Valley Polarization Control and Coherent Exciton Dynamics in Transition Metal Dichalcogenides

Abstract

Monolayer transition metal dichalcogenides (TMDCs) have attracted significant attention due to their exceptional electronic and optical properties, including direct band gaps, pronounced spin-orbit coupling, and strong excitonic effects arising from reduced dielectric screening. These atomically thin semiconductors exhibit valley-specific optical selection rules, enabling selective excitation and manipulation of exciton populations in distinct momentum-space valleys (K and K'). However, rapid valley depolarization processes mediated by intervalley scattering and the formation of spin-forbidden, optically inactive dark exciton states present considerable challenges for practical valleytronic and optoelectronic applications. Addressing these challenges requires rigorous theoretical descriptions capable of accurately capturing the underlying microscopic mechanisms governing exciton dynamics. To this end, we develop a comprehensive theoretical framework to describe and control the dynamics of valley polarization and coherent exciton states in monolayer TMDCs.

We first establish the quantum light-matter and open-quantum-system formalisms that underpin our analysis. This includes quantization of the electromagnetic field and a Jaynes-Cummings cavity-QED model to describe exciton-photon strong coupling under the rotating-wave and dipole approximations. We then derive a Lindblad master equation, starting from a microscopic exciton-phonon Hamiltonian and invoking the Born-Markov and secular approximations, to incorporate phonon-assisted relaxation, decoherence, and bright-to-dark exciton conversion induced by the solid-state environment. With these tools in hand, we develop a coherent optical response formalism to bridge the microscopic theory with observable spectra. Using perturbative nonlinear optics methods, the third-order response functions of the exciton system are constructed, enabling simulation of multi-dimensional spectroscopic signals (such as pump-probe spectra). This theoretical spectroscopy approach provides insight into exciton dynamics by predicting how coherent optical signals emerge and evolve when excitons interact with sequences of ultrafast pulses, all within our model.

We then investigate valley exciton dynamics under chiral cavity coupling as a route to manipulate the valley degree of freedom. In this scheme, a monolayer TMDC is embedded in a single-mode chiral optical cavity that interacts preferentially with excitons of one valley polarization. Within this cavity-QED setup, right-circularly polarized cavity photons are made resonant with, say, K-valley A excitons, while excitons in the K' valley remain uncoupled. The resulting strong light-matter interaction gives rise to mixed exciton-photon quasiparticles (exciton-polaritons) characterized by a Rabi-splitting of the exciton resonance. We show that this exciton-cavity hybridization can be used to control and prolong valley polarization. Specifically, by splitting the degeneracy between K and K' valley excitonic states, the cavity suppresses the rapid intervalley exchange process that would otherwise equilibrate valley populations. We solve the coupled exciton-photon dynamics (using an extended Jaynes-Cummings model) together with the Lindblad master equation for phonon-induced scattering. This allows us to track the time-dependent valley polarization of excitons, including the transient population of dark exciton states that are optically inactive but get populated via phonon-mediated spin flips. Our theoretical results reveal oscillatory and long-lived components in the valley polarization decay when the cavity coupling is introduced, indicating that a chiral cavity can significantly enhance valley polarization retention. We also extend the model to incorporate exciton-exciton interactions and biexciton (two-exciton) states, providing a more complete picture of multi-exciton processes in the cavity. A biexciton Hamiltonian is introduced to examine how bound two-exciton complexes evolve in the presence of cavity coupling and how they contribute to the nonlinear optical response. Through these studies, we demonstrate a viable strategy for coherent control of exciton populations and valley pseudospin in 2D materials via engineered light-matter coupling.

Next, we turn to the physics of coherent excitons and the emergence of optical gain in monolayer TMDCs at high excitation densities. Conventionally, achieving net optical gain in a semiconductor requires creating a population-inverted electron-hole plasma above the Mott density (the critical density at which excitons dissociate into free carriers). Here, we show that collective phase coherence among excitons can enable stimulated emission—and hence optical gain—without requiring conventional interband population inversion. We formulate coupled dynamical equations for the coherent and incoherent exciton populations, incorporating key many-body effects such as exciton-exciton annihilation (Auger-like recombination) and ongoing exciton-phonon dephasing. Using the nonlinear response formalism adapted to this exciton system, we calculate the transient polarization and simulate pump-probe spectra as a function of excitation density. The simulated spectra exhibit clear signatures of optical gain once a sizable coherent exciton population is present, in contrast to an incoherent exciton gas, which yields only bleaching or absorption. We also analyze the first-order coherence function and interference fringes of the exciton field to quantify the emergence of phase coherence. The theory predicts that in this coherent regime, the exciton system can behave as a laser-like gain medium: the emission from the exciton population becomes phase-coherent and grows in intensity without a sharp threshold. This finding implies that a form of laser-like emission with an extremely low threshold could be achieved via excitonic coherence alone, even in the absence of conventional band-to-band population inversion. In other words, our model suggests that monolayer TMDCs can support coherent light amplification through collective exciton dynamics well below the Mott transition density.

In summary, our work spans a broad range of quantum light-matter interaction topics to illuminate excitonic phenomena in monolayer TMDCs from a theoretical perspective. We integrate cavity quantum electrodynamics, open quantum system theory, and nonlinear optical spectroscopy into a unified framework to address the interplay between valley pseudospin, spin-forbidden dark states, lattice vibrations, and many-exciton correlations. By formulating solvable Hamiltonian models and master equations, and by deriving explicit coherent response functions, we provide fundamental insights into how exciton populations evolve, relax, and can be controlled via light in two-dimensional semiconductors. The results deepen our understanding of how to manipulate valley polarization through tailored light-matter coupling and how collective exciton coherence can enable novel optical functionalities. These theoretical advances lay the groundwork for future valleytronic and quantum photonic applications based on exciton dynamics in atomically thin materials.

Date of Award	18 Nov 2025
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Dangyuan LEI (Supervisor)