Anomalous valley Hall dynamics of exciton-polaritons

Technical Summary: Anomalous Optical Valley Hall Dynamics of Exciton-Polaritons

Problem Statement
Transition-metal dichalcogenides (TMDs) possess a valley degree of freedom addressable by circularly polarized light, offering a binary index for valleytronics. However, conventional electronic and excitonic platforms face significant limitations in achieving high-speed valley Hall transport due to large effective masses and reliance on electrical control, which restrict both speed and coherence. While exciton-polaritons—hybrid light-matter quasiparticles formed under strong coupling—offer a route to overcome these limitations via ultralight effective mass and long-range coherence, previously reported optical valley Hall effects in polaritonic systems have been predominantly driven by cavity-induced mechanisms (e.g., TE-TM splitting). These conventional mechanisms result in a $\pi$-periodic Hall response locked to the valley pseudospin texture, constraining the symmetry and tunability of valley transport. A critical gap remains in observing a macroscopic, spatially separated valley Hall effect (a $2\pi$-periodic evolution) that mimics conventional charge and spin Hall effects, enabling direct real-space quantification of drift velocity and intuitive valley addressing.

Methodology
The authors investigated a monolayer tungsten disulfide (WS$_2$) embedded within a hybrid optical microcavity. The sample structure consists of a 30-pair SiO$_2$/TiO$_2$ distributed Bragg reflector (DBR) bottom mirror, a 105 nm SiO$_2$ spacer, the CVD-grown WS$_2$ monolayer, a 75 nm PMMA spacer, and a 50 nm silver top mirror. The system was tuned to a negative detuning of approximately $-75$ meV, placing the lower polariton branch in a photon-like regime with a Rabi splitting of $\sim$40 meV.

Experiments were conducted at 5 K using polarization- and time-resolved real-space imaging. The system was excited using 150 fs laser pulses at 532 nm under two conditions:

1. Linearly polarized excitation: To simultaneously populate both inequivalent valleys and observe symmetry-breaking spatial separation.
2. Circularly polarized excitation ($\sigma^+$ and $\sigma^-$): To assess valley polarization retention, drift directionality, and lifetime dynamics.

Detection involved angle-resolved reflectance spectroscopy to confirm strong coupling and time-resolved photoluminescence (PL) using a streak camera to track the spatiotemporal evolution of the degree of polarization (DOP). Theoretical modeling incorporated strain-induced synthetic pseudomagnetic fields and coupled rate equations based on Hopfield coefficients to explain the observed dynamics.

Key Contributions and Results

• Observation of Anomalous Optical Valley Hall Effect: Under linearly polarized excitation, the authors directly visualized a symmetry-breaking spatial separation of polaritons from opposite valleys. Unlike the $\pi$-periodic patterns typical of cavity-driven effects, the observed polarization texture formed a "Tai Chi-like" pattern, indicating a $2\pi$-periodic evolution and a direct real-space analogue of conventional Hall effects.
• Ultrafast Drift Velocity: The separation of valley populations occurred on an ultrafast timescale. By tracking the displacement of the valley-polarized population over time, the authors extracted a Hall drift velocity of $1.69 \times 10^5$ m/s. This velocity is one to two orders of magnitude higher than values reported in conventional electronic or excitonic valley Hall systems, attributed to the ultralight effective mass of the polaritons ($m_p \sim 10^{-4} m_e$).
• Mechanism Identification: The observed behavior could not be accounted for by conventional cavity-induced TE-TM splitting. Instead, the results point to a strain-induced synthetic pseudomagnetic field acting on the excitonic component of the polaritons. Residual strain in the large-area CVD-grown WS$_2$ monolayer generates an effective field that drives valley-dependent precession and transverse deflection.
• Circular Excitation Dynamics: Under circularly polarized excitation, the valley-polarized polaritons exhibited a distinct ring-like spatial pattern with a diameter increasing with pump power. The drift distance showed an approximately linear dependence on the logarithm of pump power. Furthermore, a lateral asymmetry was observed where the side of the ring corresponding to the excitation helicity showed enhanced polarization, consistent with valley-dependent directional steering.
• Angle-Dependent Valley Lifetime: The study revealed that valley polarization lifetime is strongly dependent on the emission angle. At larger emission angles (e.g., 25$^\circ$), the valley lifetime nearly doubled compared to normal emission (0$^\circ$), reaching 15.51 ps under $\sigma^+$ excitation. This enhancement is attributed to the evolving balance between excitonic and photonic components (Hopfield coefficients) at higher in-plane momenta, which suppresses depolarization while maintaining efficient population from the excitonic reservoir.

Significance and Claims
The paper establishes exciton-polaritons as a high-speed, optically accessible platform for valley transport. The authors claim that their results demonstrate a direct, scalable analogue of the conventional Hall effect in a polaritonic system, characterized by:

1. Exceptional Dynamical Response: The ability to achieve valley Hall drift velocities approaching $10^5$ m/s, enabling access to ultrafast valley transport regimes inaccessible in purely electronic systems.
2. Symmetry Breaking: The observation of a macroscopic, spatially separated valley domain structure that breaks the symmetry constraints of previous cavity-driven mechanisms.
3. Tunability and Robustness: The demonstration of tunable drift via pump power and robust valley polarization retention up to room temperatures (implied by the suppression of intervalley scattering via radiative decay), alongside extended lifetimes at specific angles.

The authors conclude that these findings open pathways toward tunable valleytronic and topological photonic devices, leveraging the hybrid light-matter nature of polaritons for ultrafast information processing and manipulation.