Search

Non-Hermitian systems are known to possess unique singularities in the scattering spectra such as exceptional points, bound states in the continuum, Diabolic points, and anapole states, which are usually considered to be independent. Here, we demonstrate the fundamental relationships between non-Hermitian singularities and observe them experimentally in the scattering spectra. We reveal that exceptional points appear in the anapole regime, and diabolic points are associated with superscattering. We confirm our findings with microwave experiments by measuring the scattering spectra of subwavelength Mie-resonant ceramic rings. Our study underpins the generic behavior of non-Hermitian singularities in the scattering spectra of subwavelength resonators, uncovering their novel applications in non-Hermitian nonlinear optics and topological photonics., Comment: 19 pages, 5 figures

Resonant states underlie a variety of metastructures that exhibit remarkable capabilities for effective control of acoustic waves at subwavelength scales. The development of metamaterials relies on the rigorous mode engineering providing the implementation of the desired properties. At the same time, the application of metamaterials is still limited as their building blocks are frequently characterized by complicated geometry and can't be tuned easily. In this work, we consider a simple system of coupled Helmholtz resonators and study their properties associated with the tuning of coupling strength and symmetry breaking. We numerically and experimentally demonstrate the excitation of quasi-bound state in the continuum in the resonators placed in a free space and in a rectangular cavity. It is also shown that tuning the intrinsic losses via introducing porous inserts can lead to spectral splitting or merging of quasi-\textit{bound states in the continuum} and occurrence of \textit{exceptional points}. The obtained results will open new opportunities for the development of simple and easy-tunable metastructures based on Helmholtz resonances.

Periodic optical structures, such as diffraction grating and numerous photonic crystals, are one of the staples of modern nanophotonics for the manipulation of electromagnetic radiation. The array of subwavelength dielectric rods is one of the simplest platforms, which, despite its simplicity exhibits extraordinary wave phenomena, such as diffraction anomalies and narrow reflective resonances. Despite the well-documented properties of infinite periodic systems, the behavior of these diffractive effects in systems incorporating a finite number of elements is studied to a far lesser extent. Here we study theoretically and numerically the evolution of collective spectral features in finite arrays of dielectric rods. We develop an analytical model of light scattering by a finite array of circular rods based on the coupled dipoles approximation and analyze the spectral features of finite arrays within the developed model. Finally, we validate the results of the analytical model using full-wave numerical simulations.

Efficient trapping of light in nanostructures is essential for the development of optical devices that are based on the interaction between light and matter. In this work, we show theoretically and experimentally that one-dimensional arrays of subwavelength dielectric Mie-resonant particles can support collective resonances with increased $Q$-factors. We demonstrate that the increase of the $Q$-factor can be explained by interaction between the collective electric and magnetic dipole modes of the chain resulting in appearance of the inflection point at the band edge. The considered effect is studied experimentally in the chain of high-index ceramic cylinders in the microwave spectral range., Comment: 11 pages, 8 figures

Bound states in the continuum (BICs) and exceptional points (EPs) are unique singularities of non-Hermitian systems. BICs demonstrate enhancement of the electromagnetic field at the nanoscale, while EPs exhibit high sensitivity to small perturbations. Here, we demonstrate that several BICs can be merged into one EP, forming an EP-BIC. The resulting state inherits properties from both BICs and EP, namely, it does not radiate and shows extremely high sensitivity to perturbations. We validate the developed theory with numerical simulations and demonstrate the formation of second and third-order EP-BICs in stacked dielectric metasurfaces. We also show that the losses of the resulting leaky resonances exhibit an anomalous behavior when the unit cell is broken, which differs from the asymptotics commonly attributed to BICs.

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., Comment: 16 pages, 4 figures

The control of light through all-optical means is a fundamental challenge in nanophotonics and a key effect in optical switching and logic. The optical bistability effect enables this control and can be observed in various planar photonic systems such as microdisk and photonic crystal cavities and waveguides. However, the recent advancements in flat optics with wavelength-thin optical elements require nonlinear elements based on metastructures and metasurfaces. The performance of these systems can be enhanced with high-Q bound states in the continuum (BIC), which leads to intense harmonic generation, improved light-matter coupling, and pushes forward sensing limits. In this study, we report on the enhanced thermo-optical nonlinearity and the observation of optical bistability in an all-dielectric metasurface membrane with BICs. Unlike many other nanophotonic platforms, metasurfaces allow for fine control of the quality factor of the BIC resonance by managing the radiative losses. This provides an opportunity to control the parameters of the observed hysteresis loop and even switch from bistability to optical discrimination by varying the angle of incidence. Additionally, we propose a mechanism of nonlinear critical coupling that establishes the conditions for maximal hysteresis width and minimal switching power, which has not been reported before. Our work suggests that all-dielectric metasurfaces supporting BICs can serve as a flat-optics platform for optical switching and modulation based on strong thermo-optical nonlinearity.

In the field of optics and nanophotonics, simulation of electromagnetic scattering plays a major role in the study of complex nanostructures and optical devices. The numerical analysis of scattering spectra, even for nanocavities with simple geometry, is associated with significant computational difficulties. However, when the system exhibits certain symmetries, it becomes possible to simplify the problem through the process of separation of variables, which leads to a decrease in its dimension. In this paper, we aim to provide a practical guide to a fast simulation of linear and non-linear scattering problems in COMSOL Multiphysics for axisymmetric objects including computation of scattering cross-section as well as its multipolar decomposition, optical forces, and second harmonic generation. We also accompany the provided guide with the ready-to-run COMSOL models.

Acoustic metamaterials and phononic crystals represent a promising platform for the development of noise-insulating systems characterized by a low weight and small thickness. Nevertheless, the operational spectral range of these structures is usually quite narrow, limiting their application as substitutions of conventional noise-insulating systems. In this work, the problem is tackled by demonstration of several ways for the improvement of noise-insulating properties of the periodic structures based on coupled Helmholtz resonators. It is shown that tuning of local coupling between the resonators leads to the formation of ultra-broad stop-bands in the transmission spectra. This property is linked to band structures of the equivalent infinitely periodic systems and is discussed in terms of band-gap engineering. The local coupling strength is varied via several means, including introduction of the so-called chirped structures and lossy resonators with porous inserts. The stop-band engineering procedure is supported by genetic algorithm optimization and the numerical calculations are verified by experimental measurements.

Raman lasers is an actively developing field of nonlinear optics aiming to create efficient frequency converters and various optical sensors. Due to the growing importance of ultracompact chip-scale technologies, there is a constant demand for optical devices miniaturization, however, the development of a nanoscale Raman laser remains a challenging endeavor. In this work, we propose a fully subwavelength Raman laser operating in visible range based on a gallium phosphide nanocylinder resonator supporting a quasi bound state in the continuum (quasi-BIC). We perform precise spectral matching of nanoparticle's high-$Q$ modes with the pump and detuned Raman emission wavelengths. As a result of our simulations, we demonstrate a design of Raman nanolaser, ready for experimental realization, with the lasing threshold expected to be as low as $P_{\mathrm{th}} \approx 21~\mathrm{mW}$. The suggested configuration, to the best of our knowledge, represents the very first prototype of a low-threshold Raman nanolaser with all the dimensions smaller than the operational wavelength., Comment: 7 pages for main document including 4 figures, 2 pages for supplementary document including 1 figure

Guided waves are the perfect carriers of electromagnetic signals in planar miniaturized devices due to their high localization and controlled propagation direction. However, it is still a challenge to control the polarization of propagating guided waves. In this work, we discover both theoretically and experimentally the broadband polarization TE-TM degeneracy of highly localized guided waves propagating along an all-dielectric metasurface and a subwavelength chain of dielectric high-index cylinders. Using the discovered near-field polarization degree of freedom, we demonstrate experimentally the polarization transformation for guided waves propagating along a subwavelength chain of dielectric cylinders at any frequency within the finite spectral range. Namely, we implement the simplest planar near-field polarization device -- the quarter-wave-retardation (linear-to-circular) polarization transformer of guided waves. The results obtained discover the polarization degree of freedom for guided waves paving the way towards numerous applications in flat optics and planar photonics., Comment: 16 pages, 8 figures

Metasurfaces with bound states in the continuum (BICs) have proven to be a powerful platform for drastically enhancing light-matter interactions, improving biosensing, and precisely manipulating near- and far-fields. However, engineering metasurfaces to provide an on-demand spectral and angular position for a BIC remains a prime challenge. A conventional solution involves a fine adjustment of geometrical parameters, requiring multiple time-consuming calculations. In this work, to circumvent such tedious processes, we develop a physics-inspired, inverse design method on all-dielectric metasurfaces for an on-demand spectral and angular position of a BIC. Our suggested method predicts the core-shell particles that constitute the unit cell of the metasurface, while considering practical limitations on geometry and available materials. Our method is based on a smart combination of a semi-analytical solution, for predicting the required dipolar Mie coefficients of the meta-atom, and a machine learning algorithm, for finding a practical design of the meta-atom that provides these Mie coefficients. Although our approach is exemplified in designing a metasurface sustaining a BIC, it can, also, be applied to many more objective functions. With that, we pave the way toward a general framework for the inverse design of metasurfaces in specific and nanophotonic structures in general., Comment: 20 pages, 5 figures, Supplementary Material

Bound states in the continuum provide a remarkable example of how a simple problem solved about a century ago in quantum mechanics can drive the research on a whole spectrum of resonant phenomena in wave physics. Due to their huge radiative lifetime, bound states in the continuum have found multiple applications in various areas of physics devoted to wave processes, including hydrodynamics, atomic physics, and acoustics. In this review paper, we present a comprehensive description of bound states in the continuum and related effects, focusing mainly on photonic dielectric structures. We review the history of this area, basic physical mechanisms in the formation of bound states in the continuum, and specific examples of structures supporting such states. We also discuss their possible applications in optics, photonics, and radiophysics., Comment: 80 pages, 21 figures

Transition metal dichalcogenides (TMDs) is a promising platform for new generation optoelectronics and nanophotonics due to their unique optical properties. However, in contrast to direct bandgap TMDs monolayers, bulk samples have an indirect bandgap that restricts their application as light emitters. On the other hand, the high refractive index of these materials seems ideal for creating high-quality nanophotonic resonators with a strong Purcell effect. In this work, we fabricate Whispering-gallery mode (WGM) resonators from bulk (i.e., indirect bandgap) MoSe$_2$ using resistless scanning probe lithography and study their optical properties. Micro-photoluminescence($\mu$-PL) investigation revealed WGM spectra of resonators with an enhancement factor of 100 compared to pristine flake. Scattering experiments and modeling also confirm the WGM nature of spectra observed. Temperature dependence of PL revealed two components of photoluminescence. The first one quenches with decreasing temperature, the second one does not and becomes dominant. Therefore, this suggests that resonators amplify both direct and temperature-activated indirect PL. Thus, here we demonstrated the novel approach to fabricating nanophotonic resonators from bulk TMDs and obtaining PL from indirect bandgap materials. We believe that the suggested approach and structures have great prospects in nanophotonics., Comment: Main text: 18 pages, 5 figures. Supplementary: 3 pages, 2 figures

Bianisotropy of metaatoms is usually associated with their nonlocal response and the mutual coupling between electric and magnetic dipole moments induced by the incident field. In this work, we generalize the theory of bianisotropy beyond the dipole response to the cases of arbitrary high-order multipole resonances. We demonstrate that bianisotropy is exclusively caused by the absence of the inversion symmetry of metaatoms. The strength of the bianisotropy response is normally increased with the size of a meta-atom but its origin is fully connected to the symmetry of the structure. As an important example of bianisotropic particle, we consider a triangular prism and show how accounting for the higher-order multipoles prevents the violation of the Onsager-Casimir conditions for kinetic coefficients appearing in the dipole approximation. The developed theory is an important step towards a deeper insight into the scattering properties of nanoantennas and meta-atoms., Comment: 6 pages, 4 figures, Supplemental Materials

Noise pollution remains a challenging problem requiring the development of novel systems for noise insulation. Extensive work in the field of acoustic metamaterials has led to occurrence of various ventilated structures which, however, are usually demonstrated for rather narrow regions of the audible spectrum. In this work, we further extend the idea of metamaterial-based systems developing a concept of a metahouse chamber representing a ventilated structure for broadband noise insulation. Broad stop bands originate from strong coupling between pairs of Helmholtz resonators constituting the structure. We demonstrate numerically and experimentally the averaged transmission -18 dB within the spectral range from 1500 to 16500 Hz. The sparseness of the structure together with the possibility to use optically transparent materials suggest that the chamber may be also characterized by partial optical transparency depending on the mutual position of structural elements. The obtained results are promising for development of novel noise-insulating structures advancing urban science.

We develop a theory of bound states in the continuum (BICs) in multipolar lattices -- periodic arrays of resonant multipoles. We predict that BICs are completely robust to changes in lattice parameters remaining pinned to specific directions in the $k$-space. The lack of radiation for BICs in such structures is protected by the symmetry of multipoles forming the lattice. We also show that some multipolar lattices can host BICs forming a continuous line in the $k$-space and such BICs carry zero topological charge. The developed approach sets a direct fundamental relation between the topological charge of BIC and the asymptotic behavior of the Q-factor in its vicinity. We believe that our theory is a significant step towards gaining deeper insight into the physics of BICs and the engineering of high-Q states in all-dielectric metasurfaces., Comment: 7 pages, 4 figures, Supplemental material

The sensitivity of a refractive index sensor based on an optical bound state in the continuum is considered. Applying Zel'dovich perturbation theory we derived an analytic expression for bulk sensitivity of an all-dielectic sensor utilizing symmetry protected in-$\Gamma$ optical bound states in a dielectric grating. The upper sensitivity limit is obtained. A recipe is proposed for obtaining the upper sensitivity limit by optimizing the design of the grating. The results are confirmed through direct numerical simulations.

In the recent years, we observe a dramatic boost of research in photonics empowered by the concepts of machine learning and artificial intelligence. The corresponding photonic systems, to which this new methodology is applied, can range from traditional optical waveguides to nanoantennas and metasurfaces, and these novel approaches underpin the fundamental principles of light-matter interaction developed for a smart design of intelligent photonic devices. Concepts and approaches of artificial intelligence and machine learning penetrate rapidly into the fundamental physics of light, and they provide effective tools for the study of the field of metaphotonics driven by optically-induced electric and magnetic resonances. Here, we introduce this new field with its application to metaphotonics and also present a summary of the basic concepts of machine learning with some specific examples developed and demonstrated for metasystems and metasurfaces.

We study the effect of structural disorder on the transition from the bound states in the continuum (BICs) to quasi-BICs by the example of the periodic photonic structure composed of two layers of parallel dielectric rods. We uncover the specificity in the robustness of the symmetry-protected and accidental BICs against various types of structural disorder. We analyze how the spatial mode localization induced by the structural disorder results in an effective reduction of the system length and limits the Q factor of quasi-BICs. Our results are essential for the practical implementation of BICs especially in natural and self-assembled photonic structures, where the structural disorder plays a crucial role., Comment: 13 pages, 7 figures

We develop the theory of optical beam shifts (both Goos-Hanchen and Imbert-Fedorov) for the case of near-normal incidence, when the incident angle becomes comparable with the angular beam divergence. Such a situation naturally leads to strong enhancement of the shifts reported recently [ACS Photonics 6, 2530 (2019)]. Experimental results find complete and rigorous explanation in our generalized theory. In addition, the developed theory uncovers the unified origin of the anomalous beam shifts enhancement via the Berry phase singularity. We also propose a simple experimental scheme involving quarter-wave plate that allows to observe the giant transverse and longitudinal, spatial and angular beam shifts simultaneously. Our results can find applications in spin-orbit photonics, polarization optics, sensing applications, and quantum weak measurements.

We reveal that finite-size solid acoustic resonators can support genuine bound states in the continuum (BICs) completely localized inside the resonator. The developed theory provides the multipole classification of such BICs in the resonators of various shapes. It is shown how breaking of the resonator's symmetry turns BICs into quasi-BICs manifesting themselves in the scattering spectra as high-Q Fano resonances. We believe that the revealed novel states will push the performance limits of acoustic devices and will serve as high-Q building blocks for acoustic sensors, antennas, and topological acoustic structures., Comment: 14 pages, 5 figures

Bound states in the continuum (BICs) are non-radiating solutions of the wave equation with a spectrum embedded in the continuum of propagating waves of the surrounding space. The complete decoupling of BICs from the radiation continuum makes their excitation impossible from the far-field. Here, we develop a general theory of parametric excitation of BICs in nonlinear systems with Kerr-type nonlinearity via spontaneous symmetry breaking, which results in a coupling of a BIC and a bright mode of the system. Using the temporal coupled-mode theory and perturbation analysis, we found the threshold intensity for excitation of a BIC and study the possible stable and unstable solutions depending on the pump intensity and frequency detuning between the pump and BIC. We revealed that at some parameters of the pump beam, there are no stable solutions and the BIC can be used for frequency comb generation. Our findings can be very promising for use in nonlinear photonic devices and all-optical networks., Comment: 17 pages, 9 figures

An efficient modulation of backscattered energy is one of the key requirements for enabling efficient wireless communication channels. Typical architectures, being based on either electronically or mechanically modulated reflectors, cannot be downscaled to subwavelengths dimensions by design. Here we show that integrating high-index dielectric materials with tunable subwavelength resonators allows achieving an efficient backscattering modulation, keeping a footprint of an entire structure small. An interference between high-order Mie resonances leads to either enhancement or suppression of the backscattering, depending on a control parameter. In particular, a ceramic core-shell, driven by an electronically tunable split ring resonator was shown to provide a backscattering modulation depth as high as tens of the geometrical cross-section of the structure. The design was optimized towards maximizing reading range of radio frequency identification (RFID) tags and shown to outperform existing commercial solutions by orders of magnitude in terms of the modulation efficiency. The proposed concept of multipole engineering allows one to design a new generation of miniature beacons and modulators for wireless communication needs and other relevant applications.

Germanium self-assembled nanoislands and quantum dots are very prospective for CMOS-compatible optoelectronic integrated circuits but their luminescence intensity is still insufficient for many practical applications. Here, we demonstrate experimentally that photoluminescence of Ge nanoislands in silicon photonic crystal slab with hexagonal lattice can be dramatically enhanced due to the involvement in the emission process of the bounds states in the continuum. We experimentally demonstrate more than two orders of magnitude peak photoluminescence enhancement and more than one order of magnitude integrated PL enhancement in a photonic crystal slab compared with the non-structured sample area. We theoretically study this effect by the Fourier modal method in the scattering matrix form and demonstrate the appearance of quasi-normal guided modes in our photonic crystal slab. We also describe their symmetry in terms of group theory. Our work paves the way towards a new class of optoelectronic components compatible with silicon technology., Comment: 20 pages, 8 figures

We consider light scattering by dielectric gratings supporting optical bound states in the continuum. Due to the presence of instantaneous Kerr nonlinearity the critical field enhancement in the spectral vicinity of the bound state triggers the effect of optical bistability. The onset of bistability is explained theoretically in the framework of the temporal coupled mode theory. As the central result we cast the problem into the form of a singly field-driven nonlinear oscillator. The theoretical results are verified in comparison against full-wave numerical simulations.

Electromagnetic response of dielectric resonators with high refractive index is governed by optically induced electric and magnetic Mie resonances facilitating confinement of light with the amplitude enhancement. However, strong subwavelength trapping of light was associated traditionally only with plasmonic or epsilon-near-zero structures which however suffer from losses. Recently, an alternative localization mechanism was proposed to trap light in individual subwavelength optical resonators with a high quality factor in the regime of a supercavity mode. Here, we present the experimental observation of the supercavity modes in subwavelength ceramic resonators in the radiofrequency range. We demonstrate experimentally that the regime of supercavity mode can be achieved via precise tuning of the resonator's dimensions resulting in a huge growth of the quality factor reaching the experimental values up to 1.25x10^4, being limited only by material losses in dielectrics. We reveal that the supercavity modes can be excited efficiently by both near- and far-fields by means of dipole sources and plane waves, respectively. In both the cases, the supercavity mode manifests itself clearly via characteristic peculiarities of the Fano resonance and radiation patterns. Our work paves the way for future compact practical devices in photonics and radiophysics., Comment: 11 pages, 4 figures

Radio frequency identification (RFID) is a mature technology, which allows performing contactless data readout via wireless communication links. While communication protocols in this field are subject to regulations, there is a room of opportunities to improve the hardware realization of antennas devices, which support the technology. In particular, readout range extension and miniaturization of passive RFID tags is an important objective with far going impact on retail, security, IoT, and many others. Here we introduce a new concept of high-permittivity ceramic tag, which relies on different physical principles. Instead of using conduction currents in metallic wires to drive electronic chips and radiate electromagnetic waves, high permittivity components rely on an efficient excitation of displacement currents. Those are efficiently converted to conduction currents, powering a memory chip. The practical aspect of this approach is improved robustness to environmental fluctuations, footprint reduction, and readout range extension. In particular, our high permittivity ceramic ({\epsilon} ~ 100) elements have demonstrated a 25% reading range improvement in respect to commercial tags. In case, when state of the art readers and RFID chips are used, the readout distances of the developed ceramic tags are approaching 23 m and could be further extended with improved matching circuits.

Absorption of electromagnetic energy by a dissipative material is one of the most fundamental electromagnetic processes that underlies a plethora of applied problems, including sensing and molecular detection, radar detection, wireless power transfer, and photovoltaics. Perfect absorption is a particular regime when all of the incoming electromagnetic energy is absorbed by the system without scattering. Commonly, the incident energy is delivered to the absorbing system by a plane wave, hence perfect absorption of this wave requires an infinitely extended planar structure. Here, we demonstrate theoretically that a confined incident beam carrying a finite amount of electromagnetic energy can be perfectly absorbed by a finite size deep subwavelength scatterer on a substrate. We analytically solve the self-consistent scattering problem in the dipole approximation and find a closed-form expression for the spatial spectrum of the incident field and the required complex polarizability of the particle. All analytical predictions are confirmed with full-wave simulations., Comment: 4 figures

The resonator is one of the main building blocks of a plethora of photonic and microwave devices from nanolasers to compact biosensors and magnetic resonance scanners. The symmetry of the resonators is tightly related to their mode structure and multipole content which determines the linear and non-linear response of the resonator. Here, we develop the algorithm for the classification of eigenmodes in resonators of the simplest shapes depending on their symmetry group. For each type of mode, we find its multipole content. As an illustrative example, we apply the developed formalism to the analysis of dielectric triangular prism and demonstrate the formation of high-Q resonances originated due to suppression of the scattering through the main multipole channel. The developed approach one to engineer, predict, and explain scattering phenomena and optical properties of resonators and meta-atmos basing only on their symmetry without the need for numerical simulations and it can be used for the design of new photonic and microwave devices., Comment: 13 pages, 6 figures, 5 tables

Bound states in the continuum (BICs) represent localized modes with energies embedded in the continuous spectrum of radiating waves. BICs were discovered initially as a mathematical curiosity in quantum mechanics, and more recently were employed in photonics. Pure mathematical bound states have infinitely-large quality factors (Q factors) and zero resonant linewidth. In optics, BICs are physically limited by a finite size, material absorption, structural disorder, and surface scattering, and they manifest themselves as the resonant states with large Q factors, also known as supercavity modes or quasi-BICs. Optical BIC resonances have been demonstrated only in extended 2D and 1D systems and have been employed for distinct applications including lasing and sensing. Optical quasi-BIC modes in individual nanoresonators have been discovered recently but they were never observed in experiment. Here, we demonstrate experimentally an isolated subwavelength nanoresonator hosting a quasi-BIC resonance. We fabricate the resonator from AlGaAs material on an engineered substrate, and couple to the quasi-BIC mode using structured light. We employ the resonator as a nonlinear nanoantenna and demonstrate record-high efficiency of second-harmonic generation. Our study brings a novel platform to resonant subwavelength photonics.

Nonradiating sources of energy have traditionally been studied in quantum mechanics and astrophysics, while receiving a very little attention in the photonics community. This situation has changed recently due to a number of pioneering theoretical studies and remarkable experimental demonstrations of the exotic states of light in dielectric resonant photonic structures and metasurfaces, with the possibility to localize efficiently the electromagnetic fields of high intensities within small volumes of matter. These recent advances underpin novel concepts in nanophotonics, and provide a promising pathway to overcome the problem of losses usually associated with metals and plasmonic materials for the efficient control of the light-matter interaction at the nanoscale. This review paper provides the general background and several snapshots of the recent results in this young yet prominent research field, focusing on two types of nonradiating states of light that both have been recently at the center of many studies in all-dielectric resonant meta-optics and metasurfaces: optical {\em anapoles} and photonic {\em bound states in the continuum}. We discuss a brief history of these states in optics, their underlying physics and manifestations, and also emphasize their differences and similarities. We also review some applications of such novel photonic states in both linear and nonlinear optics for the nanoscale field enhancement, a design of novel dielectric structures with high-$Q$ resonances, nonlinear wave mixing and enhanced harmonic generation, as well as advanced concepts for lasing and optical neural networks., Comment: 22 pages, 9 figures, review article

Metasurfaces based on resonant subwavelength photonic structures enable novel ways of wavefront control and light focusing, underpinning a new generation of flat-optics devices. Recently emerged all-dielectric metasurfaces exhibit high-quality resonances underpinned by the physics of bound states in the continuum that drives many interesting concepts in photonics. Here we suggest a novel approach to explain the physics of bound photonic states embedded into the radiation continuum. We study dielectric metasurfaces composed of planar periodic arrays of Mie-resonant nanoparticles ("meta-atoms") which support both symmetry protected and accidental bound states in the continuum and employ the multipole decomposition approach to reveal the physical mechanism of the formation of such nonradiating states in terms of multipolar modes generated by isolated meta-atoms. Based on the symmetry of the vector spherical harmonics, we identify the conditions for the existence of bound states in the continuum originating from the symmetries of both the lattice and the unit cell. Using this formalism we predict that metasurfaces with strongly suppressed spatial dispersion can support the bound states in the continuum with the wavevectors forming a line in the reciprocal space. Our results provide a new way of designing high-quality resonant photonic systems based on the physics of bound states in the continuum., Comment: 13 pages, 7 figures, 2 tables

Auxiliary nanostructures introduce additional flexibility into optomechanical manipulation schemes. Metamaterials and metasurfaces capable to control electromagnetic interactions at the near-field regions are especially beneficial for achieving improved spatial localization of particles, reducing laser powers required for trapping, and for tailoring directivity of optical forces. Here, optical forces acting on small particles situated next to anisotropic substrates, are investigated. A special class of hyperbolic metasurfaces is considered in details and is shown to be beneficial for achieving strong optical pulling forces in a broad spectral range. Spectral decomposition of the Green functions enables identifying contributions of different interaction channels and underlines the importance of the hyperbolic dispersion regime, which plays the key role in optomechanical interactions. Homogenised model of the hyperbolic metasurface is compared to its metal-dielectric multilayer realizations and is shown to predict the optomechanical behaviour under certain conditions related to composition of the top layer of the structure and its periodicity. Optomechanical metasurfaces open a venue for future fundamental investigations and a range of practical applications, where accurate control over mechanical motion of small objects is required.

We discuss the recent advances in meta-optics and nanophotonics associated with the physics of bound states in the continuum (BICs). Such resonant states appear due to a strong coupling between leaky modes in optical guiding structures being supported by subwavelength high-index dielectric Mie-resonant nanoantennas or all-dielectric metasurfaces. First, we review briefly very recent developments in the BIC physics in application to isolated subwavelength particles. We pay a special attention to novel opportunities for nonlinear nanophotonics due to the large field enhancement inside the particle volume creating the resonant states with high-quality (high-$Q$) factors, the so-called quasi-BIC, that can be supported by the subwavelength particles. Second, we discuss novel applications of the BIC physics to all-dielectric optical metasurfaces with broken-symmetry meta-atoms when tuning to the BIC conditions allows to enhance substantially the $Q$ factor of the flat-optics dielectric structures. We also present the original results on nonlinear high-$Q$ metasurfaces and predict that the frequency conversion efficiency can be boosted dramatically by smart engineering of the asymmetry parameter of dielectric metasurfaces in the vicinity of the quasi-BIC regime., Comment: 8 pages, 4 figures

Hyperbolic metasurfaces have gained significant attention due to their extraordinary electromagnetic properties to control propagating plane waves, but the excitation and propagation of the surface plasmon-polaritons at hyperbolic metasurfaces, called hyperbolic plasmons, have been experimentally observed just recently. However, the advantages of the hyperbolic plasmons, such as hybrid TE-TM polarization discussed below, are not yet fully revealed and analyzed. In this paper we focus on the numerical and experimental characterization of surface waves in the frequency range from 2 to 8 GHz supported by a hyperbolic metasurface composed of anisotropic metallic Jerusalem crosses printed on a thin dielectric substrate. We show different shapes of equal frequency contours, which correspond to a plethora of excitation and propagation regimes of surface waves. The principal novelty of this work consists in the experimental demonstration of the surface waves with a hybrid, i.e. mixed TE-TM, polarization. Surface waves with a hybrid polarization are the promising tool in a number of applications and phenomena including polarization converters, plasmonic sensors, plasmon steering over a surface, optical forces, spin-orbit photonics, and highlight the impact for the on-chip and planar networks., Comment: 8 pages, 6 figures

We reveal that metasurfaces created by seemingly different lattices of (dielectric or metallic) meta-atoms with broken in-plane symmetry can support sharp high-$Q$ resonances that originate from the physics of bound states in the continuum. We prove rigorously a direct link between the bound states in the continuum and the Fano resonances, and develop a general theory of such metasurfaces, suggesting the way for smart engineering of resonances for many applications in nanophotonics and meta-optics., Comment: 6 pages, 4 figures, Supplementary Information

We propose a scheme to trap and filter electrons, valley dependently, on a scale beyond the diffraction limit, in a gapped Dirac system using a circularly polarized light beam and a microscale metallic resonator. The main mechanism allowing the selection is the valley dependent break of the time reversal symmetry. Indeed in one valley, the light partially closes the already existing gap while it opens it in the other one. This difference in the band structure close to the Dirac points, induces a change in the dynamics of the electrons, leading to a valley router behaviour of the system., Comment: 5 pages, 3 figures

Metasurfaces offer great potential to control near- and far-fields through engineering of optical properties of elementary cells or meta-atoms. Such perspective opens a route to efficient manipulation of the optical signals both at nanoscale and in photonics applications. In this paper we show that by using an effective surface conductivity tensor it is possible to unambigiously describe optical properties of an anisotropic metasurface in the far- and near-field regimes. We begin with retrieving the effective surface conductivity tensor from the comparative analysis of experimental and numerical reflectance spectra of a metasurface composed of elliptical gold nanoparticles. Afterwards restored conductivities are validated in the crosscheck versus semianalytic parameters obtained with the discrete dipole model with and without dipoles interaction contribution. The obtained effective parameters are further used for the dispersion analysis of surface plasmons localized at the metasurface. The effective medium model predicts existence of both TE- and TM-polarized plasmons in a wide range of optical frequencies and describes peculiarities of their dispersion, in particularly, topological transition from the elliptical to hyperbolic regime with eligible accuracy. The analysis in question offers a simple practical way to describe properties of metasurfaces including ones in the near-field zone by extracting effective parameters from the convenient far-field characterisation., Comment: 29 pages, 9 figures

Optical binding allows creation of mechanically stable nanoparticle configurations owing to formation of self-consistent optical trapping potentials. While the classical diffraction limit prevents achieving deeply subwavelength arrangements, auxiliary nanostructures enable tailoring optical forces via additional interaction channels. Here, a dimer configuration next to metal surface was analyzed in details and the contribution of surface plasmon polariton waves was found to govern the interaction dynamics. It was shown that the interaction channel, mediated by resonant surface waves, enables achieving subwavelength stable dimers. Furthermore, the vectorial structure of surface modes allows binding between two dipole nanoparticles along the direction of their dipole moments, contrary to vacuum binding, where a stable configuration is formed in the direction oriented perpendicularly to the polarization of dipole moments. In addition, the enhancement of optical binding stiffness for one order of magnitude was predicted owing to the surface plasmon polariton interaction channel. These phenomena pave a way for developing new flexible optical manipulators, allowing for the control over a nanoparticle trajectory on subwavelength scales and opens a room of opportunities for optical-induced anisotropic, i.e. with different periods along the field polarization as well as perpendicular to it, organization of particles on a plasmonic substrate.

Recent progress in nanoscale optical physics is associated with the development of a new branch of nanophotonics exploring strong Mie resonances in dielectric nanoparticles with high refractive index. The high-index resonant dielectric nanostructures form building blocks for novel photonic metadevices with low losses and advanced functionalities. However, unlike extensively studied cavities in photonic crystals, such dielectric resonators demonstrate low quality factors (Q-factors). Here, we uncover a novel mechanism for achieving giant Q-factors of subwavelength nanoscale resonators by realizing the regime of bound states in the continuum. We reveal strong mode coupling and Fano resonances in high-index dielectric finite-length nanorods resulting in high-Q factors at the nanoscale. Thus, high-index dielectric resonators represent the simplest example of nanophotonic supercavities, expanding substantially the range of applications of all-dielectric resonant nanophotonics and meta-optics., Comment: 5 pages, 4 figures

Nanoantennas for highly efficient excitation and manipulation of surface waves at nanoscale are key elements of compact photonic circuits. However, previously implemented designs employ plasmonic nanoantennas with high Ohmic losses, relatively low spectral resolution, and complicated lithographically made architectures. Here we propose an ultracompact and simple dielectric nanoantenna (silicon nanosphere) allowing for both directional launching of surface plasmon polaritons on a thin gold film and their demultiplexing with a high spectral resolution. We show experimentally that mutual interference of magnetic and electric dipole moments supported by the dielectric nanoantenna results in opposite propagation of the excited surface waves whose wavelengths differ by less than 50 nm in the optical range. Broadband reconfigurability of the nanoantennas operational range is achieved simply by varying the diameter of the silicon sphere. Moreover, despite subwavelength size ($<\lambda/3$) of the proposed nanoantennas, they demonstrate highly efficient and directional launching of surface waves both in the forward and backward directions with the measured front-to-back ratio having a contrast of almost two orders of magnitude within a 50 nm spectral band. Our lithography-free design has great potential as highly efficient, low-cost, and ultracompact demultiplexer for advanced photonic circuits., Comment: added relevant references; fixed typos in Supplementary eq. 8,9,11