Semiconducting ferromagnet-nonmagnet interfaces in van der Waals heterostructures present a unique opportunity to investigate magnetic proximity interactions dependent upon a multitude of phenomena including valley and layer pseudospins, moiré periodicity, or exceptionally strong Coulomb binding. Here, we report a charge-state dependency of the magnetic proximity effects between MoSe2 and CrBr3 in photoluminescence, whereby the valley polarization of the MoSe2 trion state conforms closely to the local CrBr3 magnetization, while the neutral exciton state remains insensitive to the ferromagnet. We attribute this to spin-dependent interlayer charge transfer occurring on timescales between the exciton and trion radiative lifetimes. Going further, we uncover by both the magneto-optical Kerr effect and photoluminescence a domain-like spatial topography of contrasting valley polarization, which we infer to be labyrinthine or otherwise highly intricate, with features smaller than 400 nm corresponding to our optical resolution. Our findings offer a unique insight into the interplay between short-lived valley excitons and spin-dependent interlayer tunneling, while also highlighting MoSe2 as a promising candidate to optically interface with exotic spin textures in van der Waals structures.

a three-body charged exciton, or trion [8, 9]. Although 43 nominally the trion exists only in the strict single particle 44 limit, in reality the transition between these two quasi-45 particle regimes is unclear, and likely depends heavily on 46 the degree of exciton and carrier spatial localization over 47 the monolayer, especially at low densities. This is partic-48 ularly true in the case of nonequilibrium scenarios such 49 as photoluminescence experiments, in which both species 50 may coexist [9]. 51 Valley Zeeman splitting of these excitonic complexes 52 has been reported under application of strong out-of-53 plane magnetic fields (B-fields) [3, 6, 10]. However, 54 translating the relatively large Zeeman splitting of a 55 purely matter-bound excitation into a photonic mode 56 splitting remains a fundamental challenge not only in 57 opto-valleytronics [11], but also in topological photon-58 ics. Indeed, many topological states of light have been 59 implemented in recent years [12], including using TMD 60 exciton-polaritons [13, 14]. The ultimate goal of real 61 topological protection against any type of disorder scat-62 tering and back-reflection requires time-reversal symme-63 try breaking [15, 16], with the size of the topological gap 64 limited by the effective Zeeman splitting of the photonic 65 modes. Large splittings are difficult to achieve at op-66 tical frequencies, and in the existing realizations either based on the use of magnetic proximity effects [17] or on 68 the matter-based Zeeman splitting of exciton-polaritons 69 [18, 19], the topological gap was < 1 meV, too small to 70 be clearly observable. 71 In our work, by harnessing many-body interactions in 72 a 2-dimensional Bose-Fermi mixture, we realize a giant 73 effective trion-polariton Zeeman splitting, over 5 times 74 larger than the bare (uncoupled) trion splitting, and more 75 than double the polariton linewidths, a crucial step to-76 wards elimination of unwanted coupling between chiral 77 modes [20]. We moreover demonstrate giant effective 78 non-linearity α ≈ 0.2±0.05 meV•µm 2 for trion-polaritons 79 under a magnetic field. This value is one order of magni-80 tude larger than previously reported in TMDs [5, 21] and 81 is based on an original mechanism involving free carrier 82 valley relaxation and strong light-matter coupling. Large 83 photonic non-linearities, as in this work, are crucial for 84 classical, quantum and topological photonics [12, 16]. 85 We study a MoSe 2 monolayer on a 10 nm thick film 86 of the ferromagnetic semiconductor europium sulfide 87 (EuS) which coats a dielectric distributed Bragg reflector 88 (DBR). Firstly, we characterize the MoSe 2 monolayer in 89 the half-cavity, or bare flake, configuration, at tempera-90 ture T = 4.2 K. Fig. 1a shows circular polarization re-91 solved reflectance contrast (RC = (R 0 − R)/R 0 , where R 92 and R 0 are the reflectance from the MoSe 2 and adjacent 93 EuS substrate, respectively) spectra from the sample un-94 der linearly polarized broadband illumination at out-of-95 plane ...

Van der Waals heterobilayers based on 2D transition metal dichalcogenides have been recently shown to support robust and long-lived valley polarization for potential valleytronic applications. However, the roles of the chemical composition and geometric alignment of the constituent layers in the underlying dynamics remain largely unexplored. Here we study spin–valley relaxation dynamics in heterobilayers with different structures and optical properties engineered via the use of alloyed monolayer semiconductors. Through a combination of time-resolved Kerr rotation spectroscopic measurements and theoretical modeling for Mo1 − x W x Se2/WSe2 samples with different chemical compositions and stacking angles, we uncover the contributions of the interlayer exciton recombination and charge carrier spin depolarization to the overall valley dynamics. We show that the corresponding decay rates can be tuned in a wide range in transitions from a misaligned to an aligned structure, and from a hetero- to a homo-bilayer. Our results provide insights into the microscopic spin–valley polarization mechanisms in van der Waals heterostructures for the development of future 2D valleytronic devices.

Two-dimensional semiconducting transition metal dichalcogenides embedded in optical microcavities in the strong exciton-photon coupling regime may lead to promising applications in spin and valley addressable polaritonic logic gates and circuits. One significant obstacle for their realization is the inherent lack of scalability associated with the mechanical exfoliation commonly used for fabrication of two-dimensional materials and their heterostructures. Chemical vapor deposition offers an alternative scalable fabrication method for both monolayer semiconductors and other two-dimensional materials, such as hexagonal boron nitride. Observation of the strong light-matter coupling in chemical vapor grown transition metal dichalcogenides has been demonstrated so far in a handful of experiments with monolayer molybdenum disulfide and tungsten disulfide. Here we instead demonstrate the strong exciton-photon coupling in microcavities composed of large area transition metal dichalcogenide/hexagonal boron nitride heterostructures made from chemical vapor deposition grown molybdenum diselenide and tungsten diselenide encapsulated on one or both sides in continuous few-layer boron nitride films also grown by chemical vapor deposition. These transition metal dichalcogenide/hexagonal boron nitride heterostructures show high optical quality comparable with mechanically exfoliated samples, allowing operation in the strong coupling regime in a wide range of temperatures down to 4 Kelvin in tunable and monolithic microcavities, and demonstrating the possibility to successfully develop large area transition metal dichalcogenide based polariton devices.

Focused ion beam (FIB) is an effective tool for precise nanoscale fabrication. It has recently been employed to tailor defect engineering in functional nanomaterials such as two-dimensional transition metal dichalcogenides (TMDCs), providing desirable properties in TMDC-based optoelectronic devices. However, the damage caused by the FIB irradiation and milling process to these delicate, atomically thin materials, especially in extended areas beyond the FIB target, has not yet been fully characterised. Understanding the correlation between lateral ion beam effects and optical properties of 2D TMDCs is crucial in designing and fabricating high-performance optoelectronic devices. In this work, we investigate lateral damage in large-area monolayer WS2 caused by the gallium focused ion beam milling process. Three distinct zones away from the milling location are identified and characterised via steady-state photoluminescence (PL) and Raman spectroscopy. The emission in these three zones have different wavelengths and decay lifetimes. An unexpected bright ring-shaped emission around the milled location has also been revealed by time-resolved PL spectroscopy with high spatial resolution. Our findings open up new avenues for tailoring the optical properties of TMDCs by charge and defect engineering via focused ion beam lithography. Furthermore, our study provides evidence that while some localised damage is inevitable, distant destruction can be eliminated by reducing the ion beam current. It paves the way for the use of FIB to create nanostructures in 2D TMDCs, as well as the design and realisation of optoelectrical devices on a wafer scale.