Gate voltage control of helicity-dependent photocurrent and polarization detection in (Bi_1−xSb_x)₂Te₃ topological insulator thin films

Abstract

Optical helicity provides us with an effective means to control the helicity-dependent photocurrent in the spin-momentum-locked surface states of topological insulators (TIs). Also, the TIs show potential in polarization detection as an intrinsic solid-state optical chirality detector for easier integration and fabrication. However, the complex photoresponses with the circular photogalvanic effect, the linear photogalvanic effect, and the photon drag effect in the TIs prevent them from direct chirality detection of the elliptically polarized light. Here, by fitting with the theoretical models to the measured photocurrents, the microscopic origin of different components of the helicity-dependent photocurrent has been demonstrated. We show a comprehensive study of the helicity-dependent photocurrent in (Bi1−xSb x )2Te3 thin films of different thicknesses as a function of the light incident angle and the gate-tuned chemical potential. The observation of the light incident angle dependence of the helicity-dependent photocurrent provides us with a polarization detection strategy using a TI thin film without the use of any additional optical elements, and the detection accuracy can be enhanced by gate tuning. Additionally, the Stokes parameters can be extracted by arithmetic operation of photocurrents measured with different incident angles and gating voltages for complete characterization of the polarization states of a light beam. Using this means, we realize the polarization detection and the Stokes parameters analysis with a single device. Our work provides an alternative solution to develop miniaturized intrinsic polarization-sensitive photodetectors.

Investigation of helicity-dependent photocurrent of surface states in (Bi0.7Sb0.3)2Te3 nanoplate

“…The HDPC current of the (Bi 0.7 Sb 0.3 ) 2 Te 3 nanoplate can be obtained by fitting the polarization-dependent photocurrent 057101-2 to the following equation: [4,11] J = J HDPC sin 2ϕ + L 1 sin 4ϕ + L 2 cos 4ϕ + j 0 .…”

“…The former one is due to the transfer of the angular momentum of the photon to the photo-generated carriers, while the latter one is owing to the transfer both of the angular momentum and the linear momentum of the photon to the photo-generated carriers. [11,[19][20][21][22] The bulk states of (Bi 1−x Sb x ) 2 Te 3 nanoplates belong to D 3d symmetry group, which will not generate CPGE current, and the CPGE current can only come from the surface states or the two-dimensional electron gas (2DEG) of the (Bi 1−x Sb x ) 2 Te 3 nanoplates, which have a symmetry of C 3v . [1] By measuring the HDPC current under ionic liquid top-gate voltage, it is revealed that the HDPC is mainly contributed by the surface states rather than the 2DEG (see the discussion below).…”

“…[3,[6][7][8] Three-dimensional (3D) topological insulators (TIs) have spin-momentum locked surface states, which possess large SOC and become good candidates for designing spintroninc devices. [1,6,[9][10][11][12][13] Helicity dependent photocurrent (HDPC) is an important method to investigate the surface states of 3D TIs, and it has been used to explore the properties of the surface states of Bi 2 Se 3 thin films and nanoplates, Bi 2 Te 3 thin films, Sb 2 Te 3 and (Bi 1−x Sb x ) 2 Te 3 thin films. [4,[14][15][16][17] Besides, the HDPC can further expand the versatility of the charge-spin interchange in the topological insulator by utilizing the polarization of photons.…”

“…Besides, HDPC can be attributed to the two main mechanisms, i.e., the circular photogalvanic effect (CPGE) and circular photon drag effect (CPDE). [11,[19][20][21][22] The influence of the light incident plane on the dominant mechanisms of the HDPC for 3D TI nanoplates is still unclear.…”