Interlayer excitons in transition-metal dichalcogenide heterobilayers possess intrinsic out-of-plane dipole moments, providing a platform for investigating exciton-exciton interactions at high densities. Here, we use excitation-energy-dependent photoluminescence excitation (PLE) spectroscopy to probe the nonlinear response of dipolar interlayer excitons in chemical vapor deposition-grown MoSe_2/WSe_2 heterobilayers. By tuning the excitation energy across intralayer exciton resonances at fixed excitation power, we selectively vary the population injected into the interlayer exciton states. Resonant excitation drives the system into a nonlinear regime, leading to saturation of the interlayer exciton photoluminescence and an apparent broadening of the intralayer 1s resonances in the PLE spectra. At the same time, the interlayer exciton emission exhibits a pronounced blueshift, reaching approximately 2.5 meV at 4 K and 1 meV at 75 K. The blueshift increases systematically with the interlayer exciton population and is consistent with a net repulsive exciton-exciton interaction, with contributions from dipole-dipole repulsion in the density regime investigated. Our results establish PLE as a sensitive approach for accessing the nonlinear, high-density regime of interlayer excitons and probing their interactions in van der Waals heterostructures.

Lateral heterostructures (LHSs) of 2D transition metal dichalcogenides (TMDs) offer a powerful platform to investigate photonic and electronic phenomena at atomically sharp interfaces. However, their controlled engineering, including tuning lateral domain size and integration into vertical van der Waals heterostructures with other 2D materials, remains challenging. Here, we present a facile route for the synthesis of two types of heterostructures (HSs), consisting of monolayers (MLs) of MoSe2 and WSe2-purely lateral (HS I) and hybrid lateral/vertical (HS II)-using liquid precursors of transition metal salts and chemical vapor deposition (CVD). Depending on the growth parameters, the heterostructure type and its lateral dimensions can be adjusted. We characterized properties of the HS I and HS II by complementary spectroscopic and microscopic techniques, including Raman and photoluminescence (PL) spectroscopy, optical and atomic force microscopy (AFM), and scanning and transmission electron microscopy (TEM). The PL measurements reveal strong interlayer exciton (IE) emission in the MoSe2/WSe2 region of HS II, which dominates the spectrum at 4 K and persists up to room temperature (RT). These results demonstrate high optical quality of the grown HSs, which in combination with the scalability of the developed approach, pave the way for fundamental studies and device applications based on these unique 2D quantum materials.

Transition metal dichalcogenides (TMDs) in their monolayer form offer a premier platform for next-generation optoelectronics, particularly through the local manipulation of their robust excitonic states using nanoscale electric fields. These localized states can be dynamically controlled through spatial structuring as well as through ultrafast field modulations driven by tailored optical pulses. Characterizing the resulting rapid, nanoscale charge carrier dynamics requires a technique with exceptional spatial and temporal resolution. Here, we report on the spatio-temporally resolved investigation of ground state and excited state photoemission from a lateral heterostructure built of monolayers of WSe_2 and MoSe_2 using few-cycle light pulses with a photon energy of 0.62 eV. We utilize photoemission electron microscopy to spatially resolve the highly nonlinear photoemission from the monolayer structure with few tens of nanometer resolution. By varying the laser pulse energy, we extract the nonlinearity of the photoemission process and thus the dynamic binding energy of the photoelectrons before and after optical excitation with high spatial and temporal resolution.

The van der Waals antiferromagnet CrSBr exhibits coupling of vibrational, electronic, and magnetic degrees of freedom, giving rise to distinctive quasi-particle interactions. We investigate these interactions across a wide temperature range using polarization-resolved Raman spectroscopy at various excitation energies, complemented by optical absorption and photoluminescence excitation (PLE) spectroscopy. Under 1.96 eV excitation, we observe pronounced changes in the $${A}_{g}^{1}$$, $${A}_{g}^{2}$$, and $${A}_{g}^{3}$$ Raman modes near the Néel temperature, coinciding with modifications in the oscillator strength of excitonic transitions and clear resonances in PLE. The distinct temperature evolution of Raman tensor elements and polarization anisotropy of Raman modes indicates that they couple to different excitonic and electronic states. The suppression of the excitonic states' oscillation strength above the Néel temperature could be related to the magnetic phase transition, thereby connecting these excitonic states and Raman modes to a specific spin alignment. We develop a simple model that describes how magnetic order impacts excitonic states and hence the intensity and polarization of the Raman scattering signal. These observations make CrSBr a versatile platform for probing quasi-particle interactions in low-dimensional magnets and provide insights for applications in quantum sensing and quantum communication.

Spin-orbit interactions couple light polarization to its transverse structure. Although widely studied at interfaces, in tilted anisotropic plates, and under tight focusing, their role in paraxial confocal systems remains unclear. We show that a commercial quarter-wave plate between crossed polarizers at nominal normal incidence converts the rejected Gaussian field into a Hermite-Gaussian-like two-lobe mode. Its reduced overlap with the collecting fiber mode enhances the polarization extinction ratio by over two orders of magnitude. For an ideal plate at normal incidence, the leading momentum-space contribution vanishes by symmetry. An effective angular model reproduces the symmetry but not the absolute intensity or the beam-size dependence. We trace this discrepancy to first-order real-space nonuniformities of the plate, amplified by the confocal geometry and isolated through beam-size scaling. Cross-polarization confocal detection thus renders weak spatially varying polarization responses observable as transverse-mode transformations.

Semiconducting transition metal dichalcogenides (TMDs), such as MoSe2and WSe2, exhibit unique optical and electronic properties. Vertical stacking of layers of one or more TMDs, to create heterostructures, has expanded the fields of moiré physics and twistronics. Bottom-up fabrication techniques, such as chemical vapor deposition, have advanced the creation of heterostructures beyond what was possible with mechanical exfoliation and stacking. These techniques now enable the fabrication oflateralheterostructures (LHs), where two or more monolayers are covalently bonded in the plane of their atoms. At their atomically sharp interfaces, lateral heterostructures exhibit additional phenomena, such as the formation of charge-transfer excitons, in which the electron and hole reside on opposite sides of the interface. Due to the energy landscape created by differences in the band structures of the constituent materials, unique effects such as unidirectional exciton transport and excitonic lensing can be observed in LHs. This review outlines recent progress in exciton dynamics and spectroscopy of TMD-based LHs and offers an outlook on future developments in excitonics in this promising system.

Spin-orbit interactions of light couple polarization and spatial degrees of freedom, underpinning phenomena such as the spin Hall effect of light. Although widely explored at interfaces and in tightly focused beams, their impact in nominally paraxial confocal systems remains largely unexamined. Here we show that a single quarter-wave plate embedded in a simple confocal geometry between polarizers can strongly reshape the transverse structure of a Gaussian beam. We observe an enhancement of the polarization extinction ratio by more than two orders of magnitude, accompanied by a transformation of the Gaussian intensity profile into a first-order Hermite-Gaussian-like two-lobe mode. The orientation of this pattern is continuously tunable via rotation of the wave plate, evidencing polarization-controlled reorientation of the transverse field. To explain these observations, we introduce a minimal extension of Jones matrix formalism incorporating complex parameters that quantitatively reproduces the measurements. Our results uncover a previously overlooked form of spin-orbit-mediated mode control in standard confocal optics and establish a simple route to on-demand spatial mode engineering for applications in resonant spectroscopy, optical imaging and quantum optics.

Moiré superlattices in twisted bilayers enable strong reconstruction of electronic band structures, giving rise to correlated phases with high tunability. Extending this concept to van der Waals magnets, we show that twisting induces spatially varying interlayer exchange interactions that can stabilize complex magnetic responses. Here, we demonstrate robust magnetic hysteresis in bilayer CrSBr upon a twist of  ~ 3°, observed as a hysteretic evolution of exciton energies that directly track the underlying magnetic configuration in field-dependent photoluminescence measurements. An analytic two-sublattice model captures this behaviour, attributing it to a twist-induced reduction of interlayer exchange that stabilizes both parallel and antiparallel spin states over a broad field range. Spatially resolved measurements reveal local variations in hysteresis loops, consistent with position-dependent modulation of magnetic parameters. In certain regions, coherent averaging over the moiré unit cell yields an effective monodomain-like response. Our results establish twist engineering as a route to programmable magnetism in two-dimensional antiferromagnets.

In the last decade atomically thin 2D materials have emerged as a perfect platform for studying and tuning light-matter interaction and electronic properties in nanostructures. The optoelectronic properties in layered materials such as transition-metal-dichalcogenides (TMDs) are governed by excitons, Coulomb bound electron-hole pairs, even at room temperature. The energy, wave function extension, spin and valley properties of optically excited conduction electrons and valence holes are controllable via multiple experimentally accessible knobs, such as lattice strain, varying atomic registries, dielectric engineering as well as electric and magnetic fields. This results in a multitude of fascinating physical phenomena in optics and transport linked to excitons with very specific properties, such as bright and dark excitons, interlayer and charge transfer excitons as well as hybrid and moiré excitons. In this book chapter we introduce general optoelectronic properties of 2D materials and energy landscapes in TMD monolayers as well as their vertical and lateral heterostructures, including twisted TMD hetero- and homobilayer bilayers with moiré excitons and lattice recombination effects. We review the recently gained insights and open questions on exciton diffusion, strain- and field-induced exciton drift. We discuss intriguing non-linear many-particle effects, such as exciton halo formation, negative and anomalous diffusion, the surprising anti-funneling of dark excitons.

Controlling the motion of neutral excitons in optically active media is a mandatory development to enable the conception of advanced circuits and devices for applications in excitronics, quantum photonics, and optoelectronics. Recently, proof of unidirectional exciton transport from high- to low-bandgap material is evidenced using a high-quality lateral heterostructure separating transition metal dichalcogenide monolayers (TMD-MLs). In this paper, by combining room-temperature micro-photoluminescence far-field imaging with a statistical description of exciton transport, the underlying excitonic local distribution and fluxes taking place near lateral heterojunctions are unveiled. The complex 2D excitonic transport properties found near a linear interface separating WSe2 from MoSe2 TMD-MLs are studied and reveal two distinct diffusion regimes profoundly affecting the effective diffusion length. Then, it is shown that combining two and three of these interfaces, allows advanced in-plane control of the excitonic distribution and flux over large distances. Exciton focalization and trapping, allowing an increase in the local exciton density up to three orders of magnitude are demonstrated. Finally, flux collimation is achieved with the formation of parallel current lines extending a few micrometers away from the source. We believe that the deterministic shaping and positioning of the exciton distribution and flux shown here will be key toward the conception of realistic excitronic devices.

CrSBr is an air-stable magnetic van der Waals semiconductor with strong magnetic anisotropy, where the interaction of excitons with the magnetic order enables the optical identification of different magnetic phases. Here, we study the magnetic anisotropy of multilayer CrSBr inside a three-axis vector magnet and correlate magnetic order and optical transitions in emission and absorption. We identify layer-by-layer switching of the magnetization through drastic changes in the optical emission and absorption energy and strength as a function of the applied magnetic field. We correlate optical transitions in reflection spectra with photoluminescence (PL) emission using transfer-matrix analysis and find that ferromagnetic and antiferromagnetic order between layers can coexist in the same crystal. In the multipeak PL emission, the intensity of energetically lower-lying transitions reduces monotonously with increasing field strength, whereas energetically higher-lying transitions around the bright exciton XB brighten close to the saturation field. Using this contrasting behavior, we can therefore correlate transitions with each other.

Lateral heterostructures of two-dimensional (2D) transition metal dichalcogenides feature atomically sharp, covalently stitched 1D interfaces that enable direct band-to-band coupling. This perspective highlights their unique ability to control quasiparticles, excitons, and spins, with implications for optoelectronics, excitonic and valleytronic devices, tunneling field-effect transistors, neuromorphic computing, spintronics, and quantum circuits. It also outlines challenges in scalable synthesis, interface engineering, and 2D–3D integration, charting paths toward future quantum technologies.

Silicon-based dielectric nanoantennas provide an effective platform for engineering light-matter interactions in van der Waals semiconductors. Here, we demonstrate near-field coupling between monolayer MoS2 and silicon nanoantennas arranged in hexagonal lattices with tunable geometric parameters, leading to a three-fold enhancement in photoluminescence and an excitation-wavelength-dependent emission that aligns with Mie-resonant modes. Raman spectroscopy reveals an up to 8-fold enhancement in the vibrational modes of MoS2, while second-harmonic generation exhibits a 20 to 30-fold increase in efficiency, closely correlating with the presence of the underlying nanoantennas. Our experiments and simulations quantify the tunable benefits of the near-field interactions, taking into account thin-film interference and strain-induced effects. Our findings present dielectric nanoantennas as a promising platform for tailoring linear and nonlinear optical properties in 2D materials, with potential applications in nanophotonic devices and integrated photonics.

Semiconducting CrSBr is a layered A-type antiferromagnet, with individual layers antiferromagnetically coupled along the stacking direction. Due to its unique orthorhombic crystal structure, CrSBr exhibits highly anisotropic mechanical and optoelectronic properties acting itself as a quasi-1D material. CrSBr demonstrates complex coupling phenomena involving phonons, excitons, magnons, and polaritons. Here we show through polarization-resolved resonant Raman scattering the intricate interaction between the vibrational and electronic properties of CrSBr. For samples spanning from few-layer to bulk thickness, we observe that the polarization of the A_g^2 Raman mode can be rotated by 90 degrees, shifting from alignment with the crystallographic a (intermediate magnetic) axis to the b (easy magnetic) axis, depending on the excitation energy. In contrast, the A_g^1 and A_g^3 modes consistently remain polarized along the b axis, regardless of the laser energy used. We access real and imaginary parts of the Raman tensor in our analysis, uncovering resonant electron-phonon coupling.

Lateral heterostructures built of monolayers of transition-metal dichalcogenides host a thin one-dimensional interface exhibiting a large energy offset. Recently, the formation of spatially separated charge-transfer (CT) excitons at the interface has been demonstrated, but their impact on technologically important exciton propagation across the interface has remained in the dark. In this theoretical work, we microscopically investigate the spatiotemporal exciton dynamics in the exemplary hBN-encapsulated WSe2-MoSe2 lateral heterostructure. We reveal a highly interesting interplay of energy-offset-driven unidirectional exciton drift across the interface and efficient capture into energetically lower CT excitons at the interface. This interplay triggers a counterintuitive thermal control of exciton transport with less efficient propagation at lower temperatures, opposite to conventional semiconductors. We predict clear signatures of this intriguing exciton propagation in both far- and near-field photoluminescence experiments. Our results present an advance in the microscopic understanding of technologically relevant unidirectional exciton transport in lateral heterostructures.

AbstractThe fundamental properties of an exciton are determined by the spin, valley, energy, and spatial wavefunctions of the Coulomb-bound electron and hole. In van der Waals materials, these attributes can be widely engineered through layer stacking configuration to create highly tunable interlayer excitons with static out-of-plane electric dipoles, at the expense of the strength of the oscillating in-plane dipole responsible for light-matter coupling. Here we show that interlayer excitons in bi- and tri-layer 2H-MoSe2 crystals exhibit electric-field-driven coupling with the ground (1s) and excited states (2s) of the intralayer A excitons. We demonstrate that the hybrid states of these distinct exciton species provide strong oscillator strength, large permanent dipoles (up to 0.73 ± 0.01 enm), high energy tunability (up to ~200 meV), and full control of the spin and valley characteristics such that the exciton g-factor can be manipulated over a large range (from −4 to +14). Further, we observe the bi- and tri-layer excited state (2s) interlayer excitons and their coupling with the intralayer excitons states (1s and 2s). Our results, in good agreement with a coupled oscillator model with spin (layer)-selectivity and beyond standard density functional theory calculations, promote multilayer 2H-MoSe2 as a highly tunable platform to explore exciton-exciton interactions with strong light-matter interactions.

Controlling the motion of neutral excitons in optically active media is a mandatory development to enable the conception of advanced circuits and devices for applications in excitronics, quantum photonics, and optoelectronics. Recently, a proof of unidirectional exciton transport from high- to low-band-gap material has been evidenced using a high-quality lateral heterostructure separating transition metal dichalcogenide monolayers (TMD-MLs). In this paper, by combining room-temperature micro-photoluminescence far-field imaging with a statistical description of exciton transport, we unveil the underlying excitonic local distribution and fluxes taking place near lateral heterojunctions. We study the complex 2D excitonic transport properties found near a linear interface separating WSe$_2$ from MoSe$_2$ TMD-MLs and reveal two distinct diffusion regimes profoundly affecting the effective diffusion length. Then, we show that combining two and three of these interfaces, allows advanced in-plane control of the excitonic distribution and flux over large distances. We demonstrate exciton focalization and trapping, allowing an increase in the local exciton density up to three orders of magnitude. Finally, we achieve flux collimation with the formation of parallel current lines extending a few micrometers away from the source. We believe that the deterministic shaping and positioning of the exciton distribution and flux here will be a key towards the conception of realistic excitronic devices.

We investigate the diffusion process of negatively charged excitons (trions) in WSe2 transition metal dichalcogenide monolayer. We measure time-resolved photoluminescence spatial profiles of these excitonic complexes which exhibit a non-linear diffusion process with an effective negative diffusion behavior. Specifically, we examine the dynamics of the two negatively charged bright excitons (intervalley and intravalley trion) as well as the dark trion. The time evolution allows us to identify the interplay of different excitonic species: the trionic species appear after the neutral excitonic ones, consistent with a bimolecular formation mechanism. Using the experimental observations, we propose a phenomenological model suggesting the coexistence of two populations: a first one exhibiting a fast and efficient diffusion mechanism and a second one with a slower dynamics and a less efficient diffusion process. These two contributions could be attributed to hot and cold trion populations.

Ultrafast laser fields are able to widely tune the physical properties of semiconductors by generating virtual states. Using strong fields at energies below the optical bandgap, control of excitons in two-dimensional semiconductors has now been demonstrated.

The existence of bound charge transfer (CT) excitons at the interface of monolayer lateral heterojunctions has been debated in literature, but contrary to the case of interlayer excitons in vertical heterostructure their observation still has to be confirmed. Here, we present a microscopic study investigating signatures of bound CT excitons in photoluminescence spectra at the interface of hBN-encapsulated lateral MoSe 2 -WSe 2 heterostructures. Based on a fully microscopic and material-specific theory, we reveal the many-particle processes behind the formation of CT excitons and how they can be tuned via interface- and dielectric engineering. For junction widths smaller than the Coulomb-induced Bohr radius we predict the appearance of a low-energy CT exciton. The theoretical prediction is compared with experimental low-temperature photoluminescence measurements showing emission in the bound CT excitons energy range. We show that for hBN-encapsulated heterostructures, CT excitons exhibit small binding energies of just a few tens meV and at the same time large dipole moments, making them promising materials for optoelectronic applications (benefiting from an efficient exciton dissociation and fast dipole-driven exciton propagation). Our joint theory-experiment study presents a significant step towards a microscopic understanding of optical properties of technologically promising 2D lateral heterostructures.