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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, $\chi^{(2)}$, 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 $\chi^{(2)}$ 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 ($\sim$ 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.

Transition metal dichalcogenide (TMD) materials have attracted substantial interest due to their remarkable excitonic, optical, electrical, and mechanical properties, which are highly dependent on their crystal structure. Controlling the crystal structure of these materials is essential for fine-tuning their performance, $\textit{e.g.}$, linear and nonlinear optical, as well as charge transport properties. While various phase-switching TMD materials, like molybdenum telluride (MoTe$_2$), are available, their transitions are often irreversible. Here, we investigate the mechanism of a light-induced reversible phase transition in mono- and bilayer flakes of rhenium disulfide (ReS$_2$). Our observations, based on scanning transmission electron microscopy, nonlinear spectroscopy, and density functional theory calculations, reveal a transition from the ground T$''$ (double distorted T) to the metastable 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 towards actively manipulating the crystal structure of ReS$_2$ and possibly its heterostructures.

Casimir torque -- a rotational motion caused by the minimization of the zero-point energy -- is a problem that attracts significant theoretical and experimental interest. Recently, it has been realized using liquid crystal phases and natural anisotropic substrates. However, for natural materials, the torque reaches substantial values only at van der Waals distances of ~10 nm. Here, we employ Casimir self-assembly using templated gold nanostructures of triangular symmetry for the purpose of rotational self-alignment at truly Casimir distances (100 -- 200 nm separation). The joint action of repulsive electrostatic and attractive Casimir potentials leads to the formation of a stable quantum trap, giving rise to a tunable Fabry-Perot microcavity. This cavity self aligns both laterally and rotationally to maximize the overlap area between the templated and floating triangular flakes. The rotational self-alignment is remarkably sensitive to the equilibrium distance between the two triangles as well as their area, which opens possibilities for active control through manipulating the electrostatic screening. Our self-assembled and self-aligned Casimir microcavities could find future use as a versatile and tunable platform for nanophotonic, polaritonic, and optomechanical applications., Comment: 26 pages, 13 figures

We study the self-hybridization between Mie modes supported by water droplets with stretching and bending vibrations in water molecules. Droplets with radii $>2.7~\mu m$ 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 in nature 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., Comment: Main text: 6 pages, 3 figures. Supplemental: 9 pages, 8 figures

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 MoS$_2$, 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-MoS$_2$ highly attractive for nonlinear nanophotonics. We demonstrate this by fabricating 3R-MoS$_2$ 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.

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., Comment: 4 figures

Transition metal dichalcogenides (TMDs) attract significant attention due to their exceptional optical and excitonic properties. It was understood already in the 1960s, and recently rediscovered, that many TMDs possess 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, which 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 WS$_2$, WSe$_2$, MoS$_2$, MoSe$_2$, MoTe$_2$, as well as, in-plane anisotropic ReS$_2$, WTe$_2$, and metallic TaS$_2$, TaSe$_2$, and NbSe$_2$. The extracted parameters demonstrate high-index ($n$ up till $\approx 4.84$ for MoTe$_2$), significant anisotropy ($n_{\parallel}-n_{\perp} \approx 1.54$ for MoTe$_2$), 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 $\sim$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.

Casimir forces in quantum electrodynamics emerge between microscopic metallic objects because of the confinement of the vacuum electromagnetic fluctuations occurring even at zero temperature. Their generalization at finite temperature and in material media are referred to as Casimir--Lifshitz forces. These forces are typically attractive, leading to the widespread problem of stiction between the metallic parts of micro- and nanodevices. Recently, repulsive Casimir forces have been experimentally realized but their reliance on specialized materials prevents their dynamic control and thus limits their further applicability. Here, we experimentally demonstrate that repulsive critical Casimir forces, which emerge in a critical binary liquid mixture upon approaching the critical temperature, can be used to actively control microscopic and nanoscopic objects with nanometer precision. We demonstrate this by using critical Casimir forces to prevent the stiction caused by the Casimir--Lifshitz forces. We study a microscopic gold flake above a flat gold-coated substrate immersed in a critical mixture. Far from the critical temperature, stiction occurs because of dominant Casimir--Lifshitz forces. Upon approaching the critical temperature, however, we observe the emergence of repulsive critical Casimir forces that are sufficiently strong to counteract stiction. This experimental demonstration can accelerate the development of micro- and nanodevices by preventing stiction as well as providing active control and precise tunability of the forces acting between their constituent parts., Comment: 27 pages, 5 figures

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.

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., Comment: 27 pages, 13 figures. New material including analytical calculations and discussions about approximations

Hybrid light-matter states, polaritons, are one of the central concepts in modern quantum optics and condensed matter physics. Polaritons emerge as a result of strong interaction between an optical mode and a material resonance, which is frequently realized in molecular, van der Waals, or solid-state platforms (1-7). However, this route requires accurate (nano)fabrication and often lacks simple means for tunability, which could be disadvantageous in some applications. Here, we use a different approach to realize polaritonic states by employing a stable equilibrium between two parallel gold nanoflakes in an aqueous solution (8). Such plates form a self-assembled Fabry-Perot microcavity with the fundamental optical mode in the visible spectral range. The equilibrium distance between the plates is determined by a balance between attractive Casimir and repulsive electrostatic forces (9-11) and can be controlled by concentration of ligand molecules in the solution, temperature, and light pressure, which allows active and facile tuning of the cavity resonance by external stimuli. Using this Casimir approach, we demonstrate self-assembled polaritons by placing an excitonic medium in the microcavity region, as well as observe their laser-induced modulations in and out of the strong coupling regime. These Casimir microcavities can be used as sensitive and tunable polaritonic platforms for a variety of applications, including opto-mechanics (12), nanomachinery (13), and cavity-induced effects, like polaritonic chemistry (14).

Strong coupling between various kinds of material excitations and optical modes has recently shown potential to modify chemical reaction rates in both excited and ground states. The ground-state modification in chemical reaction rates has usually been reported by coupling a vibrational mode of an organic molecule to the vacuum field of an external optical cavity, such as a planar Fabry-P\'erot microcavity made of two metallic mirrors. However, using an external cavity to form polaritonic states might: (i) limit the scope of possible applications of such systems, and (ii) be unnecessary. Here we highlight the possibility of using optical modes sustained by materials themselves to self-couple to their own electronic or vibrational resonances. By tracing the roots of the corresponding dispersion relations in the complex frequency plane, we show that electronic and vibrational polaritons are natural eigenstates of bulk and nanostructured resonant materials that require no external cavity. Several concrete examples, such as a slab of excitonic material and a spherical water droplet in vacuum are shown to reach the regime of such cavity-free self-strong coupling. The abundance of cavity-free polaritons in simple and natural structures questions their relevance and potential practical importance for the emerging field of polaritonic chemistry, exciton transport, and modified material properties., Comment: 14 pages, 6 figures

Vibrational strong coupling is emerging as a promising tool to modify molecular properties, by making use of hybrid light-matter states known as polaritons. Fabry-Perot cavities filled with organic molecules are typically used, and the molecular concentration limits the maximum reachable coupling strength. Developing methods to increase the coupling strength beyond the molecular concentration limit are highly desirable. In this letter, we investigate the effect of adding a gold nanorod array into a cavity containing pure organic molecules, using FT-IR microscopy and numerical modeling. Incorporation of the plasmonic nanorod array, that acts as artificial molecules, leads to an order of magnitude increase in the total coupling strength for the cavity filled with organic molecules. Additionally, we observe a significant narrowing of the plasmon linewidth inside the cavity. We anticipate that these results will be a step forward in exploring vibropolaritonic chemistry and may be used in plasmon based bio-sensors.

Segmentation of certain hollow organs, such as the bladder, is especially hard to automate due to their complex geometry, vague intensity gradients in the soft tissues, and a tedious manual process of the data annotation routine. Yet, accurate localization of the walls and the cancer regions in the radiologic images of such organs is an essential step in oncology. To address this issue, we propose a new class of hollow kernels that learn to 'mimic' the contours of the segmented organ, effectively replicating its shape and structural complexity. We train a series of the U-Net-like neural networks using the proposed kernels and demonstrate the superiority of the idea in various spatio-temporal convolution scenarios. Specifically, the dilated hollow-kernel architecture outperforms state-of-the-art spatial segmentation models, whereas the addition of temporal blocks with, e.g., Bi-LSTM, establishes a new multi-class baseline for the bladder segmentation challenge. Our spatio-temporal model based on the hollow kernels reaches the mean dice scores of 0.936, 0.736, and 0.712 for the bladder's inner wall, the outer wall, and the tumor regions, respectively. The results pave the way towards other domain-specific deep learning applications where the shape of the segmented object could be used to form a proper convolution kernel for boosting the segmentation outcome., Comment: 18 pages total. Main: 12 figures and 3 tables (main and supplemental). D.V.D is corresponding author

Large optical anisotropy observed in a broad spectral range is of paramount importance for efficient light manipulation in countless devices. Although a giant anisotropy was recently observed in the mid-infrared wavelength range, for visible and near-infrared spectral intervals, the problem remains acute with the highest reported birefringence values of 0.8 in BaTiS3 and h-BN crystals. This inspired an intensive search for giant optical anisotropy among natural and artificial materials. Here, we demonstrate that layered transition metal dichalcogenides (TMDCs) provide an answer to this quest owing to their fundamental differences between intralayer strong covalent bonding and weak interlayer van der Walls interaction. To do this, we carried out a correlative far- and near-field characterization validated by first-principle calculations that reveals an unprecedented birefringence of 1.5 in the infrared and 3 in the visible light for MoS2. Our findings demonstrate that this outstanding anisotropy allows for tackling the diffraction limit enabling an avenue for on-chip next-generation photonics., Comment: 20 pages, 5 figures

The ability to extract materials just a few atoms thick has led to discovery of graphene, monolayer transition metal dichalcogenides (TMDs), and other important two-dimensional materials. The next step in promoting understanding and utility of the flatland physics beyond the state-of-the-art is to study one-dimensional edges of such two-dimensional materials as well as to control the edge-plane ratio. Edges typically exhibit properties that are unique and distinctly different from those of planes and bulk. Thus, controlling them allows to design principally new materials with synthetic edge-plane-bulk characteristics, that is, TMD metamaterials. However, the enabling technology to study such metamaterials experimentally in a precise and systematic way has not yet been developed. Here we report a facile and controllable anisotropic wet etching method that allows scalable fabrication of TMD metamaterials with atomic precision. We show that TMDs can be etched along certain crystallographic axes, such that the obtained edges are atomically sharp and exclusively zigzag-terminated. This results in hexagonal nanostructures of predefined order and complexity, including few nanometer thin nanoribbons and nanojunctions. The method thus enables future studies of a broad range of TMD metamaterials with tailored functionality through atomically precise control of the structure.

Ultrastrong coupling is a distinct regime of electromagnetic interaction that enables a rich variety of intriguing physical phenomena. Traditionally, this regime has been reached by coupling intersubband transitions of multiple quantum wells, superconducting artificial atoms, or two-dimensional electron gases to microcavity resonators. However, employing these platforms requires demanding experimental conditions such as cryogenic temperatures, strong magnetic fields, and high vacuum. Here, we use plasmonic nanorods array positioned at the antinode of the resonant optical Fabry-P\'erot microcavity to reach the ultrastrong coupling (USC) regime at ambient conditions and without the use of magnetic fields. From optical measurements we extract the value of the interaction strength over the transition energy as high as g/{\omega}~0.55, deep in the USC regime, while the nanorods array occupies only ~4% of the cavity volume. Moreover, by comparing the resonant energies of the coupled and uncoupled systems, we indirectly observe up to ~10% modification of the ground-state energy, which is a hallmark of USC. Our results suggest that plasmon-microcavity polaritons are a promising new platform for room-temperature USC realizations in the optical and infrared range., Comment: 4 figures

Strong coupling between a single quantum emitter and an electromagnetic mode is one of the key effects in quantum optics. In the cavity QED approach to plasmonics, strongly coupled systems are usually understood as single-transition emitters resonantly coupled to a single radiative plasmonic mode. However, plasmonic cavities also support non-radiative (or "dark") modes, which offer much higher coupling strengths. On the other hand, realistic quantum emitters often support multiple electronic transitions of various symmetry, which could overlap with higher order plasmonic transitions -- in the blue or ultraviolet part of the spectrum. Here, we show that vacuum Rabi splitting with a single emitter can be achieved by leveraging dark modes of a plasmonic nanocavity. Specifically, we show that a significantly detuned electronic transition can be hybridized with a dark plasmon pseudomode, resulting in the vacuum Rabi splitting of the bright dipolar plasmon mode. We develop a simple model illustrating the modification of the system response in the "dark" strong coupling regime and demonstrate single photon non-linearity. These results may find important implications in the emerging field of room temperature quantum plasmonics.

The ability to differentiate chiral molecules of different handedness is of great importance for chemical and life sciences. Since most of the relevant chiral molecules have their chiral transitions in the UV region, detecting their circular dichroism (CD) signal is associated with practical experimental challenges of performing optical measurements in that spectral range. To address this problem, here, we study the possibility of shifting CD signal of a model chiral medium by reaching the strong coupling regime with an optical microcavity. Specifically, we show that by strongly coupling chiral plasmonic nanoparticles to a non-chiral Fabry-P\'erot microcavity one can imprint the hybrid mode splitting, the hallmark of strongly coupled systems, on the CD spectrum of the coupled system and thereby effectively shift the chiral resonance of the model system to lower energies. We first predict the effect using analytical transfer-matrix method as well as numerical finite-difference time-domain (FDTD) simulations. Furthermore, we confirm the validity of theoretical predictions in a proof-of-principle experiment involving chiral plasmonic nanoparticles coupled to a Fabry-P\'erot microcavity.

All-dielectric subwavelength structures utilizing Mie resonances provide a novel paradigm in nanophotonics for controlling and manipulating light. So far, only spontaneous emission enhancement was demonstrated with single dielectric nanoantennas, whereas stimulated emission was achieved only in large lattices supporting collective modes. Here, we demonstrate the first single-particle all-dielectric monolithic nanolaser driven by Mie resonances in visible and near-IR frequency range. We employ halide perovskite CsPbBr$_3$ as both gain and resonator material that provides high optical gain (up to $\sim 10^4$ cm$^{-1}$) and allows simple chemical synthesis of nanocubes with nearly epitaxial quality. Our smallest non-plasmonic Mie-resonant single-mode nanolaser with the size of 420 nm operates at room temperatures and wavelength 535 nm with linewidth $\sim 3.5$ meV. These novel lasing nanoantennas can pave the way to multifunctional photonic designs for active control of light at the nanoscale.

Polaritons are compositional light-matter quasiparticles that have recently enabled remarkable breakthroughs in quantum and nonlinear optics, as well as in material science. Despite the enormous progress, however, a direct nanometer-scale visualization of polaritons has remained an open challenge. Here, we demonstrate that plasmon-exciton polaritons, or plexcitons, generated by a hybrid system composed of an individual silver nanoparticle and a few-layer transition metal dichalcogenide can be spectroscopically mapped with nanometer spatial resolution using electron energy loss spectroscopy in a scanning transmission electron microscope. Our experiments reveal important insights about the coupling process, which have not been reported so far. These include nanoscale variation of Rabi splitting and plasmon-exciton detuning, as well as absorption-dominated extinction signals, which in turn provide the ultimate evidence for the plasmon-exciton hybridization in the strong coupling regime. These findings pioneer new possibilities for in-depth studies of polariton-related phenomena with nanometer spatial resolution.

Strong light-matter interactions in both the single-emitter and collective strong coupling regimes attract significant attention due to emerging quantum and nonlinear optics applications, as well as opportunities for modifying material-related properties. Further exploration of these phenomena requires an appropriate theoretical methodology, which is demanding since polaritons are at the intersection between quantum optics, solid state physics and quantum chemistry. Fortunately, however, nanoscale polaritons can be realized in small plasmon-molecule systems, which in principle allows treating them using ab initio methods, although this has not been demonstrated to date. Here, we show that time-dependent density-functional theory (TDDFT) calculations can access the physics of nanoscale plasmon-molecule hybrids and predict vacuum Rabi splitting in a system comprising a few-hundred-atom aluminum nanoparticle interacting with one or several benzene molecules. We show that the cavity quantum electrodynamics approach holds down to resonators on the order of a few cubic nanometers, yielding a single-molecule coupling strength exceeding 200 meV due to a massive vacuum field value of 4.5 V/nm. In a broader perspective, our approach enables parameter-free in-depth studies of polaritonic systems, including ground state, chemical and thermodynamic modifications of the molecules in the strong-coupling regime, which may find important use in emerging applications such as cavity enhanced catalysis., Comment: 6 pages, 5 figures

Monolayer transition metal dichalcogenides (TMDCs) have recently been proposed as a unique excitonic platform for advanced optical and electronic functionalities. However, in spite of intense research efforts, it has been largely overlooked that, in addition to displaying rich exciton physics, TMDCs also possess a very high refractive index. This opens a possibility to utilize these materials for constructing resonant nanoantennas based on subwavelength geometrical modes. Here we show that nanodisks fabricated from exfoliated multilayer WS$_2$ support distinct Mie resonances and so-called anapole states that can be easily tuned in wavelength over the visible and near-infrared spectral range by varying the nanodisks' size and aspect ratio. We argue that the TMDC material anisotropy and the presence of excitons substantially enrich nanophotonics by complementing traditional approaches based on plasmonics and well-known high-index materials such as silicon. As a proof-of-concept, we demonstrate a novel regime of light-matter interaction, anapole-exciton polaritons, which we realize within a single WS$_2$ nanodisk. Our results thus suggest that nanopatterned TMDCs are promising materials for high-index nanophotonics applications with enriched functionalities and superior prospects.

Polaritons are compositional light-matter quasiparticles that arise as a result of strong coupling between a vacuum field of a resonant optical cavity and electronic excitations in quantum emitters. Reaching such a regime is often hard, as it requires materials possessing high oscillator strengths to interact with the relevant optical mode. Two dimensional transition metal dichalcogenides (TMDs) have recently emerged as promising candidates for realization of the strong coupling regime at room temperature. However, these materials typically provide coupling strengths in the range of 10-40 meV, which may be insufficient for reaching strong coupling with low quality factor resonators. Here, we demonstrate a universal scheme that allows a straightforward realization of strong and ultra-strong coupling regime with 2D materials and beyond. By intermixing plasmonic excitations in nanoparticle arrays with excitons in a WS2 monolayer inside a resonant metallic microcavity, we fabricate a hierarchical system with the combined Rabi splitting exceeding 500 meV at room temperature. Photoluminescence measurements of the coupled systems show dominant emission from the lower polariton branch, indicating the participation of excitons in the coupling process. Strong coupling has been recently suggested to affect numerous optical- and material-related properties including chemical reactivity, exciton transport and optical nonlinearities. With the universal scheme presented here, strong coupling across a wide spectral range is within easy reach and therefore exploring these exciting phenomena can be further pursued in a much broader class of materials., Comment: 4 Figures; includes supporting information

Realizing strong coupling between a single quantum emitter (QE) and an optical cavity is of crucial importance in the context of various quantum optical applications. While Rabi splitting of single quantum emitters coupled to high-Q diffraction limited cavities have been reported in numerous configurations, attaining single emitter Rabi splitting with a plasmonic nanostructure is still elusive. Here, we establish the analytical condition for strong coupling between a single QE and a plasmonic nanocavity and apply it to study various plasmonic arrangements that were shown to enable Rabi splitting. We investigate numerically the optical response and the resulting Rabi splitting in metallic nanostructures such as bow-tie nanoantennas, nanosphere dimers and nanospheres on a surface and find the optimal geometries for emergence of the strong coupling regime with single QEs. We also provide a master equation approach to show the saturation of a single QE in the gap of a silver bow-tie nanoantenna. Our results will be useful for implementation of realistic quantum plasmonic nanosystems involving single QEs., Comment: 9 pages, 7 figures

Intermixed light-matter quasiparticles - polaritons - possess unique optical properties owned to their compositional nature. These intriguing hybrid states have been extensively studied over the past decades in a wide range of realizations aiming at both basic science and emerging applications. However, recently it has been demonstrated that not only optical, but also material-related properties, such as chemical reactivity and charge transport, may be significantly altered in the strong coupling regime of light-matter interactions. Here, we show that a nanoscale system, comprised of a plasmonic nanoprism strongly coupled to excitons in J-aggregated form of organic chromophores, experiences modified excited state dynamics and therefore modified photo-chemical reactivity. Our experimental results reveal that photobleaching, one of the most fundamental photochemical reactions, can be effectively controlled and suppressed by the degree of plasmon-exciton coupling and detuning. In particular, we observe a 100-fold stabilization of organic dyes for the red-detuned nanoparticles. Our findings contribute to understanding of photochemical properties in the strong coupling regime and may find important implications for the performance and improved stability of optical devices incorporating organic dyes., Comment: 5 figures; includes Supplementary Materials

Monolayer transition metal dichalcogenides (TMDs) show an optimal surface-to-volume ratio and are thus promising candidates for novel molecule sensor devices. It was recently predicted that a certain class of molecules exhibiting a large dipole moment can be detected through the activation of optically inaccessible (dark) excitonic states in absorption spectra of tungsten-based TMDs. In this work, we investigate the molecule signatures in photoluminescence spectra in dependence of a number of different experimentally accessible quantities, such as excitation density, temperature as well as molecular characteristics including the dipole moment and its orientation, molecule-TMD distance, molecular coverage and distribution. We show that under certain optimal conditions, even room temperature detection of molecules can be achieved.

Absorption of electromagnetic energy by a material is a phenomenon that underlies many applied problems, including molecular sensing, photocurrent generation and photodetection. Commonly, the incident energy is delivered to the system through a single channel, for example by a plane wave incident on one side of an absorber. However, absorption can be made much more efficient by exploiting wave interference. A coherent perfect absorber is a system in which complete absorption of electromagnetic radiation is achieved by controlling the interference of multiple incident waves. Here, we review recent advances in the design and applications of such devices. We present the theoretical principles underlying the phenomenon of coherent perfect absorption and give an overview of the photonic structures in which it can be realized, including planar and guided-mode structures, graphene-based systems, parity- and time-symmetric structures, 3D structures and quantum-mechanical systems. We then discuss possible applications of coherent perfect absorption in nanophotonics and, finally, we survey the perspectives for the future of this field.

Formation of dressed light-matter states in optical structures, manifested as Rabi splitting of the eigen energies of a coupled system, is one of the key effects in quantum optics. In pursuing this regime with semiconductors, light is usually made to interact with excitons $-$ electrically neutral quasiparticles of semiconductors, meanwhile interactions with charged three-particle states $-$ trions $-$ have received little attention. Here, we report on strong interaction between plasmons in silver nanoprisms and charged excitons $-$ trions $-$ in monolayer tungsten disulphide (WS$_{2}$). We show that the plasmon-exciton interactions in this system can be efficiently tuned by controlling the charged versus neutral exciton contribution to the coupling process. In particular, we show that a stable trion state emerges and couples efficiently to the plasmon resonance at low temperature by forming three bright intermixed plasmon-exciton-trion polariton states. Our findings open up a possibility to exploit electrically charged trion polaritons $-$ previously unexplored mixed states of light and matter in nanoscale hybrid plasmonic systems., Comment: 31 pages, 11 figures, Main text, Supplementary Info

We present magnetotransport measurements in HgTe quantum well with inverted band structure, which expected to be a two-dimensional topological insulator having the bulk gap with helical gapless states at the edge. The negative magnetoresistance is observed in the local and nonlocal resistance configuration followed by the periodic oscillations damping with magnetic field. We attribute such behaviour to Aharonov-Bohm effect due to magnetic flux through the charge carrier puddles coupled to the helical edge states. The characteristic size of these puddles is about 100 nm., Comment: 6 pages, 6 figures

Realizing strong light-matter interactions between individual 2-level systems and resonating cavities in atomic and solid state systems opens up possibilities to study optical nonlinearities on a single photon level, which can be useful for future quantum information processing networks. However, these efforts have been hampered by the unfavorable experimental conditions, such as cryogenic temperatures and ultrahigh vacuum, required to study such systems and phenomena. Although several attempts to realize strong light-matter interactions at room-temperature using so-called plasmon resonances have been made, successful realizations on the single nanoparticle level are still lacking. Here, we demonstrate strong coupling between plasmons confined within a single silver nanoprism and excitons in molecular J-aggregates at ambient conditions. Our findings show that the deep subwavelength mode volumes, $V$, together with high quality factors, $Q$, associated with plasmons in the nanoprisms result in strong coupling figure-of-merit -- $Q/\sqrt{V}$ as high as $\sim6\times10^{3}$~$\mu$m$^{-3/2}$ -- a value comparable to state-of-art photonic crystal and microring resonator cavities, thereby suggesting that plasmonic nanocavities and specifically silver nanoprisms can be used for room-temperature quantum optics.

We study experimentally the transport properties of "inverted" semiconductor HgTe-based quantum well, which is related to the two-dimensional topological insulator, in diffusive transport regime. We perform nonlocal electrical measurements in the absence of the magnetic field and observe large signal due to the edge states. It demonstrates, that the edge states can propagate over long distance 1 mm, and, therefore, there is no difference between local and non local electrical measurements in topological insulator. In the presence of the in-plane magnetic field we find strong decrease of the local resistance and complete suppression of the nonlocal resistance. We attribute this observation to the transition between topological insulator and bulk metal induced by the in-plane magnetic field., Comment: 4.5 pages, 4 figures