Linearly polarized light enables chiral edge transport in quasi-2D Dirac materials

TL;DR

This study shows that linearly polarized high-frequency light can induce Floquet topological phases in quasi-2D Dirac materials, owing to second-order momentum terms from intersurface hybridization. Using ultrathin Bi$_2$Se$_3$ films, the authors derive an effective Floquet Hamiltonian with a light-induced mass term, revealing a transition to a Chern insulator with chiral edge states and a quantized Hall conductance without needing circular polarization or magnetic doping. The transition occurs at experimentally accessible light intensities and thickness-dependent thresholds, highlighting the pivotal role of confinement and interlayer coupling in non-equilibrium band topology. Overall, the work broadens Floquet engineering’s scope to linearly polarized driving in quasi-2D systems, enabling tunable, dissipationless edge transport for potential optoelectronic and quantum-information applications.

Abstract

Floquet engineering with high-frequency light offers dynamic control over topological phases in quantum materials. While in 3D Dirac systems circularly polarized light is known to induce topological phase transitions via gap opening, linearly polarized light (LPL) has generally been considered ineffective. Here we show that in quasi-2D Dirac materials the second-order momentum term arising from the intersurface coupling can induce a topological phase transition under LPL, leading to chiral edge channels. Considering an ultrathin Bi$_2$Se$_3$ film as a representative system, we show that this transition occurs at experimentally accessible light intensities. Our results thus promote quasi-2D materials as viable platforms for light-controlled topological phases, expanding the potential of Floquet topological engineering.

Figures (5)

• Figure 1: Electromagnetic spectrum is shown with corresponding wavelength and angular frequency ranges. NIR region is selected to satisfy the high-frequency condition relative to the $\sim$0.3 eV bandgap of Bi$_2$Se$_3$, enabling effective Floquet engineering. Plotted are the refractive index (extracted from Ref. fang2020layer), and the maximum permissible optical field amplitude and intensity, for Bi$_2$Se$_3$ films of various thicknesses (2–5 QLs as ultrathin film and 8 QLs as a representative thick film) within the NIR regime.
• Figure 2: Floquet band structure and Berry curvature of a 3QL Bi$_2$Se$_3$ film under LPL. The applied light induces a bandgap modulation in the surface states, leading to a topological phase transition. The sign reversal of the Berry curvature at critical field amplitude ($A = 0.88$ nm$^{-1}$) confirms the emergence of a light-induced topological phase.
• Figure 3: Light-induced modulation of the surface-state bandgap in Bi$_2$Se$_3$ ultrathin (2-5 QLs) and thick films under LPL. Bandgap variation versus optical field amplitude $A$ exhibits gap closing and reopening, indicating a topological phase transition. The inset shows the threshold amplitude $A_{\text{th}}$ for each thickness in the ultrathin regime. The thick film remains topologically unaffected by the applied field, in the entire range considered.
• Figure 4: Average $z$-component of the spin expectation value $\langle S_z \rangle$ versus optical field amplitude $A$, for ultrathin and thick Bi$_2$Se$_3$ films. While $\langle S_z \rangle$ remains zero for the thick film due to the preserved helical spin texture, it varies non-monotonically in ultrathin films, changing sign at a threshold amplitude $A_{\text{th}}$, indicating a light-induced topological phase transition.
• Figure 5: Longitudinal ($\sigma_{xx}$) and Hall ($\sigma_{xy}$) conductances of Bi$_2$Se$_3$ ultrathin films (2–5 QLs) under LPL as a function of the optical field amplitude $A$, with $E_F = 0.001$ eV and temperature $\Theta = 0.001$. The quantized Hall plateau $\sigma_{xy} = -e^2/h$ and vanishing $\sigma_{xx}$ for $A > A_{\text{th}}$ confirm the emergence of chiral edge states with Chern number $C = -1$.