Ultrafast helicity control of surface currents in topological insulators with near-unity fidelity

Abstract

In recent years, a class of solid-state materials, called three-dimensional topological insulators, has emerged. In the bulk, a topological insulator behaves like an ordinary insulator with a band gap. At the surface, conducting gapless states exist showing remarkable properties such as helical Dirac dispersion and suppression of backscattering of spin-polarized charge carriers. The characterization and control of the surface states via transport experiments is often hindered by residual bulk contributions. Here we show that surface currents in Bi2Se3 can be controlled by circularly polarized light on a picosecond timescale with a fidelity near unity even at room temperature. We reveal the temporal separation of such ultrafast helicity-dependent surface currents from photo-induced thermoelectric and drift currents in the bulk. Our results uncover the functionality of ultrafast optoelectronic devices based on surface currents in topological insulators.

Subpicosecond spin dynamics of excited states in the topological insulator Bi2Te3

“…As a consequence of the weak surface scattering these findings enabled, on the one hand, the observation of spin-polarized electrical currents originating from TSSs using circularly-polarized light in the time domain [6,30,31]. On the other hand, the use of linearly-polarized fs-infrared pulses enabled the suppression of the charge current on the surface [5,16], presumably giving rise to pure ultrafast spin currents evolving on a different time scale [16], so that the transient Dirac cone can be dynamically populated by the same number of excited elec- …”

“…Their metallic surface hosts Dirac-cone spinpolarized topological surface states (TSSs) that can be used as channels in which to drive pure spin currents or spin-polarized electrical currents on ultrafast time scales [4][5][6][7]. While under equilibrium conditions the helical spin texture of TSSs has been widely studied using spin and angle-resolved photoemission (SARPES) [8][9][10][11][12], very little is known about the nonequilibrium spin properties of TSSs following optical excitation by intense fs-laser fields.…”

Revealing the ultrafast light-to-matter energy conversion before heat diffusion in a layered Dirac semimetal

“…Dirac materials are also usable as wide-band mode lockers that create ultrashort laser pulses of any color, owing to an ability to absorb light over a broad range of wavelengths [10][11][12]; Broad-band lasing may also be realized [12][13][14][15]. Investigations of the optical responses of Dirac fermions have opened pathways to take control of their charge [16][17][18], spin [19][20][21] and topological properties [22,23] by light.…”

“…When founding the functions of optically nonequilibrated Dirac materials [10][11][12][13][14][15][16][17][18][19][20][21][22][23], it becomes important to understand how the energy of light is transferred to matter and converted thereafter. In fact, there is still no counterpart for the well-established 2-, 3-, or 4-level rate equations that nicely describes the population inversions in conventional lasers.…”

Anomalous Hall Coulomb drag of massive Dirac fermions

“…Dirac fermions in 2D are described by the Hamiltonian H D = A σ · (k ×ẑ) + M σ z , with σ = (σ x , σ y , σ z ) the usual Pauli matrices, k = (k x , k y ) the 2D wave vector, A stems from the Fermi velocity and M a generic mass term. In the limit M → 0 the quasi-particle dispersion is linear, a feature that has aroused intense interest experimentally [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] and theoretically [22][23][24][25][26][27][28][29][30][31][32][33][34]. These studies have illuminated the considerable potential of Dirac fermions for spintronics, thermoelectricity, magnetoelectronics and topological quantum computing [35].…”