. We distinguish the different ultrafast photocurrents driven by the linear photogalvanic effect, circular photogalvanic effect, and thermoelectric effect. Our results potentially enable an all-optical modulation of THz emission without any external bias field, which could play an important role in the development of topological insulator-based high-speed THz optoelectronic and opto-spintronic devices.

Topological Surface‐Dominated Spintronic THz Emission in Topologically Nontrivial Bi1−xSbx Films

“…Such a misorientation can affect the photocarrier dynamics and even become the dominant mechanism of THz radiation. [ 22 , 23 ] To verify the misorientation of the Bi 1− x Sb x film and substrate, we conducted ω − φ 2D scans for the Bi 1− x Sb x (003) and sapphire (006) lattice planes. Figure 1b shows a schematic of the experimental geometry of the ω − φ 2D scans.…”

“…[ 25 ] If the shift current contributes to the THz radiation, the φ dependence of the THz amplitude (peak‐to‐peak value of the THz signal) must exhibit a threefold symmetry for the single crystalline Bi 1− x Sb x because the shift current depends on the crystal symmetry. [ 18 , 23 ] However, the φ dependence of the THz amplitude for the 10 nm thick Bi 0.8 Sb 0.2 exhibited a simple onefold symmetry (Figure 2c ), indicating that the THz emission from the shift current was negligible in Bi 0.8 Sb 0.2 . The negligible shift current signal results from the emitted THz pulses from each twin domain being cancelled out (Figure S3 and Note S2 , Supporting Information).…”

Large In-Plane Anisotropic Terahertz Emission Induced by Asymmetric Polarization in Low-Symmetric PdSe2

“…This anisotropy from the nonlinear optical response is greatly larger than that of BP (∼0.23), GeP (∼0.22), and PdSe 2 (∼0.3) in the linear optical regime. The large anisotropy ratio is usually observed in low-dimensional materials, such as carbon nanotubes and Sb 2 Te 3 . This further indicates that PdSe 2 has low in-plane symmetry with only onefold symmetry at the azimuthal angle due to the intrinsic symmetry breaking.…”

“…The large anisotropy ratio is usually observed in low-dimensional materials, such as carbon nanotubes 46 and Sb 2 Te 3 . 47 This further indicates that PdSe 2 has low in-plane symmetry with only onefold symmetry at the azimuthal angle due to the intrinsic symmetry breaking. Due to the symmetry breaking by the asymmetric polarization, the data can be fitted by E THz ∝ L cos φ + M, as shown in Figure 4b.…”

“…Along the positive puckered direction as shown in the upper panel of Figure d, the polarization is possible from Pd to Se, while the polarization possibly changes from Se to Pd along the negative puckered direction, as shown at the bottom panel of Figure d. This asymmetric nonlinear polarization could also happen in low-dimensional and layered materials, such as single-wall carbon nanotubes, Sb 2 Te 3 , graphite, Bi 2 Se 3 , and GeSe . Even though some explanations such as the prioritized direction induced by stacking fault have been proposed, the more microscopic explanations are desirable in the future.…”