Anomalous valley Hall dynamics of exciton-polaritons

TL;DR

The paper tackles the need for ultrafast valley transport in two-dimensional TMD systems by demonstrating an anomalous optical valley Hall effect in a monolayer WS2 exciton-polariton microcavity. Using polarization- and time-resolved real-space imaging, the authors observe a symmetry-breaking spatial separation of valley-polaritized polaritons under linearly polarized driving and measure an ultrafast Hall drift velocity on the order of one hundred thousand meters per second, inconsistent with conventional cavity-induced mechanisms. They attribute the effect to a strain-induced synthetic pseudomagnetic field acting on the excitonic component of the polaritons, supported by angle-resolved valley dynamics and circular-polarization measurements showing valley-dependent drift and extended lifetimes. The results establish exciton-polaritons as a high-speed, optically accessible platform for tunable valley transport, with implications for valleytronic and topological photonic devices.

Abstract

The valley degree of freedom in atomically thin transition-metal dichalcogenides provides a natural binary index for information processing. Exciton-polaritons formed under strong light-matter coupling offer a promising route to overcome the limited lifetime and transport of bare valley excitons. Here we report an anomalous optical valley Hall effect in a monolayer WS2 exciton-polariton system. Using polarization- and time-resolved real-space imaging, we directly visualize a symmetry-breaking spatial separation of polaritons from opposite valleys under linearly polarized excitation, accompanied by an ultrafast Hall drift velocity on the order of 10^5 m/s. This behaviour cannot be accounted for by conventional cavity-induced mechanisms and instead points to a strain-induced synthetic pseudomagnetic field acting on the excitonic component of polaritons. Our results establish exciton-polaritons as a high-speed and optically accessible platform for valley transport, opening pathways towards tunable valleytronic and topological photonic devices.

Figures (12)

• Figure 1: Sample structure and optical characterization.a, Schematic of the hybrid optical microcavity consisting of a 30-pair DBR as the bottom mirror and a silver top mirror, with a monolayer WS${}_2$ embedded at the antinode of the cavity field. b, Optical microscope image of the cavity region. The dotted outline delineates the CVD-grown WS${}_2$ monolayer. c, Angle-resolved reflectance spectra of the microcavity. The solid white curves denote the upper (UP) and lower polariton (LP) branches obtained from a coupled-oscillator fit, while the blue dashed lines mark the exciton resonance, $E_X$, and the bare cavity photon dispersion, $E_c$. The extracted detuning is $\Delta \approx–75~\mathrm{meV}$. d, Polarization-resolved PL spectra of the lower polariton branch under $150~\mathrm{fs}$ circularly polarized $532~\mathrm{nm}$ excitation, yielding a degree of polarization of approximately $\pm25\%$.
• Figure 1: Angle-resolved reflectance spectra of the sample measured at room temperature. A distinct anticrossing behavior is observed, confirming the strong coupling regime in the system. As the temperature increases to room temperature, the exciton energy exhibits a redshift, resulting in a detuning value of $-42 ~\mathrm{meV}$. The solid white curves denote the upper (UP) and lower polariton (LP) branches obtained from a coupled-oscillator fit, while the blue dashed lines mark the exciton resonance, $E_X$, and the bare cavity photon dispersion, $E_c$.
• Figure 1: The simulated valley polarization distribution in k space. Polaritons at $-\mathrm{k}_x$ and $\mathrm{k}_x$ exhibit negative and positive valley polarization, respectively.
• Figure 2: Dynamics of the anomalous optical valley Hall effect.a, Experimental real-space map of the valley polarization under linearly (vertically) polarized excitation along the Y-direction. The green circle indicates the excitation laser spot, and the double-headed arrow denotes the polarization direction of the incident light. b, Simulated real-space valley-polarization map under comparable excitation conditions, incorporating strain-induced symmetry breaking via an effective pseudomagnetic field. c, Time-resolved real-space snapshots illustrating the evolution of the valley Hall deflection. Dashed red and blue lines indicate the evolution paths of the $\sigma^{+}$ and $\sigma^{-}$ components of the valley polarization. d, Displacement of the valley-polarized polariton population as a function of time at an excitation power of $30~\mathrm{mW}$. A linear fit (red line) yields a Hall drift velocity of $1.69~\times 10^{5}~\mathrm{m/s}$. The inset shows a line cut of the real-space DOP distribution at $5~\mathrm{ps}$, where the experimental data are represented by circles, the bold light-blue band indicates the $95\%$ prediction interval, and the navy-blue curve corresponds to a two-Gaussian fit.
• Figure 2: Time-resolved PL at -$25^\circ$ under circular excitation. Red and blue dotted curves correspond to PL collected in the $\sigma^+$ and $\sigma^-$ channels, respectively, while solid curves represent theoretical fits. Green dotted curves show the temporal decay of valley polarization with single-exponential fits (solid lines). The extracted lifetimes are 15.75 and $15.59~\mathrm{ps}$.