Excitons In Transition Metal Dichalcogenides Research Articles

Strong Linearly Polarized Light Emission by Coupling Out-of-Plane Exciton to Anisotropic Gap Plasmon Nanocavity.

With exceptional quantum confinement, 2D monolayer semiconductors support a strong excitonic effect, making them an ideal platform for exploring light-matter interactions and as building blocks for novel optoelectronic devices. Different from the well-known in-plane excitons in transition metal dichalcogenides (TMD), the out-of-plane excitons in indium selenide (InSe) usually show weak emission, which limits their applications as light sources. Here, by embedding InSe in an anisotropic gap plasmon nanocavity, we have realized plasmon-enhanced linearly polarized photoluminescence with an anisotropic ratio up to ∼140, corresponding to degree of polarization (DoP) of ∼98.6%. Such polarization selectivity, originating from the polarization-dependent plasmonic enhancement supported by the "nanowire-on-mirror" nanocavity, can be well tuned by the InSe thickness. Moreover, we have also realized an InSe-based light-emitting diode with polarized electroluminescence. Our research highlights the role of excitonic dipole orientation in designing nanophotonic devices and paves the way for developing InSe-based optoelectronic devices with polarization control.

Large Rabi splitting energy in resonant quasi-BIC WSe2 metasurfaces

Strong coupling between excitons in transition metal dichalcogenides and cavities has attracted much attention in recent years, while the Rabi splitting values are usually small, which limits its further applications. In this work, we propose a WSe2 metasurface that supports quasi bound states in the continuous (QBIC) to realize a large Rabi splitting value by adjusting the effective overlap of the exciton with the resonance modes. The designed metasurface consists of composite nanoholes of WSe2. By changing the relative position of the nanoholes and modifying the symmetry of the structure, two QBIC modes are excited, and they mainly localize the electric field inside the device. By tuning the oscillator strength of WSe2, the strong coupling between excitons in WSe2 and QBIC modes is realized, resulting in Rabi splitting energies of 468 meV and 472 meV, respectively. This work provides insights into enhancing light-matter interactions and expands the potential applications of strong coupling.

(Invited) Exciton Transport in Strained 2D Semiconductors

Being atomically thin, flexible, and exhibiting considerable light emission and ultrafast non-equilibrium dynamics, semiconducting transition metal dichalcogenides (TMDs) have been considered as promising candidates for next-generation optoelectronic devices. The optical and electronic properties of TMDs are governed by a rich landscape of tightly bound excitons, including regular bright excitons, as well as optically inaccessible dark exciton states. Recently, strain engineering of monolayer TMDs has been introduced to tune their optical properties, such as the exciton transition energy, exciton-phonon coupling, or the Stokes shift [1-3].Transport of charge carriers is crucial for nanoelectronics. In conventional materials, electronic transport can be conveniently controlled by external electric fields. However, the tightly bound excitons, being neutral particles, are only weakly affected by electrical fields. We demonstrate that mechanical strain can also be used to manipulate the transport of excitons in TMDs. To this end, we apply homogeneous tensile strain to a WS2 monolayer by bending the substrate [1], which causes a redshift of the X0 exciton photoluminescence. By measuring the spatiotemporal photoluminescence after near-resonant excitation with femtosecond laser pulses, we map the spread of excitons and extract the strain-dependent diffusion coefficient [4].Furthermore, we demonstrate the propagation of excitons in an inhomogeneous strain landscape. We create inhomogeneous tensile strain in TMD monolayers by transferring them onto patterned substrates with nanopillars or by a nanoimprint technique [5]. Due to the redshift of the exciton resonances with applied strain, excitons are expected to move towards high-strain regions in an inhomogeneous strain field - the so-called "funneling" effect. We verify this behavior for the "bright" TMD material monolayer MoSe2. In the case of the "dark" monolayer WS2, we observe exactly the opposite effect. Here, the excitons are expelled from the high-strain regions ("anti-funneling") [6]. By comparing our experimental results with a microscopic theory, we explain this observation by the drift of momentum-dark KΛ excitons, which, in contrast to bright excitons, shift to higher energies with strain.Our joint experiment-theory study highlights the dominant role of momentum-dark excitons for the dynamics in monolayer TMDs and provides crucial design guidelines for TMD devices based on exciton transport.

Brightening and Directionality Control of Dark Excitons through Quasi-Bound States in the Continuum

Thanks to their long lifetime, spin-forbidden dark excitons in transition metal dichalcogenides are promising candidates for storage applications in opto-electronics and valleytronics. To date, their study has been hindered by inefficient generation mechanisms and the necessity for elaborate detection schemes. In this work, we propose a new hybrid platform that simultaneously addresses both challenges. We study an all-dielectric metasurface with two symmetrically protected quasi-bound states in the continuum to enhance both the excitation and emission of dark excitons in a tungsten diselenide monolayer under normal light incidence. Our simulations show a giant photoluminescence signal enhancement (∼520) along with directional emission, thus offering distinct advantages for opto-electronic and valleytronic devices.

Valley-addressable monolayer lasing through spin-controlled Berry phase photonic cavities.

The spin-valley coupling between circularly polarized light and valley excitons in transition metal dichalcogenides provides the opportunity to generate and manipulate spin information by exploiting the valley degree of freedom. Here, we demonstrate a room-temperature valley-addressable tungsten disulfide monolayer laser in which the spin of lasing is controlled by the spin of pump without magnetic fields. This effect was achieved by integrating a tungsten disulfide monolayer into a photonic cavity that supports two orthogonal spin modes with high quality factors. The spin-pumped lasing effectively broke the population symmetry of valley excitons, resulting in highly coherent emission with valley-switchable radiation modes due to distinct laser thresholds. Our scheme provides a nanophotonic platform to develop versatile coherent spin-light sources operating at room temperature by actively manipulating spin-valley coupling in light-matter interactions.

Exciton-assisted electron tunnelling in van der Waals heterostructures.

The control of elastic and inelastic electron tunnelling relies on materials with well-defined interfaces. Two-dimensional van der Waals materials are an excellent platform for such studies. Signatures of acoustic phonons and defect states have been observed in current-to-voltage measurements. These features can be explained by direct electron-phonon or electron-defect interactions. Here we use a tunnelling process that involves excitons in transition metal dichalcogenides (TMDs). We study tunnel junctions consisting of graphene and gold electrodes separated by hexagonal boron nitride with an adjacent TMD monolayer and observe prominent resonant features in current-to-voltage measurements appearing at bias voltages that correspond to TMD exciton energies. By placing the TMD outside of the tunnelling pathway, we demonstrate that this tunnelling process does not require any charge injection into the TMD. The appearance of such optical modes in electrical transport introduces additional functionality towards van der Waals material-based optoelectronic devices.

Tunable Couplings of Photons with Bright and Dark Excitons in Monolayer Semiconductors on Plasmonic-Nanosphere-on-Mirror Cavities

Tunable exciton–photon couplings are of great importance for cavity quantum electrodynamics (QED), polariton chemistry, optical computing, and bosonic lasing. In this regard, two-dimensional (2D) excitons in transition metal dichalcogenides (TMDs) have attracted tremendous interest─the huge oscillator strengths endow sensitive responses to optical modes, and their excitonic properties can be actively tuned by external stimuli. However, tunable coupling with spin-forbidden dark excitons in monolayer TMDs has rarely been demonstrated, and how the bright/dark-exciton–photon couplings coexist in one system is still a mystery. Herein, we utilize waveguide and antenna modes in a gold nanosphere-on-mirror cavity to match the in-plane and out-of-plane dipole moments of bright and dark excitons in monolayer WS2, respectively, and simultaneously. Strong bright-exciton–photon couplings with a coupling strength of up to 60 meV are observed at room temperature. We show that the waveguide mode can be tuned in wavelengths by controlling the spacer thickness and demonstrate the anti-crossing feature of polaritons. Next, by increasing the excitation angle, we dynamically enlarge the relative contribution from the antenna mode coupled with dark excitons and suppress the waveguide mode coupled with bright states. Moreover, a tunable bright exciton linewidth is achieved as a result of suppressed homogeneous broadening caused by the dominant dark exciton decays under strong dark-exciton–photon couplings. This is not only proof of tunable couplings with dark excitons but also an insightful finding that reveals the novel interactions between bright- and dark-exciton–photon hybrids in a single optical cavity, opening new possibilities in tunable QED.

Customizing Radiative Decay Dynamics of Two-Dimensional Excitons via Position- and Polarization-Dependent Vacuum-Field Interference.

Embodying bosonic and interactive characteristics in two-dimensional space, excitons in transition metal dichalcogenides (TMDCs) have garnered considerable attention. The utilization of the strong-correlation effects, long-range transport, and valley-dependent properties requires customizing exciton decay dynamics. Vacuum-field manipulation allows radiative decay engineering without disturbing intrinsic material properties. However, conventional flat mirrors cannot customize the radiative decay landscape in TMDC's plane or support vacuum-field interference with desired spectrum and polarization properties. Here, we present a meta-mirror platform resolving the issues with more optical degrees of freedom. For neutral excitons of the monolayer MoSe2, the optical layout formed by meta-mirrors manipulated the radiative decay rate in space by 2 orders of magnitude and revealed the statistical correlation between emission intensity and spectral line width. Moreover, the anisotropic meta-mirror demonstrated polarization-dependent radiative decay control. Our platform would be promising to tailor two-dimensional distributions of lifetime, density, diffusion, and polarization of TMDC excitons in advanced opto-excitonic applications.

Cavity-assisted boosting of self-hybridization between excitons and photonic bound states in the continuum in multilayers of transition metal dichalcogenides

Strong coupling between excitons in transition metal dichalcogenides (TMDs) and quasibound states in the continuum (QBIC) has attracted much attention in recent years. However, the coupling strength is often limited due to the spatial mismatch at the location of the TMDs and the maximum field strength of the QBIC mode. Here, we report a cavity-assisted boosting of self-hybridization between excitons (X) and the QBIC mode at room temperature by embedding a two-dimensional (2D) metasurface composed of bulk ${\mathrm{WS}}_{2}$ into a microcavity. We demonstrate that the self-hybridized BIC-X coupling strength in this 2D metasurface can be dramatically enhanced with the assistance of a Fabry-P\'erot cavity. Full wave simulations demonstrate a giant Rabi splitting up to 240 meV, which is twice as high as the QBIC-X self-hybridization in the 2D metasurface system. A coupled oscillator model containing a $3\ifmmode\times\else\texttimes\fi{}3$ Hamiltonian matrix combined with a near-field analysis reveals the underlying mechanism of the greatly enlarged coupling strength: The cavity provides a strong out-of-plane field confinement and the QBIC mode concentrates in an in-plane electric field, which greatly facilitates the spatial overlap of excitons with the localized field. Importantly, we also demonstrate that the coupling strength of the hybrid system can be readily tuned by controlling the excitonic oscillator strength of the bulk TMD material. This provides a powerful approach for manipulating the self-hybridization process. We believe that the cavity-based configuration proposed in this paper can serve as a universal recipe for achieving strong light-matter interactions, thus paving the way for the design of tunable exciton-polariton photonic devices with high performance.

Polaritonic Chern Insulators in Monolayer Semiconductors.

Systems with strong light-matter interaction open up new avenues for studying topological phases of matter. Examples include exciton polaritons, mixed light-matter quasiparticles, where the topology of the polaritonic band structure arises from the collective coupling between matter wave and optical fields strongly confined in periodic dielectric structures. Distinct from light-matter interaction in a uniform environment, the spatially varying nature of the optical fields leads to a fundamental modification of the well-known optical selection rules, which were derived under the plane wave approximation. Here we identify polaritonic Chern insulators by coupling valley excitons in transition metal dichalcogenides to photonic Bloch modes in a dielectric photonic crystal slab. We show that polaritonic Dirac points, which are markers for topological phase transition points, can be constructed from the collective coupling between valley excitons and photonic Dirac cones in the presence of both time-reversal and inversion symmetries. Lifting exciton valley degeneracy by breaking time-reversal symmetry leads to gapped polaritonic bands with nonzero Chern numbers. Through numerical simulations, we predict polaritonic chiral edge states residing inside the topological gaps. Our Letter paves the way for the further study of strong exciton-photon interaction in nanophotonic structures and for exploring polaritonic topological phases and their practical applications in polaritonic devices.

Tip-Enhanced Dark Exciton Nanoimaging and Local Strain Control in Monolayer WSe2.

Dark excitons in transition-metal dichalcogenides, with their long lifetimes and strong binding energies, provide potential platforms from photonic and optoelectronic applications to quantum information science even at room temperature. However, their spatial heterogeneity and sensitivity to strain is not yet understood. Here, we combine tip-enhanced photoluminescence spectroscopy with atomic force induced strain control to nanoimage dark excitons in WSe2 and their response to local strain. Dark exciton emission is facilitated by out-of-plane picocavity Purcell enhancement giving rise to spatially highly localized emission, providing for higher spatial resolution compared to bright exciton nanoimaging. Further, tip-antenna-induced dark exciton emission is enhanced in areas of higher strain associated with bubbles. In addition, active force control shows dark exciton emission to be more sensitive to strain with both compressive and tensile lattice deformation facilitating emission. This interplay between localized strain and Purcell effects provides novel pathways for nanomechanical exciton emission control.

Magneto-optical measurements of the negatively charged 2s exciton in WSe2

Monolayer transition metal dichalcogenides (TMDs) host a variety of optically excited quasiparticle species that stem from two-dimensional confinement combined with relatively large carrier effective masses and reduced dielectric screening. The magnetic response of these quasiparticles gives information on their spin and valley configurations, nuanced carrier interactions, and insight into the underlying band structure. Recently, there have been several reports of $2s/3s$ charged excitons in TMDs, but very little is still known about their response to external magnetic fields. Using photoluminescence excitation spectroscopy, we observe the presence of the $2s$ charged exciton and report its response to an applied magnetic field. We benchmark this response against the neutral exciton and find that both the $2s$ neutral and charged excitons exhibit similar behavior with $g$ factors of ${g}_{{X}_{0}^{2s}}=\ensuremath{-}5.20\ifmmode\pm\else\textpm\fi{}0.11$ and ${g}_{{X}_{\ensuremath{-}}^{2s}}=\ensuremath{-}4.98\ifmmode\pm\else\textpm\fi{}0.11$, respectively.

(Digital Presentation) Growth of Vdw Heterostructures with Tunable Moiré Patterns

Overlaying two two-dimensional (2D) lattices with different periodicities or a relative twist angle gives rise to a larger (quasi-) periodic lattice called a moiré pattern. In van der Waals (vdW) heterostructures, the moiré pattern has recently been shown to display interesting novel physical phenomena. For instance, twisted bilayer graphene near the “magic angle” of 1.1° hosts flat electronic bands and strongly correlated states such as unconventional superconductivity [1, 2]. On the other hand, the moiré pattern has been found to both localize and tune interlayer excitons in transition metal dichalcogenides (TMDCs) heterostructures with small twist angle [3-5]. Such moiré heterostructures are generally produced by mechanical exfoliation and stacking to precisely control the twist angle and moiré periodicity. However, this laborious method leads to interfacial contamination and twist angle inhomogeneities and is inherently not scalable. In principle, direct growth of van der Waals heterostructures can overcome these limitations. However, due to energetic considerations, control over the twist angle, and therefore the moiré lattice, has not yet been achieved. Herein, using TMDCs as a model system, we demonstrate the scalable growth of moiré heterostructures with continuously tunable periodicity. By controlling the substitutional alloying of larger atoms within the same group of the periodic table (S, Se), we precisely engineer the lattice parameters of the 2D layers and the moiré period. The grown moiré heterostructures have lateral size up to 200 μm and are shown to host moiré excitons. These results lay the groundwork for the integration of moiré heterostructures in emerging quantum technologies.[1] Cao, Y.; Fatemi, V.; Fang, S.; Watanabe, K.; Taniguchi, T.; Kaxiras, E.; Jarillo-Herrero, P. Nature 2018, 556 (7699), 43-50.[2] Cao, Y., Fatemi, V., Demir, A., Fang, S., Tomarken, S. L., Luo, J. Y., ... & Jarillo-Herrero, P. 2018. Nature, 556 (7699), 80-84.[3] Yu, H.; Liu, G.-B.; Tang, J.; Xu, X.; Yao, W. (2017). Science Advances 2017. 3 (11), e1701696.[4] Seyler, K. L.; Rivera, P.; Yu, H.; Wilson, N. P.; Ray, E. L.; Mandrus, D. G.; Yan, J.; Yao, W.; Xu, X. Nature 2019, 567 (7746), 66.[5] Jin, C.; Regan, E. C.; Yan, A.; Utama, M. I. B.; Wang, D.; Zhao, S.; Qin, Y.; Yang, S.; Zheng, Z.; Shi, S.; et al. Nature 2019, 567 (7746), 76.

Engineering the radiative dynamics of thermalized excitons with metal interfaces

As a platform for optoelectronic devices based on exciton dynamics, monolayer transition metal dichalcogenides (TMDCs) are often placed near metal interfaces or inside planar cavities. While the radiative properties of point dipoles at metal interfaces has been studied extensively, those of excitons, which are delocalized and exhibit a temperature-dependent momentum distribution, lack a thorough treatment. Here, we analyze the emission properties of excitons in TMDCs near planar metal interfaces and explore their dependence on exciton center-of-mass momentum, transition dipole orientation, and temperature. Defining a characteristic energy scale k B T c = (ℏk)2/2m (k being the radiative wavevector and m the exciton mass), we find that at temperatures T ≫ T c and low densities where the momentum distribution can be characterized by Maxwell–Boltzmann statistics, the modified emission rates (normalized to free space) behave similarly to point dipoles. This similarity in behavior arises due to the broad nature of wavevector components making up the exciton and point dipole emission. On the other hand, the narrow momentum distribution of excitons for T < T c can result in significantly different emission behavior as compared to point dipoles. These differences can be further amplified by considering excitons with a Bose Einstein distribution at high phase space densities, such as in a condensate phase. We find suppression or enhancement of emission relative to the point dipole case by several orders of magnitude. These insights can help optimize the performance of optoelectronic devices that incorporate 2D semiconductors near metal electrodes and can inform future studies of exciton radiative dynamics at low temperatures. Additionally, these studies show that nanoscale optical cavities are a viable pathway to generating long-lifetime exciton states in TMDCs.

Efficient ionization of two-dimensional excitons by intense single-cycle terahertz pulses

External electric fields are highly attractive for dynamical manipulation of excitons in two-dimensional materials. Here, we theoretically study the ionization of excitons in monolayer transition metal dichalcogenides (TMDs) by intense pulsed electric fields in the terahertz (THz) regime. We find that THz pulses with realistic field strengths are capable of ionizing a significant fraction of photogenerated excitons in TMDs into free charge carriers. Short THz pulses are therefore an efficient, non-invasive method of dynamically controlling the free carrier concentration in monolayer TMDs, which is useful for applications such as THz modulators. We further demonstrate that exciton ionization probabilities should be experimentally measurable by comparing free carrier absorption before and after the THz pulse. Detailed results are provided for different TMDs in various dielectric environments.

Unravelling strong electronic interlayer and intralayer correlations in a transition metal dichalcogenide

Electronic correlations play important roles in driving exotic phenomena in condensed matter physics. They determine low-energy properties through high-energy bands well-beyond optics. Great effort has been made to understand low-energy excitations such as low-energy excitons in transition metal dichalcogenides (TMDCs), however their high-energy bands and interlayer correlation remain mysteries. Herewith, by measuring temperature- and polarization-dependent complex dielectric and loss functions of bulk molybdenum disulphide from near-infrared to soft X-ray, supported with theoretical calculations, we discover unconventional soft X-ray correlated-plasmons with low-loss, and electronic transitions that reduce dimensionality and increase correlations, accompanied with significantly modified low-energy excitons. At room temperature, interlayer electronic correlations, together with the intralayer correlations in the c-axis, are surprisingly strong, yielding a three-dimensional-like system. Upon cooling, wide-range spectral-weight transfer occurs across a few tens of eV and in-plane p–d hybridizations become enhanced, revealing strong Coulomb correlations and electronic anisotropy, yielding a two-dimensional-like system. Our result shows the importance of strong electronic, interlayer and intralayer correlations in determining electronic structure and opens up applications of utilizing TMDCs on plasmonic nanolithrography.

Exciton Transitions in Monolayer WS2 Activated by Swift Heavy Ion Irradiation

The emergence of monolayer transition metal dichalcogenides (TMDCs) has provided a favorable platform for exploring a broad range of novel optoelectronic device applications and quantum phenomena due to their remarkable physical and chemical characteristics. Understanding the transition and motion of excitons in TMDCs is of fundamental interest for their optoelectronic devices. Here, we demonstrate the exciton transition in monolayer WS2 that can be fine-tuned by swift heavy ion (SHI) irradiation. The dependence of trion and exciton emissions on SHI irradiation in the as-transferred and as-grown monolayer WS2 was investigated by micro photoluminescence (PL) spectra where different PL responses were monitored. The trion-to-exciton transition in the as-transferred WS2 monolayer was ascribed to SHI irradiation that resulted in charge localization and pre-existing defect annealing, whereas the exciton-to-trion transition in the as-grown WS2 monolayer was attributed to the competition of charge localization and charge transfer caused by SHI irradiation. Femtosecond transient absorption measurements were performed to study the exciton relaxation dynamics in SHI-irradiated WS2, and it was shown that defect-assisted recombination dominates the exciton relaxation process for SHI-irradiated WS2. These results open up new opportunities to manipulate and tune the optical properties of two-dimensional TMDCs, which are of special importance for the development of optoelectronic and valleytronic devices.

Experimental observation of topological Z2 exciton-polaritons in transition metal dichalcogenide monolayers

The rise of quantum science and technologies motivates photonics research to seek new platforms with strong light-matter interactions to facilitate quantum behaviors at moderate light intensities. Topological polaritons (TPs) offer an ideal platform in this context, with unique properties stemming from resilient topological states of light strongly coupled with matter. Here we explore polaritonic metasurfaces based on 2D transition metal dichalcogenides (TMDs) as a promising platform for topological polaritonics. We show that the strong coupling between topological photonic modes of the metasurface and excitons in TMDs yields a topological polaritonic Z2 phase. We experimentally confirm the emergence of one-way spin-polarized edge TPs in metasurfaces integrating MoSe2 and WSe2. Combined with the valley polarization in TMD monolayers, the proposed system enables an approach to engage the photonic angular momentum and valley and spin of excitons, offering a promising platform for photonic/solid-state interfaces for valleytronics and spintronics.

Nonvolatile Electric Control of Exciton Complexes in Monolayer MoSe2 with Two-Dimensional Ferroelectric CuInP2S6.

Monolayer transition-metal dichalcogenides (TMDs) have provided a platform to investigate the excitonic states at the two-dimensional (2D) limit. The inherent properties of excitons in TMDs, such as the photoluminescence quantum yield, the charge states, and even the binding energy, can be effectively controlled via electrostatic gating, selective carrier doping, or substrate dielectric engineering. Here, aiming for the nonvolatile electrical tunability of excitonic states and thereby the optical property of TMDs, we demonstrate a 2D ferroelectric heterostructure with monolayer MoSe2 and ultrathin CuInP2S6 (CIPS). In the heterostructure, the electric polarization of CIPS results in continuous, global, and large electronic modulation in monolayer MoSe2. With the saturated ferroelectric polarization of CIPS, electron-doped or hole-doped MoSe2 is realized in a single device. The carrier density tunability in the heterostructure is as high as 5 × 1012 cm-2. The nonvolatile behavior of these devices up to 3 months is also characterized. Our results provide a new and practical strategy for low-power consumption and agelong tunable optoelectronic devices.

Optical dispersion of valley-hybridised coherent excitons with momentum-dependent valley polarisation in monolayer semiconductor

2D excitons in transition-metal dichalcogenides (TMDCs) offer interesting effects related to the valley pseudo-spin degree of freedom and long-range exchange interactions, as well as the coupling with light states. Several theoretical predictions have claimed that the neutral exciton of TMDCs splits into a transversal and longitudinal exciton branch, with the longitudinal one, which is the upper branch, exhibiting an extraordinary strong dispersion in the meV range within the light cone. Historically, this was linked for semiconductor quantum wells to strong far-field optical dipole coupling, or strong electronic long-range exchange interactions. Recently, experiments utilizing Fourier-space spectroscopy have shown that the exciton (exciton–polariton) dispersion can indeed be measured for high-quality hexagonal-BN-encapsulated WSe2 monolayer samples and can confirm the energy scale. Here, the exciton fine-structure’s pseudo-spin and the valley polarisation are investigated as a function of the centre-of-mass-momentum and excitation-laser detuning. For quasi-resonant excitation, a strong dispersion featuring a pronounced momentum-dependent helicity is observed. By increasing the excitation energy step-wise towards and then above the electronic band gap and the B-exciton level, the dispersion and the helicity systematically decrease due to contributions of incoherent excitons and emission from plasma. The decline of the helicity with centre-of-mass momentum can be phenomenologically modelled by the Maialle–Silva–Sham mechanism using the exciton splitting as the source of an effective magnetic field. By contributing to a better understanding of valley decoherence effects and the role of hybridised states in the optoelectronic properties, polarisation-sensitive Fourier-space investigations can support the development of future ‘optical-valleytronic’ devices.