Photonic bound states in the continuum (BICs) have emerged as a versatile tool for enhancing light-matter interactions by strongly confining light fields. Chiral BICs are photonic resonances with a high degree of circular polarisation, which hold great promise for spin-selective applications in quantum optics and nanophotonics. Here, we demonstrate a novel application of a chiral BIC for inducing strong coupling between the circularly polarised photons and spin-polarised (valley) excitons (bound electron-hole pairs) in atomically-thin transition metal dichalcogenide crystals (TMDCs). By placing monolayer WS2 onto the BIC-hosting metasurface, we observe the formation of intrinsically chiral, valley-selective exciton polaritons, evidenced by circularly polarised photoluminescence (PL) at two distinct energy levels. The PL intensity and degree of circular polarisation of polaritons exceed those of uncoupled excitons in our structure by an order of magnitude. Our microscopic model shows that this enhancement is due to folding of the Brillouin zone creating a direct emission path for high-momenta polaritonic states far outside the light cone, thereby providing a shortcut to thermalisation (energy relaxation) and suppressing depolarisation. Moreover, while the polarisation of the upper polariton is determined by the valley excitons, the lower polariton behaves like an intrinsic chiral emitter with its polarisation fixed by the BIC. Therefore, the spin alignment of the upper and lower polaritons (↑↓ and ↑↑) can be controlled by σ+ and σ- circularly polarised optical excitation, respectively. Our work introduces a new type of chiral light-matter quasi-particles in atomically-thin semiconductors and provides an insight into their energy relaxation dynamics.

We demonstrate that anisotropic magnetoresistance (AMR) in metal-ferromagnet bilayers can arise entirely from interfacial scattering, without invoking bulk spin Hall or inverse spin Hall effects. Using a minimal boundary-value formulation of the Boltzmann equation with interfacial exchange and Rashba spin-orbit coupling, we identify a resonant spin-filtering mechanism whereby one spin projection becomes immune to backscattering at a clean interface. This mechanism quantitatively reproduces the magnitude (Δρ/ρ∼10^{-4}-10^{-3}), thickness dependence, and angular symmetry conventionally attributed to spin Hall magnetoresistance in Pt/YIG. Crucially, the maximal AMR scales linearly in the smaller of the interfacial exchange or spin-orbit coupling: a parametric behavior forbidden in any spin-Hall-based theory. The effect is intrinsically sensitive to interface quality, charge transfer, and disorder, providing clear signatures that can be experimentally tested. Our results establish interfacial spin filtering as an essential and previously overlooked origin of AMR in metal-ferromagnet heterostructures, calling for a fundamental reassessment of magnetotransport mechanisms in spin-orbit-coupled metals.

Over the last decade, layered crystals, referred to as van der Waals (vdW) materials, have attracted tremendous interest due to their unique properties in their single and few layer forms. Their bulk counterparts, however, have only been recently explored as building blocks for nanophotonics. Indeed, bulk vdW materials offer promising properties such as high refractive indices and adherence to any type of substrate. We present here a variety of 1D grating structures composed of bulk transition metal dichalcogenide (TMD) WS2 as a highly tunable and versatile platform for observation of a multi-level polaritonic system. The WS2 excitons are simultaneously strongly coupled with the two grating photonic modes, including the bound state in the continuum (BIC) of the lower energy mode, giving rise to polariton-BICs. The polaritonic dispersion shapes can be varied in a straightforward fashion by choosing WS2 films of different thicknesses and changing the gratings periods.

Deterministic and versatile approaches to sample preparation on nanoscopic scales are important in many fields including photonics, electronics, biology and material science. However, challenges exist in meeting many nanostructuring demands–particularly in emerging optical materials and component architectures. Here, we report a nanofabrication workflow that overcomes long-standing challenges in deterministic and top-down sample preparation procedures. The salient feature is a carbon mask with a low sputter yield that can be readily shaped using high resolution electron beam processing techniques. When combined with focused ion beam processing, the masking technique yields structures with ultra-smooth, near-vertical side walls. We target different material platforms to showcase the broad utility of the technique. As a first test case, we prepared nanometric gaps in evaporated Au. Gap widths of 7 plus/minus 2 nm, aspect ratios of 17, and line edge roughness values of 3sigma = 2.04 nm are achieved. Furthermore, the gap widths represent an order of magnitude improvement on system resolution limits. As a second test case, we designed and fabricated dielectric resonators in the ternary compounds MnPSe3 and NiPS3; a class of van der Waals material resistant to chemical etch approaches. Nanoantenna arrays with incrementally increasing diameter were fabricated in crystalline, exfoliated flakes. The optical response was measured by dark field spectroscopy and is in agreement with simulations. The workflow reported here leverages established techniques in material processing without the need for custom or specialized hardware. It is broadly applicable to functional materials and devices, and extends high speed focused ion beam milling to true sub-10 nm length scales.

We investigate the impact of the quantized mechanical motion of optically trapped atoms, arranged in proximity to a one-dimensional waveguide, on the propagation of polariton modes. Our study identifies a regime of resonant phonon-assisted mixing between lower and upper polaritons, resulting in a pronounced polaron effect. This effect is characterized by the formation of new band gaps and the appearance of weakly dispersive states within the original polariton band gap. The polaron spectrum, which can be directly probed via resonant elastic scattering, provides novel opportunities for quantum optical applications. These findings open avenues for enhanced control in state-of-the-art waveguide quantum electrodynamics experiments with cold atoms.

Ultrafast all-optical logic devices based on nonlinear light-matter interactions hold the promise to overcome the speed limitations of conventional electronic devices. Strong coupling of excitons and photons inside an optical resonator enhances such interactions and generates new polariton states which give access to unique nonlinear phenomena, such as Bose-Einstein condensation, used for all-optical ultrafast polariton transistors. However, to reach the threshold for condensation high quality factors and high pulse energies are required. Here we demonstrate all-optical switching exploiting the ultrafast transition from the strong to the weak coupling regime in low-Q microcavities embedding bilayers of transition metal dichalcogenides with high optical nonlinearities and fast exciton relaxation times. We observe a collapse of polariton gaps as large as 55 meV, and their revival, lowering the threshold for optical switching below 4 pJ per pulse, while retaining ultrahigh switching frequencies. As an additional degree of freedom, the switching can be triggered pumping either the intra- or the interlayer excitons of the bilayers at different wavelengths, speeding up the polariton dynamics, owing to unique interspecies excitonic interactions. Our approach will enable the development of compact ultrafast all-optical logical circuits and neural networks, showcasing a new platform for polaritonic information processing based on manipulating the light-matter coupling.

We consider an altermagnet subject to the electron attractive potential mediated by the dispersive phonons. While altermagnetism suppresses superconductivity, scattering of electrons on the Fermi surface by thermal phonons suppresses altermagnetism. We show that this leads to the re-entrant superconductivity over temperature and to the stabilization of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) finite momentum superconducting state at low temperatures. The effect is mediated by the retardation effects and no d-wave pairing interaction is required.

Halide perovskites such as methylammonium lead bromide (MAPbBr3) host tightly bound three-dimensional excitons that are robust at room temperature. The excellent optical properties of MAPbBr3 allow the design of optical single-mode waveguides and cavities in the frequency range close to excitonic transitions. Taken together, these results indicate that MAPbBr3 is an excellent platform for probing exciton-polariton nonlinear phenomena at room temperature. Here, we investigate the ultrafast nonequilibrium dynamics of polaritons under nonresonant femtosecond-pulsed excitation. We demonstrate the presence of a stimulated acoustic phonon-assisted scattering regime above the threshold pump fluence, characterized by the explosive growth of emission intensity, a redshift of the emission spectral maximum, spectral narrowing, and subpicosecond emission dynamics. Our theoretical findings are confirmed by experimental measurements.

Transition metal dichalcogenides (TMDs) mono layers exhibit a unique band structure that allows for valley-selective optical excitation at the KlK’ points of the Brillouin zone. The formation of K and K’ excitons, which possess high oscillator strength and strong photoluminescence, can be controlled by circularly polarized light, which opens exciting possibilities for spintronics and quantum information technologies [1]. In the experiment, however, the degree of circular polarization of KlK’ excitons is usually quenched due to non-elastic scattering events, which can be suppressed by strong exciton-photon coupling [2]. Chiral photonic bound states in the continuum (BICs) emerge as a promising candidate for further enhancing the polarization properties in TMDs due to their polarization selectivity and strong confinement of photons [3].

Despite the apparent simplicity, the problem of refraction of electromagnetic waves at the planar interface between two media has an incredibly rich spectrum of unusual phenomena. An example is the paradox that occurs when an electromagnetic wave is incident on the interface between a hyperbolic medium and an isotropic dielectric. At certain orientations of the optical axis of the hyperbolic medium relative to the interface, the reflected and transmitted waves are completely absent. In this paper, we formulate the aforementioned paradox and present its resolution by introduction of infinitesimal losses in a hyperbolic medium. We show that the reflected wave exists, but became extremely decaying as the loss parameter tends to zero. As a consequence, all the energy scattered into the reflected channel is absorbed at the interface. We support our reasoning with analytical calculations, numerical simulations, and an experiment with self-complementary metasurfaces in the microwave region. In addition to the great fundamental interest, this paradox resolution discovers a plethora of applications for the reflectors, refractors, absorbers, lenses, antennas, camouflage and holography applications.

We study the Hanle and spin polarization recovery effects on resident electrons in a monolayer MoSe2 on EuS. We demonstrate that localized electrons provide the main contribution to the spin dynamics signal at low temperatures below 15 K for small magnetic fields of only a few mT. The spin relaxation of these electrons is determined by random effective magnetic fields due to a contact spin interaction, namely the hyperfine interaction with the nuclei in MoSe2 or the exchange interaction with the magnetic ions of the EuS film. From the magnetic field angular dependence of the spin polarization we evaluate the anisotropy of the intervalley electron g factor and the spin relaxation time. The nonzero in-plane g factor |gx| approximate to 0.1, the value of which is comparable to its dispersion, is attributed to randomly localized electrons in the MoSe2 layer.

Experimental investigations of AlAs/(Al,Ga)As/GaAs vertical-cavity surface-emitting lasers in multimode generation regime are performed. A high degree of circular polarization (>70

In the framework of the Floquet theory of periodically driven quantum systems, it is demonstrated that irradiation of graphene by a circularly polarized electromagnetic field induces an attractive area in the core of repulsive potentials. Consequently, the quasi-stationary electron states bound by the repulsive potentials appear. The difference between such field-induced states in graphene and usual systems with the parabolic dispersion of electrons is discussed and possible manifestations of these states in electronic transport and optical spectra of graphene are considered.

Confinement of reactants within nanoscale spaces of low-dimensional materials has been shown to provide reorientation of strained reactants or stabilization of unstable reactants for synthesis of molecules and tuning of chemical reactivity. While few studies have reported chemistry within zero-dimensional pores and one-dimensional nanotubes, organic reactions in confined spaces between two-dimensional materials have yet to be explored. Here, we demonstrate that reactants confined between atomically thin sheets of graphene or hexagonal boron nitride experience pressures as high as 7 gigapascal, which allows the propagation of solvent-free organic reactions that ordinarily do not occur under standard conditions. Specifically, we show that cyclodehydrogenation of hexaphenylbenzene without catalysts as a proof of concept and oxidative polymerization of dopamine into sheet-like crystalline structure are enabled by the effective high pressure experienced by the reactants between the graphene layers. Our results demonstrate a facile, general approach for performing high-pressure chemistry based on confinement of reactants within two-dimensional materials.

Within the Floquet theory of periodically driven quantum systems, we demonstrate that an off-resonant high-frequency electromagnetic field can induce the Lifshitz phase transition in periodical structures described by the one-dimensional repulsive Hubbard model with the nearest and next-nearest-neighbor hopping. The transition changes the topology of electron energy spectrum at the Fermi level, transforming it from the two Fermi points to the four Fermi points, which facilitates the emergence of the superconducting fluctuations in the structure. Possible manifestations of the effect and conditions of its experimental observability are discussed.

Dielectric nanoresonators have been shown to circumvent the heavy optical losses associated with plasmonic devices; however, they suffer from less confined resonances. By constructing a hybrid system of both dielectric and metallic materials, one can retain low losses, while achieving stronger mode confinement. Here, we use a high refractive index multilayer transition-metal dichalcogenide WS2 exfoliated on gold to fabricate and optically characterize a hybrid nanoantenna-on-gold system. We experimentally observe a hybridization of Mie resonances, Fabry-Perot modes, and surface plasmon-polaritons launched from the nanoantennas into the substrate. We measure the experimental quality factors of hybridized Mie-plasmonic (MP) modes to be up to 33 times that of standard Mie resonances in the nanoantennas on silica. We then tune the nanoantenna geometries to observe signatures of a supercavity mode with a further increased Q factor of over 260 in experiment. We show that this quasi-bound state in the continuum results from strong coupling between a Mie resonance and Fabry-Perot-plasmonic mode in the vicinity of the higher-order anapole condition. We further simulate WS2 nanoantennas on gold with a 5 nm thick hBN spacer in between. By placing a dipole within this spacer, we calculate the overall light extraction enhancement of over 107, resulting from the strong, subwavelength confinement of the incident light, a Purcell factor of over 700, and high directivity of the emitted light of up to 50%. We thus show that multilayer TMDs can be used to realize simple-to-fabricate, hybrid dielectric-on-metal nanophotonic devices granting access to high-Q, strongly confined, MP resonances, along with a large enhancement for emitters in the TMD-gold gap.

We consider optical response in multi-band, multi-layer two-dimensional superconductors. Within a simple model, we show that linear response to AC gating can detect collective modes of the condensate, such as Leggett and clapping modes. We show how trigonal warping of the superconducting order parameter can help facilitate detection of clapping modes. Taking rhombohedral trilayer graphene as an example, we consider several possible pairing mechanisms and show that all-electronic mechanisms may produce in-gap clapping modes. These modes, if present, should be detectable in the absorption of microwaves applied via the gate electrodes, which are necessary to enable superconductivity in this and many other settings; their detection would constitute strong evidence for unconventional pairing. Last, we show that absorption at frequencies above the superconducting gap 2 |Δ| also contains a wealth of information about the gap structure. Our results suggest that linear spectroscopy can be a powerful tool for the characterization of unconventional two-dimensional superconductors.

Two-dimensional semiconductors offer a compelling platform for excitons with robust interaction with light, owing to their confined nature and their numerous manipulable degrees of freedom. In bilayers, interlayer excitons (IX) combine these degrees of freedom with high interactions due to their out-of-plane alignment. However, their oscillator strength is often negligible. Interlayer hybridization provides IX with a significant oscillator strength. Here, we examine the ultrafast dynamics of these hybrid IX in bilayer and trilayer MoSe2. We find that IX are particularly strong in trilayers. These unexplored excitonic species exhibit fundamentally different dynamics from IX in bilayers, with delayed rise times of over 2 ps and significantly longer lifetimes. We attribute this to the origin of this excitonic species and confirm it with theory. Our findings offer insights into high oscillator strength, long-living interlayer excitons in trilayers, superior to their bilayer counterparts.

We theoretically consider an ensemble of quantum dimers placed inside an optical cavity. We predict two effects: first, an exchange of angular momentum between the dimers mediated by the emission and re-absorption of the cavity photons leads to the alignment of dimers. Furthermore, the optical angular momentum of the vacuum state of the chiral cavity is transferred to the ensemble of dimers which leads to the synchronous rotation of the dimers at certain levels of light-matter coupling strength.

Deeply subwavelength lasers (or nanolasers) are highly demanded for compact on-chip bioimaging and sensing at the nanoscale. One of the main obstacles for the development of single-particle nanolasers with all three dimensions shorter than the emitting wavelength in the visible range is the high lasing thresholds and the resulting overheating. Here we exploit exciton-polariton condensation and mirror-image Mie modes in a cuboid CsPbBr3 nanoparticle to achieve coherent emission at the visible wavelength of around 0.53 μm from its ultra-small (≈0.007 μm3 or ≈λ3/20) semiconductor nanocavity. The polaritonic nature of the emission from the nanocavity localized in all three dimensions is proven by direct comparison with corresponding one-dimensional and two-dimensional waveguiding systems with similar material parameters. Such a deeply subwavelength nanolaser is enabled not only by the high values for exciton binding energy (≈35 meV), refractive index (>2.5 at low temperature), and luminescence quantum yield of CsPbBr3, but also by the optimization of polaritons condensation on the Mie resonances with quality factors improved by the metallic substrate. Moreover, the key parameters for optimal lasing conditions are intermode free spectral range and phonons spectrum in CsPbBr3, which govern polaritons condensation path. Such chemically synthesized colloidal CsPbBr3 nanolasers can be potentially deposited on arbitrary surfaces, which makes them a versatile tool for integration with various on-chip systems.