Ellipticity-Controlled Bright-Dark Coherence Transition in Monolayer WSe2

Abstract

The generation of exciton valley coherence typically requires linearly polarized (LP) light as an external coherent drive, whereas circularly polarized (CP) light fails to induce coherence. Here, we develop a unified, microscopically-grounded open-quantum-system framework within a five-level model incorporating bright-dark exciton interactions in monolayer WSe2, and demonstrate that the polarization ellipticity of the excitation field provides selective control over distinct exciton species contributing to valley coherence. Specifically, LP and CP excitations generate bright and dark coherence, respectively, with continuous ellipticity tuning enabling controlled transitions between these states. We further reveal dual magnetic advantages for manipulating dark coherence even in the absence of initial coherence: (i) an out-of-plane magnetic field suppresses coherence decay and (ii) an in-plane field enables its optical readout, with quantitatively realistic field strengths. These findings provide a powerful mechanism for accessing hidden dark states via ellipticity-driven coherence transfer, and establish a new pathway for harnessing bright-dark valley-coherence transitions in future quantum control.

Figures (4)

• Figure 1: (a) Five-level model with bright and dark excitons. Coherent intervalley coupling in the dark manifold arises from SR exchange; Incoherent processes originate from LR exchange and system-reservoir coupling, yielding Lindblad rates $\Gamma_i$: $\Gamma_{b}^{K,K'}$ ($\Gamma_{d}^{K,K'}$) for bright (dark) recombination, $\Gamma_{H,L}$ for bright-exciton intervalley scattering, and $\Gamma_{bd}^{K(K')}$ for bright-dark scattering. (b) Energies of spin-unlike excitons $|\text{K}_d\rangle$, $|\text{K}'_d\rangle$ and exchange-dressed eigenstates $|\text{G}\rangle$ (grey), $|\text{D}\rangle$ ("truly" dark) versus out-of-plane field. (c) Exciton dispersions. I: Spin-like and spin-unlike valley excitons (red/green); I$\rightarrow$II: LR and SR hybridize valely states into valley-mixed bright (blue/pink) and dark (cyan/orange) excitons; II$\rightarrow$III: Out-of-plane field tunes valley mixing and splitting; III$\rightarrow$IV: In-plane field additionally mix bright and dark sectors. Color gradients indicate momentum-dependent composition.
• Figure 2: Time evolutions of VC for bright [$\mathcal{C}_b(\rho)$: red line] and dark [$\mathcal{C}_d(\rho)$: blue line] excitons under LP (a), CP (b) and EP (c) excitations. The green circles in (a) and (b) refer to the coherence times $\uptau_l^{\mathcal{C}}$. (d) Poincare sphere schematic and optical polarization state representation. Here $\chi$ and $\psi$ respectively represent the orientation angle of major axis for the polarization ellipse and the ellipticity angle. (e) Summary of exciton coherence driven by different polarized lights.
• Figure 3: The time evolutions of dark coherence for different out-of-plane (a) and in-plane (b) magnetic field strength. The yellow circles in (a) and (b) refer to the maximum coherence intensity $\mathcal{C}_d(\rho)_{\max}$. The inset in (b) shows the ratio of bright-dark mixing [Eq. \ref{['eqcoherlu']}]. The dark coherence at $t=2$ (c) and $8$ ps (d) as a function of magnetic field strength and azimuth angle $\theta(\pi)=\arctan(B_{\parallel}/B_{\perp})$. Several values of contour lines of coherence intensity $\mathcal{C}_d(\rho)$ are also shown.
• Figure 4: The time evolution of dark coherence for different intravalley scattering times $\uptau_{bd}$ (a), intervalley scattering times $\uptau_{\text{v}}$ (b), exchange interactions $\updelta$ (c) and temperatures $T$ (d). The pink circles in (a), (b) and (d) refer to the maximum coherence intensities $\mathcal{C}_d(\rho)_{\max}$, while the green circle in (c) refers to the coherence time $\uptau_d^{\mathcal{C}}$.