Excitons in recently discovered two-dimensional magnetic semiconductors have emerged as a promising vehicle for optoelectronic and spin-photonic applications. To exploit novel possibilities magnetic degrees of freedom offer, insight into the interplay of magnetism, lattice and optical excitations becomes essential. We consider Chromium Sulphur Bromide, which has two kinds of excitons, XB at 1.8 eV and XA at 1.38 eV. Here we show, through a combination of many body perturbation theory and experiment, that XB is an order of magnitude more sensitive to magnetic and lattice perturbations than XA. We trace the difference to the latter being localised (Frenkel-like), while the former is delocalised (Wannier-Mott-like) - a coexistence rarely seen in two-dimensional materials. This finding is supported by the strong temperature and magnetic field (up to 85 Tesla) dependent shifts in optical response for XB (much smaller for XA), and we show it is related to XB's tendency for delocalisation (in-plane and out-of-plane) and enhanced coupling with Ag phonon modes.

The coupling of light with excitations in matter is one of the most important concepts to make photons interact, crucial for the development of efficient optoelectronic devices. In materials with exceptionally strong light-matter interaction, excitons can hybridize with photons without the need of an external cavity. Here, we report the electrical excitation of such self-hybridized polaritons in the van der Waals antiferromagnet CrSBr. We exploit an unconventional excitation via energy transfer from tunneling electrons in graphene tunnel junctions to strongly bound excitons in proximate CrSBr layers. This enables us to excite CrSBr crystals ranging in thickness from a bilayer up to 250 nanometers, with the strong linear polarization of the electroluminescence confirming the excitonic origin. We assign the electrically excited emission to self-hybridized exciton polaritons, highlighting the strong coupling between optical excitations and confined photon modes in CrSBr. Our findings not only offer an efficient method to generate polaritons electrically but also create opportunities for future spintronic devices.

van der Waals heterostructures (vdWHs) composed of transition-metal dichalcogenides (TMDs) and layered magnetic semiconductors offer great opportunities to manipulate the exciton and valley properties of TMDs. Here, we present magneto-photoluminescence (PL) studies in a WSe2 monolayer (ML) on a CrSBr crystal, an anisotropic layered antiferromagnetic semiconductor. Our results reveal the unique behavior of each of the ML-WSe2 PL peaks under a magnetic field that is distinct from the pristine case. An intriguing feature is the clear enhancement of the PL intensity that we observe each time the external magnetic field tunes the energy of an exciton in CrSBr into resonance with one of the optical states of WSe2. This result suggests a magnetic field-controlled resonant energy transfer (RET) beyond other effects reported in similar structures. Our work provides deep insight into the importance of different mechanisms in magnetic vdWHs and underscores its great potential for light harvesting and emission enhancement of two-dimensional materials.

Triggered by advances in atomic-layer exfoliation and growth techniques, along with the identification of a wide range of extraordinary physical properties in self-standing films consisting of one or a few atomic layers, two-dimensional (2D) materials such as graphene, transition metal dichalcogenides (TMDs), and other van der Waals (vdW) crystals now constitute a broad research field expanding in multiple directions through the combination of layer stacking and twisting, nanofabrication, surface-science methods, and integration into nanostructured environments. Photonics encompasses a multidisciplinary subset of those directions, where 2D materials contribute remarkable nonlinearities, long-lived and ultraconfined polaritons, strong excitons, topological and chiral effects, susceptibility to external stimuli, accessibility, robustness, and a completely new range of photonic materials based on layer stacking, gating, and the formation of moire patterns. These properties are being leveraged to develop applications in electro-optical modulation, light emission and detection, imaging and metasurfaces, integrated optics, sensing, and quantum physics across a broad spectral range extending from the far-infrared to the ultraviolet, as well as enabling hybridization with spin and momentum textures of electronic band structures and magnetic degrees of freedom. The rapid expansion of photonics with 2D materials as a dynamic research arena is yielding breakthroughs, which this Roadmap summarizes while identifying challenges and opportunities for future goals and how to meet them through a wide collection of topical sections prepared by leading practitioners.

The discovery of two-dimensional van der Waals magnets has greatly expanded our ability to create and control nanoscale quantum phases. A unique capability emerges when a two-dimensional magnet is also a semiconductor that features tightly bound excitons with large oscillator strengths that fundamentally determine the optical response and are tunable with magnetic fields. Here we report a previously unidentified type of optical excitation-a magnetic surface exciton-enabled by the antiferromagnetic spin correlations that confine excitons to the surface of CrSBr. Magnetic surface excitons exhibit stronger Coulomb attraction, leading to a higher binding energy than excitons confined in bulk layers, and profoundly alter the optical response of few-layer crystals. Distinct magnetic confinement of surface and bulk excitons is established by layer- and temperature-dependent exciton reflection spectroscopy and corroborated by ab initio many-body perturbation theory calculations. By quenching interlayer excitonic interactions, the antiferromagnetic order of CrSBr strictly confines the bound electron-hole pairs within the same layer, regardless of the total number of layers. Our work unveils unique confined excitons in a layered antiferromagnet, highlighting magnetic interactions as a vital approach for nanoscale quantum confinement, from few layers to the bulk limit.