We theoretically investigate the infrared optical properties of a three-terminal silicene-based device under the irradiation of a circularly polarized terahertz (THz) electromagnetic field, utilizing the intersubband transition theorem at low temperatures. It has been demonstrated that the electronic structure of bulked silicene can be tuned to the topological insulated (TI) and band insulated (BI) state, respectively, via the back-gate voltage induced staggered sublattice potential. Furthermore, with the enhancement of the staggered sublattice potential, the refractive index, extinction coefficient and optical conductivity from the TI state spin-up and -down subbands are observed to be red-and blue-shifted, respectively, while those from the BI state spin-up and -down subbands are proved to be continually blue-shifted. The collective excitations and individual electron-hole pair excitations induced a TI and BI state electron energy loss spectrum (EELS) with a similar red-and/or blue-shift behaviour as the refractive index, extinction coefficient and optical conductivity. The obtained results may be useful in the design of the spintronic and optoelectronic devices based on silicene.

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We theoretically investigate the electrically controllable spin polarization and selective efficiency of the edge state Dirac electron in a two-dimensional topological insulator (TI) sandwiched between ferromagnetic (FM) electrodes by using the method of Keldysh nonequilibrium Green's function. A nearly full spin polarization of the topological edge state with giant inversion of ∼80% is observed, which is much higher than the value previously reported. Moreover, the selective efficiency for spin-up electrons under the modulation of the parallel configuration of FM electrodes has been demonstrated to be larger than 95% for the first time, while that for spin-down electrons in the antiparallel case is higher than 90% in a wide energy range, owing to the inter-edge spin tunneling induced backscattering and spin dephasing effect. The obtained results may provide a deeper understanding of the TI edge states and a valuable guidance to design spin switch and filter with high on-off speed and selective efficiency based on TIs.

Transport properties on the surface of a topological insulator (TI) under the modulation of a two-dimensional (2D) ferromagnet/ferromagnet junction are investigated by the method of wave function matching. The single ferromagnetic barrier modulated transmission probability is expected to be a periodic function of the polarization angle and the planar rotation angle, that decreases with the strength of the magnetic proximity exchange increasing. However, the transmission probability for the double ferromagnetic insulators modulated n-n junction and n-p junction is not a periodic function of polarization angle nor planar rotation angle, owing to the combined effects of the double ferromagnetic insulators and the barrier potential. Since the energy gap between the conduction band and the valence band is narrowed and widened respectively in ranges of 0 ≤ θ < π/2 and π/2 < θ ≤ π, the transmission probability of the n-n junction first increases rapidly and then decreases slowly with the increase of the magnetic proximity exchange strength. While the transmission probability for the n-p junction demonstrates an opposite trend on the strength of the magnetic proximity exchange because the band gaps contrarily vary. The obtained results may lead to the possible realization of a magnetic/electric switch based on TIs and be useful in further understanding the surface states of TIs.