Ultrafast manipulation of topologically enhanced surface transport driven by mid-infrared and terahertz pulses in Bi2Se3

Abstract

Topology-protected surface transport of ultimate thinness in three-dimensional topological insulators (TIs) is breaking new ground in quantum science and technology. Yet a challenge remains on how to disentangle and selectively control surface helical spin transport from the bulk contribution. Here we use the mid-infrared and terahertz (THz) photoexcitation of exclusive intraband transitions to enable ultrafast manipulation of surface THz conductivity in Bi2Se3. The unique, transient electronic state is characterized by frequency-dependent carrier relaxations that directly distinguish the faster surface channel than the bulk with no complication from interband excitations or need for reduced bulk doping. We determine the topological enhancement ratio between bulk and surface scattering rates, i.e., γBS/γSS ~3.80 in equilibrium. The ultra-broadband, wavelength-selective pumping may be applied to emerging topological semimetals for separation and control of the protected transport connected with the Weyl nodes from other bulk bands.

Light control of surface–bulk coupling by terahertz vibrational coherence in a topological insulator

“…We also observe a decrease of Δε 1 with increasing probe frequency in the same range. In our previous study of intraband photoexcitation responses in Bi 2 Se 3 , mainly using mid-infrared pulses, 23 we identified a similar bipolar spectral shape of Δσ 1 (ω) dynamics, which arises from the combined responses due to competing surface and bulk states. There we showed that the above behavior can be used to distinguish the intrinsic transport properties of u mode that strongly influence surface state transport.…”

“…E THz ≠ 0, the spectral-temporal characteristics of the complex conductivity response functions are measured by our THz pump-THz probe conductivity spectroscopy that allows us to differentiate surface from bulk contributions as shown in ref. 23 . In this way we measure the transport lifetimes of Dirac currents, τ Dirac , and bulk ones, τ Bulk , which reveals the interaction between topological states and bulk coherent phonons.…”

“…At the same time, we keep the surface and bulk carrier densities fixed, consistent with a physical picture of sub-gap photoexcitation that is decoupled from interband transitions. 23 A unique set of model parameters is obtained by simultaneously fitting conductivity and permittivity in the measured broad frequency range. By only varying the surface τ S and bulk τ B scattering times (i.e., 1/γ), we are able to consistently reproduce the time-dependent experimental results, as shown in Fig.…”

“…This fitting implies that the THz pumping used here gives rise to much smaller changes of surface and bulk quasiparticle densities as compared to the scattering time changes, unlike for high frequency pumping as discussed in ref. 23 . We can thus separate the intrinsic surface and bulk transport properties in both time and frequency domains.…”

Dynamical Separation of Bulk and Edge Transport in HgTe-Based 2D Topological Insulators

“…Topological signatures may be enhanced in dynamical studies by exploiting the difference in transport or scattering timescales between topological and bulk carriers. Recent efforts have for example demonstrated that dynamical experiments in the THz range [13] or using timeresolved ARPES [14] allow the investigation and manipulation of sensitive topological states despite bulk background carriers. Transposing this idea to a microwave transport regime, we propose microwave drives to dynamically isolate quasi-ballistic topological edge modes while bulk carriers have an evanescent contribution since their diffusivity collapses near the gap of the material.…”