Anisotropic Terahertz Emission from Bi2Se3 Thin Films with Inclined Crystal Planes

Abstract

We investigate the surface states of topological insulator (TI) Bi2Se3 thin films grown on Si nanocrystals and Al2O3 substrates by using terahertz (THz) emission spectroscopy. Compared to bulk crystalline Bi2Te2Se, film TIs exhibit distinct behaviors in the phase and amplitude of emitted THz radiation. In particular, Bi2Se3 grown on Al2O3 shows an anisotropic response with a strong modulation of the THz signal in its phase. From x-ray diffraction, we find that the crystal plane of the Bi2Se3 films is inclined with respect to the plane of the Al2O3 substrate by about 0.27°. This structural anisotropy affects the dynamics of photocarriers and hence leads to the observed anisotropic response in the THz emission. Such relevance demonstrates that THz emission spectroscopy can be a sensitive tool to investigate the fine details of the surface crystallography and electrostatics of thin film TIs.

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.…”

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

“…Along the positive puckered direction as shown in the upper panel of Figure 4d, 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 4d. This asymmetric nonlinear polarization could also happen in lowdimensional and layered materials, such as single-wall carbon nanotubes, 46 Sb 2 Te 3 , 47 graphite, 48 Bi 2 Se 3 , 49 and GeSe. 50 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.…”

Surface and interface states of Bi2Se3 thin films investigated by optical second-harmonic generation and terahertz emission

“…The emitted THz wave is guided by a set of parabolic mirrors, and its transient electric field profiles are detected by a photoconductive antenna. 28,29 The time-profiles of the THz electric field emitted from the top surfaces of Bi 2 Se 3 are displayed in Fig. 4(b).…”

“…Except for the Bi 2 Se 3 film grown on Al 2 O 3 without the capping layer, the THz electric field exhibits isotropic behavior upon the variation of U (not shown). 9,29 This leads us to exclude optical rectification as a possible mechanism of the THz emission and to consider the surge current mechanism as a primary emission source of the THz emission from Bi 2 Se 3 . Upon the formation of photoexcited carriers, a transient electric dipole-generating THz wave is formed by a mobility difference between an electron and a hole (photo-Dember effect) or by a chargeacceleration due to a built-in field.…”