The recent discovery of ferroelectric nematic liquid crystals (FNLCs) has led to exciting materials which combine polar long-range orientational order and giant spontaneous electric polarization Ps with 3D fluidity. While this fluidity opens new perspectives for directional control of Ps in space and time, it also requires confinement of FNLCs. We investigate whether and how ferroelectric nematics might be confined as a cylindrical core in a hollow polymer fiber by coaxial electrospinning and whether the ferroelectric order is preserved in this process. We find that it is, in fact, exceptionally easy to spin fibers with a continuous FNLC core, because the polarization stiffening effect counteracts the Rayleigh-Plateau instability. Polarization-resolved second-harmonic generation measurements reveal that the polar order is preserved in the core. While in thin fibers with a core diameter on the order of 1 μm, Ps is uniformly aligned along the fiber axis, we find a twist of Ps along the diameter in thicker fibers. The latter is analyzed in light of the current debate on the twisted ground states of ferroelectric nematics. The confinement of FNLCs in macroscopically polar fibers offers new research opportunities and might be an important step toward application in, e.g., actuators or energy harvesting devices.

Ambient humidity poses a significant challenge in the development of practical room‐temperature NO2 gas sensors. Herein, atomically precise zigzag edges are employed in multilayer MoS2, fabricated using electron beam lithography and anisotropic wet etching, to achieve highly sensitive and selective gas‐sensing performance, that is, humidity‐tolerant at elevated temperatures and humidity‐enhanced at room temperature under ultraviolet illumination. Notably, exposure to 2.5 parts per billion (ppb) NO2 at 70% relative humidity under ultraviolet illumination and at room temperature resulted in a 33‐fold increase in response and a 6‐fold faster recovery compared to 0% relative humidity, leading to response values exceeding 1100%. The optimized samples demonstrate a theoretical detection limit ranging from 4 to 400 parts per trillion (ppt) NO2. The enhanced NO2 sensing capabilities of MoS2 edges have been further confirmed through first‐principles calculations. This study expands the applications of nanostructured MoS2 and highlights its potential for detecting NO2 at sub‐ppb levels in complex scenarios, such as high‐humidity conditions.

Dielectric metasurfaces that combine high-index materials with optical nonlinearities are widely recognized for their potential in various quantum and classical nanophotonic applications. However, the fabrication of high-quality metasurfaces poses significant material-dependent challenges, as their designs are often susceptible to disorder, defects, and scattering losses, which are particularly prone to occur at the edges of nanostructured features. Additionally, the choice of the material platforms featuring second-order optical nonlinearities, χ^(2), is limited to broken-inversion symmetry crystals such as GaAs, GaP, LiNbO_3, and various bulk van der Waals materials, including GaSe and NbOCl_2. Here, we use a combination of top-down lithography and anisotropic wet etching of a specially stacked van der Waals crystal – 3R-MoS_2, which exhibits both a high refractive index and exceptional χ^(2) nonlinearity, to produce metasurfaces consisting of perfect equilateral triangle nanoholes with atomically precise zigzag edges. Due to the geometry of the triangle, the etching process is accompanied by a transition from an in-plane C_4 symmetric structure to a broken-in-plane symmetry configuration, thereby allowing for the realization of the quasi-bound-state-in-the-continuum (q-BIC) concept. The resulting ultrathin metasurface (∼ 20-25 nm) demonstrates a remarkable enhancement in second-harmonic generation (SHG) – over three orders of magnitude at specific wavelengths and linear polarization directions compared to a host flake.

Abstract 2D transition metal dichalcogenide (TMD) materials have attracted interest due to their remarkable excitonic, optical, electrical, and mechanical properties, which are dependent on their crystal structure. Consequently, controlling the crystal structure of these materials is essential for fine-tuning their performance, e.g., linear and nonlinear optical, as well as charge transport properties. While various phase-switching TMD materials are available, their transitions are often irreversible. Here, we investigate the mechanism of a light-induced reversible phase transition in mono- and bilayer rhenium disulfide (ReS2). Our observations, based on transmission electron microscopy, nonlinear spectroscopy, and density functional theory, reveal a transition from the ground $${\rm{T}}^{\prime\prime}$$ T ″ (double-distorted T) to the metastable $${\rm{H}}^{\prime}$$ H ′ (distorted H) phase under femtosecond laser irradiation or influence of highly-energetic electrons. We show that the formation of sulfur vacancies facilitates this phenomenon. Our findings pave the way toward manipulating the crystal structure of ReS2 and possibly its heterostructures.

Triggered by the development of exfoliation and the identification of a wide range of extraordinary physical properties in self-standing films consisting of one or few atomic layers, two-dimensional (2D) materials such as graphene, transition metal dichalcogenides (TMDs), and other van der Waals (vdW) crystals currently constitute a wide research field protruding in multiple directions in combination with layer stacking and twisting, nanofabrication, surface-science methods, and integration into nanostructured environments. Photonics encompasses a multidisciplinary collection of those directions, where 2D materials contribute with polaritons of unique characteristics such as strong spatial confinement, large optical-field enhancement, long lifetimes, high sensitivity to external stimuli (e.g., electric and magnetic fields, heating, and strain), a broad spectral range from the far infrared to the ultraviolet, and hybridization with spin and momentum textures of electronic band structures. The explosion of photonics with 2D materials as a vibrant research area is producing breakthroughs, including the discovery and design of new materials and metasurfaces with unprecedented properties as well as applications in integrated photonics, light emission, optical sensing, and exciting prospects for applications in quantum information, and nanoscale thermal transport. This Roadmap summarizes the state of the art in the field, identifies challenges and opportunities, and discusses future goals and how to meet them through a wide collection of topical sections prepared by leading practitioners.

The field of nanophotonics requires high-quality materials for the fabrication of resonant structures that can confine light down to the nanoscale. Metallic nanostructures often used for this purpose exhibit high optical losses, so high-refractive-index dielectrics such as silicon (Si) and III–V semiconductors are widely used instead. Recently, layered materials, often referred to as ‘van der Waals materials’ for the forces holding atomic planes together in bulk crystals, have been introduced as alternative dielectric building blocks for nanophotonics. Compared to traditional semiconductors, these materials exhibit higher refractive indices and transparency in the visible and near-infrared favourable for compact waveguides; strong birefringence and large nonlinear optical coefficients attractive for nonlinear optics; and out-of-plane van der Waals adhesive forces enabling novel tuning techniques and heterointegration approaches for the realization of previously inaccessible photonic structures. Recently, these properties of quasi-bulk van der Waals materials (as opposed to their widely studied monolayers) have been applied in a variety of photonic structures and devices, which will be discussed here. We report on recent progress in utilizing layered materials in waveguiding, wavefront shaping, Purcell enhancement, quantum nanophotonics, lasing, nonlinear optics, and strong light–matter coupling, as well as offer a snapshot of future developments in hybrid and tunable nanophotonics, three-dimensional photonic structures, optical trapping, polariton devices and van der Waals integrated nanophotonic circuits. This Review reports the recent progress in utilizing van der Waals layered materials in various nanophotonics applications and provides an overview of their future developments in hybrid and tunable nanophotonics, 3D photonic structures, optical trapping, polariton devices and van der Waals integrated nanophotonic circuits.

Semiconducting transition metal dichalcogenides (TMDs) have attracted significant attention for their potential to develop high-performance, energy-efficient, and nanoscale electronic devices. Despite notable advancements in scaling down the gate and channel length of TMD field-effect transistors (FETs), the fabrication of sub-30 nm narrow channels and devices with atomic-scale edge control still poses challenges. Here, we demonstrate a crystallography-controlled nanostructuring technique to fabricate ultranarrow tungsten disulfide (WS2) nanoribbons as small as sub-10 nm in width. The WS2 nanoribbon junctions having different widths display diodic current-voltage characteristics, providing a way to create and tune nanoscale device properties by controlling the size of the structures. The transport properties of the nanoribbon FETs are primarily governed by narrow channel effects, where the mobility in the narrow channels is limited by edge scattering. Our findings on nanoribbon devices hold potential for developing future-generation nanometer-scale van der Waals semiconductor-based devices and circuits.

Second-order nonlinearity in solids gives rise to a plethora of unique physical phenomena ranging from piezoelectricity and optical rectification to optical parametric amplification, spontaneous parametric down-conversion, and the generation of entangled photon pairs. Monolayer transition metal dichalcogenides (TMDs), such as MoS2, exhibit one of the highest known second-order nonlinear coefficients. However, the monolayer nature of these materials prevents the fabrication of resonant objects exclusively from the material itself, necessitating the use of external structures to achieve optical enhancement of nonlinear processes. Here, we exploit the 3R phase of a molybdenum disulfide multilayer for resonant nonlinear nanophotonics. The lack of inversion symmetry, even in the bulk of the material, provides a combination of a massive second-order susceptibility, an extremely high and anisotropic refractive index in the near-infrared region (n > 4.5), and low absorption losses, making 3R-MoS2 highly attractive for nonlinear nanophotonics. We demonstrate this by fabricating 3R-MoS2 nanodisks of various radii, which support resonant anapole states, and observing substantial (> 100-fold) enhancement of second-harmonic generation in a single resonant nanodisk compared to an unpatterned flake of the same thickness. The enhancement is maximized at the spectral overlap between the anapole state of the disk and the material resonance of the second-order susceptibility. Our approach unveils a powerful tool for enhancing the entire spectrum of optical second-order nonlinear processes in nanostructured van der Waals materials, thereby paving the way for nonlinear and quantum high-index TMD-nanophotonics.

Casimir torque, a rotational motion driven by zero-point energy minimization, is a problem that attracts notable research interest. Recently, it has been realized using liquid crystal phases and natural anisotropic substrates. However, for natural materials, substantial torque occurs only at van der Waals distances of ~10 nm. Here, we use Casimir self-assembly with triangular gold nanostructures for rotational self-alignment at truly Casimir distances (100 to 200 nm separation). The interplay of repulsive electrostatic and attractive Casimir potentials forms a stable quantum trap, giving rise to a tunable Fabry-Pérot microcavity. This cavity self-aligns both laterally and rotationally to maximize area overlap between templated and floating flakes. The rotational self-alignment is sensitive to the equilibrium distance between the two triangles and their area, offering possibilities for active control via electrostatic screening manipulation. Our self-assembled Casimir microcavities present a versatile and tunable platform for nanophotonic, polaritonic, and optomechanical applications.

We study the self-hybridization between Mie modes supported by water droplets with stretching and bending vibrations in water molecules. Droplets with radii >2.7 μm are found to be polaritonic on the onset of the ultrastrong light-matter coupling regime. Similarly, the effect is observed in larger deuterated water droplets at lower frequencies. Our results indicate that polaritonic states are ubiquitous and occur in water droplets in mists, fogs, and clouds. This finding may have implications not only for polaritonic physics but also for aerosol and atmospheric sciences. Published by the American Physical Society 2024

Recent technological advancements have enabled strong light-matter interaction in highly dissipative cavity-emitter systems. However, in these systems, which are well described by the Tavis-Cummings model, the considerable loss rates render the realization of many desirable nonlinear effects, such as saturation and photon blockade, problematic. Here we present another effect occurring within the Tavis-Cummings model: a nonlinear response of the cavity for resonant external driving of intermediate strength, which makes use of large cavity dissipation rates. In this regime, $(N+1)$-photon processes dominate when the cavity couples to $N$ emitters. We explore and characterize this effect in detail, and provide a picture of how the effect occurs due to destructive interference between the emitter ensemble and the external drive. We find that a central condition for the observed effect is large cooperativity, i.e., the product of the cavity and emitter decay rates is much smaller than the collective cavity-emitter interaction strength squared. Importantly, this condition does not require strong coupling. We also find an analytical expression for the critical drive strength at which the effect appears. Our results have potential for quantum state engineering, e.g., photon filtering, and could be used for the characterization of cavity-emitter systems where the number of emitters is unknown. In particular, our results open the way for investigations of unique quantum-optics applications in a variety of platforms that neither require high-quality cavities nor strong coupling.

The intriguing properties of polaritons resulting from strong and ultrastrong light–matter coupling have been extensively investigated. However, most research has focused on spectroscopic characteristics of polaritons, such as their eigenfrequencies and Rabi splitting. Here, we study the decay rates of a plasmon–microcavity system in the strong and ultrastrong coupling regimes experimentally and numerically. We use a classical scattering matrix approach, approximating our plasmonic system with an effective Lorentz model, to obtain the decay rates through the imaginary part of the complex quasinormal mode eigenfrequencies. Our classical model automatically includes all the interaction terms necessary to account for ultrastrong coupling without dealing with the rotating-wave approximation and the diamagnetic term. We find an asymmetry in polaritonic decay rates, which deviate from the expected average of the uncoupled system’s decay rates at zero detuning. Although this phenomenon has been previously observed in exciton–polaritons and attributed to their disorder, we observe it even in our homogeneous system. As the coupling strength of the plasmon–microcavity system increases, the asymmetry also increases and can become so significant that the lower (upper) polariton decay rate reduction (increase) goes beyond the uncoupled decay rates, γ − < γ 0,c < γ +. Furthermore, our findings demonstrate that polaritonic linewidth asymmetry is a generic phenomenon that persists even in the case of bulk polaritons.

Perfect absorption and strong coupling are two highly sought-after regimes of light-matter interactions. Both regimes have been studied as separate phenomena in excitonic 2D materials, particularly in MoS2. However, the structures used to reach these regimes often require intricate nanofabrication. Here, we demonstrate the occurrence of perfect absorption and strong coupling in thin MoS2 multilayers supported by a glass substrate. We measure reflection spectra of mechanically exfoliated MoS2 flakes at various angles beyond the light-line via Fourier plane imaging and spectroscopy and find that absorption in MoS2 monolayers increases up to 74% at the C-exciton by illuminating at the critical angle. Perfect absorption is achieved for ultrathin MoS2 flakes (4-8 layers) with a notable angle and frequency sensitivity to the exact number of layers. By calculating zeros and poles of the scattering matrix in the complex frequency plane, we identify perfect absorption (zeros) and strong coupling (poles) conditions for thin (<10 layers) and thick (>10 layers) limits. Our findings reveal rich physics of light-matter interactions in bare MoS2 flakes, which could be useful for nanophotonic and light harvesting applications.

The interaction between molecular electronic transitions and electromagnetic fields can be enlarged to the point where distinct hybrid light-matter states, polaritons, emerge. The photonic contribution to these states results in increased complexity as well as an opening to modify the photophysics and photochemistry beyond what normally can be seen in organic molecules. It is today evident that polaritons offer opportunities for molecular photochemistry and photophysics, which has caused an ever-rising interest in the field. Focusing on the experimental landmarks, this review takes its reader from the advent of the field of polaritonic chemistry, over the split into polariton chemistry and photochemistry, to present day status within polaritonic photochemistry and photophysics. To introduce the field, the review starts with a general description of light-matter interactions, how to enhance these, and what characterizes the coupling strength. Then the photochemistry and photophysics of strongly coupled systems using Fabry-Perot and plasmonic cavities are described. This is followed by a description of room-temperature Bose-Einstein condensation/polariton lasing in polaritonic systems. The review ends with a discussion on the benefits, limitations, and future developments of strong exciton-photon coupling using organic molecules.

Transition metal dichalcogenides (TMDs) attract significant attention due to their exceptional optical, excitonic, mechanical, and electronic properties. Nanostructured multilayer TMDs were recently shown to be highly promising for nanophotonic applications, as motivated by their exceptionally high refractive indexes and optical anisotropy. Here, we extend this vision to more sophisticated structures, such as periodic arrays of nanodisks and nanoholes, as well as proof-of-concept waveguides and resonators. We specifically focus on various advanced nanofabrication strategies, including careful selection of resists for electron beam lithography and etching methods. The specific materials studied here include semiconducting WS_2, in-plane anisotropic ReS_2, and metallic TaSe_2, TaS_2 and NbSe_2. The resulting nanostructures can potentially impact several nanophotonic and optoelectronic areas, including high-index nanophotonics, plasmonics and on-chip optical circuits. The knowledge of TMD material-dependent nanofabrication parameters developed here will help broaden the scope of future applications of these materials in all-TMD nanophotonics.

Optical anapoles are intriguing charge-current distributions characterized by a strong suppression of electromagnetic radiation. They originate from the destructive interference of the radiation produced by electric and toroidal multipoles. Although anapoles in dielectric structures have been probed and mapped with a combination of near- and far-field optical techniques, their excitation using fast electron beams has not been explored so far. Here, we theoretically and experimentally analyze the excitation of optical anapoles in tungsten disulfide (WS$_2$) nanodisks using Electron Energy Loss Spectroscopy (EELS) in Scanning Transmission Electron Microscopy (STEM). We observe prominent dips in the electron energy loss spectra and associate them with the excitation of optical anapoles and anapole-exciton hybrids. We are able to map the anapoles excited in the WS$_2$ nanodisks with subnanometer resolution and find that their excitation can be controlled by placing the electron beam at different positions on the nanodisk. Considering current research on the anapole phenomenon, we envision EELS in STEM to become a useful tool for accessing optical anapoles appearing in a variety of dielectric nanoresonators.

Transition metal dichalcogenide (TMD) materials attract significant research attention thanks to their exceptional excitonic and optical properties. In this work, we analyze the formation of strained ultrasharp zigzag edges in MoS2 multilayers produced by anisotropic wet etching. The topography of the edges is determined by the relative stability of the different crystallographic directions of the multilayer as well as the interlayer interactions. Furthermore, we study the linear (Raman) and nonlinear (second-harmonic generation) spectroscopic characteristics of such edges and observe enhanced second-order nonlinearity originating from the strained zigzag edges. We also confirm that ultrasharp hexagonal nanoholes in MoS2 grow along the most stable crystallographic directions despite potential stacking faults or instabilities in the crystal quality. Our results open the way to exploit a broad range of phenomena occurring at the edges of MoS2 material, including the unique determination of crystal orientation for moire ' engineering and strongly correlated phenomena in 2D material-based systems, as well as potential applications in TMD-based electrocatalysis and gas sensing.

Transition metal dichalcogenides (TMDs) attract significant attention due to their remarkable optical and excitonic properties. It was understood already in the 1960s and recently rediscovered that many TMDs possess a high refractive index and optical anisotropy, which make them attractive for nanophotonic applications. However, accurate analysis and predictions of nanooptical phenomena require knowledge of dielectric constants along both in- and out-of-plane directions and over a broad spectral range, information that is often inaccessible or incomplete. Here, we present an experimental study of optical constants from several exfoliated TMD multilayers obtained using spectroscopic ellipsometry in the broad range of 300-1700 nm. The specific materials studied include semiconducting WS2, WSe2, MoS2, MoSe2, and MoTe2, as well as in-plane anisotropic ReS2 and WTe2 and metallic TaS2, TaSe2, and NbSe2. The extracted parameters demonstrate a high index (n up to ∼4.84 for MoTe2), significant anisotropy (n∥ - n⊥ ≈ 1.54 for MoTe2), and low absorption in the near-infrared region. Moreover, metallic TMDs show potential for combined plasmonic-dielectric behavior and hyperbolicity, as their plasma frequency occurs at around ∼1000-1300 nm depending on the material. The knowledge of optical constants of these materials opens new experimental and computational possibilities for further development of all-TMD nanophotonics.

Falko Schmidt, 2, a) Agnese Callegari, Abdallah Daddi-Moussa-Ider, 4 Battulga Munkhbat, 6 Ruggero Verre, Timur Shegai, Mikael Käll, Hartmut Löwen, Andrea Gambassi, 8 and Giovanni Volpe b) Department of Physics, University of Gothenburg, SE–41296 Gothenburg, Sweden Institute of Applied Physics, Friedrich-Schiller-University, D–07745 Jena, Germany Institut für Theoretische Physik II: Weiche Materie, Heinrich-Heine-Universität Düsseldorf, D–40225 Düsseldorf, Germany Abteilung Physik lebender Materie, Max-Planck-Institut für Dynamik und Selbstorganisation, D–37077 Göttingen, Germany Department of Physics, Chalmers University of Technology, SE–412 96 Gothenburg, Sweden Department of Photonics Engineering, Technical University of Denmark, DK–2800, Kgs. Lyngby, Denmark SISSA – International School for Advanced Studies, IT–34136 Trieste, Italy INFN, Sezione di Trieste, IT–34136 Trieste, Italy

The perfect absorption of light has been traditionally achieved by layered structures whose thickness is at least λ/4n. To achieve electromagnetically thin perfect absorbers, research has focused on metallic metasurfaces which require complex nanofabrication. MoS2 is a promising material for ultra-thin perfect absorbers due to the high oscillator strength of its excitonic transitions and layered nature. However previous efforts to increase MoS2 absorption involve intricate fabrication. Our work shows enhanced absorption in monolayers, perfect absorption and exciton-polaritons in MoS2 slabs. This work simultaneously presents a simple approach to achieve perfect absorption in ultra-thin non-metallic structures and to observe cavity-free exciton-polaritons.