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Symmetry breaking plays a crucial role in understanding the fundamental physics underlying numerous physical phenomena, including the electromagnetic response in resonators, giving rise to intriguing effects such as directional light scattering, supercavity lasing, and topologically protected states. This work demonstrates that adding a small fraction of lossy metal (as low as 1 × 10−6 in volume) to a lossless dielectric resonator breaks inversion symmetry (IS), thereby lifting its degeneracy, leading to a strong bianisotropic response. In the case of the metasurface composed of such resonators, this effect leads to unidirectional perfect absorption while maintaining nearly perfect reflection from the opposite direction. It has developed more general Onsager‐Casimir relations for the polarizabilities of particle arrays, taking into account the contributions of quadrupoles, which shows that bianisotropy is not solely due to dipoles, but also involves high‐order multipoles. The experimental validation demonstrates an extremely thin terahertz‐perfect absorber with a wavelength‐to‐thickness ratio of up to 25,000, where the material thickness is only 2% of the theoretical minimum thickness dictated by the fundamental limit. The findings can pave a new route to design devices for applications involving optical‐to‐heat conversion processes. [ABSTRACT FROM AUTHOR]

Simultaneous scattering invisibility and free-space field enhancement have been achieved based on multipolar interferences among all-dielectric nanoparticles. The scattering properties of all-dielectric nanowire quadrumers are investigated and two sorts of scattering invisibilities have been identified: the trivial invisibility where the individual nanowires are not effectively excited; and the nontrivial invisibility with strong multipolar excitations within each nanowire, which results in free-space field enhancement outside the particles. It is revealed that such nontrivial invisibility originates from not only the simultaneous excitations of both electric and magnetic resonances, but also their significant magnetoelectric cross-interactions. We further show that the invisibility obtained is both polarization and direction selective, which can probably play a significant role in various applications including non-invasive detection, sensing, and non-disturbing medical diagnosis with high sensitivity and precision. [ABSTRACT FROM AUTHOR]

For an isotropic quantum resonant scatterer, such as a quantum dot embedded in host semiconductors, we propose a method to achieve resonant electron scattering by taking physical quantities into account through the second-order expansion. All needed physical information for spherical harmonic channels in anomalous quantum resonant scattering is revealed in a parameter space as the size of quantum scatterer is comparable to the de Broglie wavelength of incoming matter wave. Our results provide the guideline to realize quantum resonant scatterers with state-of-the-art semiconductor heterostructure technology. [ABSTRACT FROM AUTHOR]

Rapid progress in nanophotonics is driven by the ability of optically resonant nanostructures to enhance near-field effects controlling far-field scattering through intermodal interference. A majority of such effects are usually associated with plasmonic nanostructures. Recently, a new branch of nanophotonics has emerged that seeks to manipulate the strong, optically induced electric and magnetic Mie resonances in dielectric nanoparticles with high refractive index. In the design of optical nanoantennas and metasurfaces, dielectric nanoparticles offer the opportunity for reducing dissipative losses and achieving large resonant enhancement of both electric and magnetic fields. We review this rapidly developing field and demonstrate that the magnetic response of dielectric nanostructures can lead to novel physical effects and applications. [ABSTRACT FROM AUTHOR]

A spaser is a nanoplasmonic counterpart of a laser, with photons replaced by surface plasmon polaritons and a resonant cavity replaced by a metallic nanostructure supporting localized plasmonic modes. By combining analytical results and first-principle numerical simulations, we provide a comprehensive study of the ultrafast dynamics of a spaser. Due to its highly-nonlinear nature, the spaser is characterized by a large number of interacting degrees of freedom, which sustain a rich manifold of different phases we discover, describe and analyze here. In the regime of strong interaction, the system manifests an irreversible ergodic evolution towards the configuration where energy is equally shared among all the available degrees of freedom. Under this condition, the spaser generates ultrafast vortex-like lasing modes that are spinning on the femtosecond scale and whose direction of rotation is dictated by quantum noise. In this regime, the spaser acquires the character of a nanoparticle with an effective spin. This opens up a range of interesting possibilities for achieving unidirectional emission from a symmetric nanostructure, stimulating a broad range of applications for nanoplasmonic lasers as unidirectional couplers and random information sources. [ABSTRACT FROM AUTHOR]

We suggest a novel strategy for spectrally selective optical shielding of arbitrary shaped volumes by arranging specifically designed two- or three-layer nanowires around an area that needs to be protected. We show that such nanowire shields preserve their functionality for almost arbitrary geometry, and we term such structures optical metacages. We analyze several designs of such optical metacages made from either metallic or dielectric materials with experimentally measured parameters. We employ a semianalytical approach and also verify our results by numerical simulations. We further study optical properties of the introduced metacages in both near- and far-field regions, as well as analyze their frequency selectivity and the vanishing backscattering regime. [ABSTRACT FROM AUTHOR]

Optical metasurfaces are thin-layer subwavelength-patterned structures that interact strongly with light. Metasurfaces have become the subject of several rapidly growing areas of research, being a logical extension of the field of metamaterials towards their practical applications. Metasurfaces demonstrate many useful properties of metadevices with engineered resonant electric and magnetic optical responses combined with low losses of thin-layer structures. Here we introduce the basic concepts of this rapidly growing research field that stem from earlier studies of frequency-selective surfaces in radiophysics, being enriched by the recent development of metamaterials and subwavelength nanophotonics. We review the most interesting properties of photonic metasurfaces, demonstrating their useful functionalities such as frequency selectivity, wavefront shaping, polarization control, etc. We discuss the ways to achieve tunability of metasurfaces and also demonstrate that nonlinear effects can be enhanced with the help of metasurface engineering. [ABSTRACT FROM AUTHOR]

We designed a metasurface made of a monolayer of spherical nanoparticles embedded in a dielectric slab, which exhibits transmission properties independent of the incidence angle. Adjusting the electromagnetic coupling between high-index dielectric and hybrid core-shell nanoparticles enables the metasurface to operate in low-pass, bandpass, as well as band-stop regimes in the visible and near-infrared spectral ranges. We demonstrate how symmetric properties of spherical nanoparticles determine the response of the metasurface, resulting in a spectral filter with a wide angular acceptance range. We study transmission characteristics of the metasurface, such as frequency selectivity, the slope of filtering at cutoff frequencies, and the robustness of the metasurface against experimental variations in geometrical parameters. Our analyses show that the proposed approach can be used to design angular-independent spectral filters with the same material platform and approach to operate in different regimes and spectral ranges. [ABSTRACT FROM AUTHOR]

We analyse scattering of light from multi-layer plasmonic nanowires and employ a genetic algorithm for optimizing the scattering cross section. We apply the mode-expansion method using experimental data for material parameters to demonstrate that our genetic algorithm allows designing realistic core-shell nanostructures with the superscattering effect achieved at any desired wavelength. This approach can be employed for optimizing both superscattering and cloaking at different wavelengths in the visible spectral range. [ABSTRACT FROM AUTHOR]

Abstract: Various generalized continuum models are developed to analyze the one-dimensional shear deformations of a Cosserat lattice. The original discrete model and the derived continuum models are applied to the analysis of long- and short-wavelength harmonic waves, localized waves, static boundary effects, and the spectrum stop bands in the wave filtering problem. It is found that the conventional single-field and gradient micropolar models are capable of describing the long-wavelength harmonic and localized waves, as well as the static non-staggered boundary effects. However, such models fail to correctly describe the short-wavelength waves, they cannot be used to find the spectrum stop bands and to study staggered boundary effects. On the other hand, the multi-field theory is able to tackle all these problems. [Copyright &y& Elsevier]

Conventional approaches to control and shape the scattering patterns of light generated by different nanostructures are mostly based on engineering of their electric response due to the fact that most metallic nanostructures support only electric resonances in the optical frequency range. Recently, fuelled by the fast development in the fields of metamaterials and plasmonics, artificial optically-induced magnetic responses have been demonstrated for various nanostructures. This kind of response can be employed to provide an extra degree of freedom for the efficient control and shaping of the scattering patterns of nanoparticles and nanoantennas. Here we review the recent progress in this research direction of nanoparticle scattering shaping and control through the interference of both electric and optically-induced magnetic responses. We discuss the magnetic resonances supported by various structures in different spectral regimes, and then summarize the original results on the scattering shaping involving both electric and magnetic responses, based on the interference of both spectrally separated (with different resonant wavelengths) and overlapped dipoles (with the same resonant wavelength), and also other higher-order modes. Finally, we discuss the scattering control utilizing Fano resonances associated with the magnetic responses. [ABSTRACT FROM AUTHOR]

We study the problem of the laser heating of plasmonic nanoparticles and demonstrate that, in sharp contrast to the common belief, a particle with a small dissipative constant absorbs much more energy than the particle with a large value of this constant. Even higher effective absorption may be achieved for core-shell nanoparticles. Our analysis uses the exact Mie solutions, and optimization of the input energy is performed at a fixed fluence with respect to the particle size, wavelength and duration of the laser pulse. We introduce a new quantity, the effective absorption coefficient of a particle, which allows one to compare quantitatively the light absorption by nanoparticles with that of a bulk material. We describe a range of parameters where a giant absorption enhancement can be observed and give practical examples of metals whose optical properties vary from weak (potassium) to strong (platinum) dissipation. [ABSTRACT FROM AUTHOR]

We introduce a novel concept of surface bound states in the continuum, i.e., surface modes embedded into the linear spectral band of a discrete lattice. We suggest an efficient method for creating such surface modes and the local bounded potential necessary to support the embedded modes. We demonstrate that the surface embedded modes are structurally stable, and the position of their eigenvalues inside the spectral band can be tuned continuously by adding weak nonlinearity. [ABSTRACT FROM AUTHOR]

We report on the influence of backward propagating waves on the twist optical Freedericksz transition when a linearly polarised light impinges at normal incidence on a nematic liquid crystal film with planar alignment. We show that the reorientation threshold oscillates as a function of the optical thickness of the nematic layer. The amplitude of these oscillations strongly depends on the refractive index changes at the film boundaries and is shown to be related to interferential effects between forward and backward propagating waves that arise from unavoidable dielectric permittivity tensor mismatch and the film boundaries. [ABSTRACT FROM AUTHOR]

We consider a class of PT-symmetric systems which include mutually matched nonlinear loss and gain (in other words, a class of PT-invariant Hamiltonians in which both the harmonic and anharmonic parts are non-Hermitian). For a basic system in the form of a dimer, symmetric and asymmetric eigenstates, including multistable ones, are found analytically. We demonstrate that, if coupled to a linear chain, such a nonlinear PT-symmetric dimer generates previously unexplored types of nonlinear Fano resonances, with completely suppressed or greatly amplified transmission, as well as a regime similar to the electromagnetically induced transparency. The implementation of the systems is possible in various media admitting controllable linear and nonlinear amplification of waves. [ABSTRACT FROM AUTHOR]

We consider a model utilizing the concept of impedance matching, which can be applied to design the coupled cascaded plasmonic cavity waveguide with desired properties. We use a transfer matrix method to obtain its transmission and dispersion diagrams. Base on this method, we demonstrate that a band-pass metal-dielectric-metal plasmonic filter with quasi-flat group velocity and tunable bandwidth can be achieved. [ABSTRACT FROM AUTHOR]

Abstract: The static and dynamic properties of a Cosserat-type lattice interface of finite thickness are studied, so that both displacements and rotational degrees of freedom are taken into account. The model allows considering interfaces with a beam-like microstructure and interfaces with finite size particles as particular cases. One-dimensional solutions describing shear and micro-rotations at the interface are obtained and discussed. Harmonic as well as localized solutions, and the properties of the interfaces as filters for elastic waves are investigated. It is shown that both systems with long- and short-wavelength localization may exist. [Copyright &y& Elsevier]

Abstract: We study the dynamical scattering in one-dimensional systems with a nonlinear side-coupled defect. Such structures exhibit the nonlinear Fano resonances, where nothing can propagate through. We developed a numerical model to study dynamical scattering. According to our analysis the scattering waves become dynamically unstable in the vicinity of the nonlinear Fano resonances, due to modulational instability caused by the presence of nonlinearity. It results in a time-growing amplitude of the nonlinear defect. We also demonstrate the existence of the nonlinear quasi-localized state, supported by such structures. [Copyright &y& Elsevier]

This study presents the polarization photodetection enhancement in Sb2Se3 nanotube (NT)‐based near‐infrared (NIR) photodetectors through simulation‐based and experimental investigations. High‐quality single‐crystal Sb2Se3 NTs are grown via chemical vapor deposition and characterized by using multiple techniques. The optical simulation reveals a remarkable difference in the light absorption ratio (specifically, absorption along the NT/nanowire (NW) against absorption perpendicular to the NT/NW) between Sb2Se3 NT and NW of the same size in the NIR region. The complementary photodetection experiments present that the fabricated Sb2Se3 NT photodetector demonstrates enhanced polarization photodetection in the NIR range, as indicated by a significantly increased dichroic ratio (3.03 at 850 nm) compared to that of similar‐sized NW counterpart (1.81 at 850 nm). Additionally, the Sb2Se3 NT photodetector exhibits exceptional performance, with a high responsivity of 4.18 A W−1 and specific detectivity of 8.94 × 1010 Jones under 830 nm light illumination. This study provides a comprehensive understanding of the microcavity resonance effect and its role in polarization photodetection enhancement, highlighting the potential of self‐assembled Sb2Se3 NTs in high‐performance near‐infrared polarized photodetection and other relevant applications. [ABSTRACT FROM AUTHOR]

Gas sensing is essential for detecting and measuring gas concentrations across various environments, with applications in environmental monitoring, industrial safety, and healthcare. The integration of two-dimensional (2D) materials, organic materials, and metal oxides has significantly advanced gas sensor technology, enhancing its sensitivity, selectivity, and response times at room temperature. This review examines the progress in optically activated gas sensors, with emphasis on 2D materials, metal oxides, and organic materials, due to limited studies on their use in optically activated gas sensors, in contrast to other traditional gas-sensing technologies. We detail the unique properties of these materials and their impact on improving the figures of merit (FoMs) of gas sensors. Transition metal dichalcogenides (TMDCs), with their high surface-to-volume ratio and tunable band gap, show exceptional performance in gas detection, especially when activated by UV light. Graphene-based sensors also demonstrate high sensitivity and low detection limits, making them suitable for various applications. Although organic materials and hybrid structures, such as metal–organic frameworks (MoFs) and conducting polymers, face challenges related to stability and sensitivity at room temperature, they hold potential for future advancements. Optically activated gas sensors incorporating metal oxides benefit from photoactive nanomaterials and UV irradiation, further enhancing their performance. This review highlights the potential of the advanced materials in developing the next generation of gas sensors, addressing current research gaps and paving the way for future innovations. [ABSTRACT FROM AUTHOR]

Structural symmetry plays a pivotal role in the emergence of symmetry-protected bound states in the continuum (BICs), often observed at the Γ-point within the first Brillouin zone. However, structural symmetry is not an absolute requirement for the formation of BICs at the Γ-point. In this work, we demonstrate that all-dielectric metasurfaces and photonic crystal slabs, made of dimer nanostructures with different sizes and shapes, can sustain BICs at the Γ-point. We show that the nature of these BICs is well preserved, irrespective of the size mismatch/difference, as long as the center-to-center distance between two nanodisks is equal to half of the lattice constants of a superunit cell. The BICs are transformed into quasi-BICs (QBICs) with finite quality (Q) factors by varying the interspacing of dimer nanodisks. Multipole decomposition indicates that this BIC is primarily governed by a toroidal dipole, with a secondary contribution from a magnetic dipole and magnetic quadrupole. Furthermore, we establish that such a BIC is robust against the shape of nanodisks. Notably, we observe that the Q-factor of QBICs for right nanodisks displaced along the y-axis is three orders of magnitude higher than those along the x-axis, suggesting an effective approach to realizing ultrahigh-Q resonances. Finally, we present an experimental demonstration of such a BIC by fabricating silicon dimer metasurfaces and photonic crystal slabs with dimer nanoholes. The trend of measured Q-factors and resonant wavelengths of QBICs shows good agreement with theoretical predictions. The maximum Q-factor is up to 22 633. These results not only advance our understanding of BICs within compound metasurfaces but also hold great promise in enhancing light–matter interactions. [ABSTRACT FROM AUTHOR]

We demonstrate how to achieve reversible nonreciprocal optical response in a periodic photonic structure with a pair of defects, one of them being a nonlinear liquid crystal defect layer. The twin defect structure is symmetric at low intensity and becomes asymmetric above an intensity threshold that corresponds to the optical reordering of the liquid crystal. We show that nonreciprocal effects can be reversed by changing the wavelength as a consequence of the wavelength dependent light localization at the defect mode inside the structure. [ABSTRACT FROM AUTHOR]

We introduce a concept of the Mach–Zehnder–Fano interferometer by inserting a cavity exhibiting Fano resonance into a conventional interferometer. By employing the scattering-matrix approach, we demonstrate that the transmission becomes sensitive to a position of the cavity such that an asymmetric structure exhibits a series of narrow resonances with almost perfect reflection. We discuss how to implement this novel geometry in two-dimensional photonic crystals and use direct numerical simulations to demonstrate novel regimes of the resonant transmission and reflection. [ABSTRACT FROM AUTHOR]

We demonstrate that one-dimensional photonic crystals with pure nematic liquid-crystal defects can operate as all-optical switching devices based on optical orientational nonlinearities of liquid crystals. We show that such a periodic structure is responsible for a modulated threshold of the optical Fréedericksz transition in the spectral domain, and this leads to all-optical switching and light-induced multistability. This effect has no quasistatics electric field analog, and it results from nonlinear coupling between light and a defect mode. [ABSTRACT FROM AUTHOR]

We report on the experimental studies of the light-induced reorientational effects in a one-dimensional periodic photonic structure with an embedded planar nematic-liquid-crystal defect. We demonstrate that in the presence of a periodic structure, the self-action of light in a liquid-crystal layer demonstrates sharp power-dependent characteristics for the intensity-dependent optical transmission. Robustness of the effect suggests its applications for all-optical tunable photonic devices. [ABSTRACT FROM AUTHOR]

Harnessing the power of symmetry‐protected bound states in the continuum (SP BICs) has become a focal point in scientific exploration, promising many interesting applications in nanophotonics. However, the practical realization of ultrahigh quality (Q) factor quasi‐BICs (QBICs) is hindered by the fabrication imperfections. In this work, an easy approach is proposed to achieve ultrahigh‐Q resonances by strategically breaking symmetry. By introducing precise perturbations within the zero eigenfield region, QBICs with consistently ultrahigh‐Q factors, beyond conventional limitations are achieved. Intriguingly, intentionally disrupting symmetry in the maximum eigenfield region leads to a rapid decline in QBIC's Q‐factors as the asymmetry parameter increases. Leveraging this design strategy, ultrahigh‐Q modes with a high Q‐factor of 36,694 in a silicon photonic crystal slab are experimentally realized. The findings establish a robust and straightforward pathway toward unlocking the full potential of SP BICs, enhancing light‐matter interactions across diverse applications. [ABSTRACT FROM AUTHOR]

Based on fundamental properties of the light scattering by a particle under a plane, linearly polarized wave illumination, we rigorously prove the existence of the ultimate upper limit for the light absorption by any partial mode and calculate this limit explicitly. The limit is a certain simple universal function of the incident light wave number, and the multipolarity of the corresponding partial mode solely. It does not depend on the optical constants of the scatterer, its size, or even its shape. First, we obtain this result for the scattering by a spherical particle. Then, we generalize it to an arbitrary finite obstacle. The results are valid for any polarization of the incident wave, any angle of its incidence, and any type of the scatterer (homogeneous, stratified, or with smoothly variable refractive index). We also prove that the maximal partial absorption cross section for any finite scatterer cannot exceed the corresponding value for a homogeneous sphere in 3D and circular cylinder in 2D. As an example, the results are applied to maximize the absorption cross section of a spherical core-shell structure. [ABSTRACT FROM AUTHOR]

An exotic invisibility accompanied by significant accessible free‐space near‐field enhancement has been achieved with scattering systems made of coupled all‐dielectric nanowires. It is revealed that such nontrivial invisibility originates from not only efficient excitations of both electric and magnetic resonances, but also strong magneto‐electric cross‐interactions between them. (Picture: Wei Liu and Andrey E. Miroshnichenko, article number 1700103, in this issue) [ABSTRACT FROM AUTHOR]

We demonstrate that spectrally diverse multiple magnetic dipole resonances can be excited in all-dielectric structures lacking rotational symmetry, in contrast to conventionally used spheres, disks, or spheroids. Such multiple magnetic resonances arise from hybrid Mie-Fabry-Perot modes, and can constructively interfere with induced electric dipole moments, thereby leading to novel multifrequency unidirectional scattering. Here we focus on elongated dielectric nanobars, whose magnetic resonances can be spectrally tuned by their aspect ratios. Based on our theoretical results, we suggest all-dielectric multimode metasurfaces and verify them in proof-of-principle microwave experiments. We also believe that the demonstrated property of multimode directionality is largely responsible for the best efficiency of all-dielectric metasurfaces that were recently shown to operate across multiple telecom bands. [ABSTRACT FROM AUTHOR]

The article discusses reports within the issue by researcher J. Lin and colleagues and F. J. Rodriguez-Fortuño and team on the polarization properties of surface plasmon polaritons under light excitation. Lin's team looked at the generation of surface plasmons by electric dipole apertures on an illuminated gold film metasurface, noting the relationship between surface plasmon interference and the direction or handedness of linear or circular polarized light sources. Rodriguez-Fortuño and colleagues studied near-field interference in the propagation of surface plasmonic wavefields by a circular polarized light source.

Optically resonant dielectric metasurfaces offer unique capability to fully control the wavefront, polarization, intensity, or spectral content of light based on the excitation and interference of different electric and magnetic Mie multipolar resonances. Recent advances of the wide accessibility in nanofabrication and nanotechnologies have led to a surge in the research field of high-quality functional optical metasurfaces, which can potentially replace or even outperform conventional optical components with ultra-thin features. Replacing conventional optical filtering components with metasurface technology offers remarkable advantages, including lower integration cost, ultra-thin compact configuration, easy combination with multiple functions, and less restriction on materials. Here, we propose and experimentally demonstrate a planar narrow bandpass filter based on the optical dielectric metasurface composed of Si nanoresonators in arrays. A broadband transmission spectral valley (around 200 nm) has been realized by combining electric and magnetic dipole resonances adjacent to each other. Meanwhile, we obtain a narrow-band transmission peak by exciting a high-quality leaky mode, which is formed by partially breaking a bound state in the continuum generated by the collective longitudinal magnetic dipole resonances in the metasurface. Owing to the in-plane inversion symmetry of our nanostructure, the radiation of this antisymmetric mode is inhibited at far field, manifesting itself a sharp Fano-shape peak in the spectrum. Our proposed metasurface-based filter shows a stable performance for oblique light incidence with small angles (within 10°). Our work implies many potential applications of nanoscale photonics devices, such as displays, spectroscopy, etc. [ABSTRACT FROM AUTHOR]

It is proposed and demonstrated that the Q factor of optical resonators can be significantly enhanced by introducing an extra anisotropic cladding. We study the optical resonances of all-dielectric core-shell nanoresonators and show that radially anisotropic claddings can be employed to squeeze more energy into the core area, leading to stronger light confinement and thus significant Q-factor enhancement. We further demonstrate that the required homogenous claddings of unusual anisotropy parameters can be realized through all-dielectric multilayered isotropic structures. It is expected that the mechanism we have revealed not only offers extra flexibilities of resonance manipulations for conventional dielectric structures, but also may shed new light onto investigations into unconventional nanostructures consisting of two-dimensional materials that are intrinsically highly anisotropic. [ABSTRACT FROM AUTHOR]

We introduce an important approach for enhancing the nonlinear response of graphene through its resonant coupling to a plasmonic metasurface via cascaded Fano resonances. Such a hybrid metasurface supports two types of subradiant resonant modes, i.e., asymmetric modes of structured metamaterial elements ("metamolecules") and graphene plasmons exhibiting strong mutual coupling and avoided dispersion crossing. We demonstrate that the tunability of graphene plasmons facilitates the strong interaction between the subradiant modes, modifying the spectral position and lifetime of the Fano resonances. We reveal that a strong resonant interaction, combined with the subwavelength localization of plasmons, leads to an enhanced nonlinear response and high efficiency of the second-harmonic generation. [ABSTRACT FROM AUTHOR]

We study optical response of all-dielectric multilayer structures and demonstrate that the total scattering of such structures can be suppressed leading to optimal invisibility cloaking. We use experimental material data and a genetic algorithm to reduce the total scattering by adjusting the material and thickness of various layers for several types of dielectric cores at telecommunication wavelengths. Our approach demonstrates 80-fold suppression of the total scattering cross-section by employing just a few dielectric layers. [ABSTRACT FROM AUTHOR]

All-dielectric metasurfaces provide a powerful platform for a new generation of flat optical devices, in particular, for applications in telecommunication systems, due to their low losses and high transparency in the infrared. However, active and reversible tuning of such metasurfaces remains a challenge. This study experimentally demonstrates and theoretically justifies a novel scenario of the dynamical reversible tuning of all-dielectric metasurfaces based on the temperature-dependent change of the refractive index of silicon. How to design an all-dielectric metasurface with sharp resonances by achieving interference between magnetic dipole and electric quadrupole modes of constituted nanoparticles arranged in a 2D lattice is shown. Thermal tuning of these resonances can cause drastic but reciprocal changes in the directional scattering of the metasurface in a spectral window of 75 nm. This change can result in a 50-fold enhancement of the radiation directionality. This type of reversible tuning can play a significant role in novel flat optical devices including the metalenses and metaholograms. [ABSTRACT FROM AUTHOR]

All-dielectric metamaterials and metasurfaces have been demonstrated as ideal platforms to manipulate electromagnetic waves and enhance light–matter interaction. Analogous to guided modes in a dielectric waveguide, high refractive index dielectric metasurface supports leaky modes (also known as quasi-normal modes), including electric and magnetic resonant modes, which are represented by a complex eigenfrequency. Such leaky modes play a dominant role in governing the optical properties of dielectric nanoparticles and metasurfaces (i.e., absorption, scattering and emission). These unique properties enable researchers to design dielectric metasurfaces with desired functionalities without resorting to computationally expensive parameter scanning with full-wave simulation. Moreover, leaky modes underpin many exciting topics, such as Fano resonances, bound states in the continuum (leaky modes with Q=+ ∞), coherent perfect absorption, parity-time symmetry, and exceptional points. This review provides an overview of the recent progress on leaky modes of all-dielectric-based metasystem, from fundamental physics to their applications. We start with surveying the fundamental physics of leaky modes and discussing leaky mode engineering in either a passive or active way. Then, we discuss the recent progress on the leaky modes' application in the perfect light absorber, solar cells, photodetectors, enhanced light emission and lasing, strong coupling, enhanced nonlinear harmonic generation, and structural color. Next, we present an overview of recent advances in bound states in the continuum and their application in enhanced light–matter​ interactions. Also, we review other interesting topics: parity-time symmetry and exceptional points, and discuss their application in manipulating light in an unprecedented way. Finally, we present our vision of the challenges and opportunities in this rapidly developing field of research. [ABSTRACT FROM AUTHOR]

It is well established that for symmetry‐protected bound states in the continuum (BICs), introducing the broken geometry symmetry in a dielectric metasurface transforms such a BIC into a quasi‐BIC (QBIC) with high‐quality factor (Q‐factor). Typically, the smaller the asymmetry parameter, the larger the Q‐factor. However, it is very challenging to fabricate such nanostructures with an ultra‐small asymmetry parameter, thus limiting the measured Q‐factor of QBIC. In this work, the authors demonstrated that BICs can be sustained at Γ‐point in an asymmetric dielectric metasurface, whose unit cell is composed of a dielectric cuboid with an off‐centre hole inside it. Multipole decompositions and near‐field distributions indicate that the toroidal dipole dominates the nature of such a QBIC. Furthermore, the authors found that such a BIC is robust against the shape of the hole. Besides, two BICs at different wavelengths can be achieved by applying either a rectangular hole or a rectangular lattice. Finally, the authors presented experimental verifications of BIC types by fabricating asymmetric silicon metasurfaces. Measurement results show that the Q‐factor of QBIC can reach almost 5,000. The results may enrich the library of BICs and find exciting applications in developing high‐performance photonics devices, such as nanolasers, biosensors and enhanced nonlinear harmonic generation. [ABSTRACT FROM AUTHOR]

We study the scattering properties of arrays of core-shell nanoparticles and reveal the existence of polarization-independent Fano resonances. Such Fano resonances occur through the interaction of a pair of degenerate orthogonal electric and artificial magnetic modes of the same strength with the spectrally narrow geometric resonance due to the diffractive coupling in the array. Furthermore, we show that for different polarizations of the incident plane waves, the electric and magnetic modes supported by each nanoparticle can be selectively controlled, providing extra freedom for near-field manipulation, with applications to nonlinear and lasing devices. [ABSTRACT FROM AUTHOR]

We present a general theory of circular dichroism in planar chiral nanostructures with rotational symmetry. It is demonstrated, analytically, that the handedness of the incident field's polarization can control whether a nanostructure induces either absorption or scattering losses, even when the total optical loss (extinction) is polarization-independent. We show that this effect is a consequence of modal interference so that strong circular dichroism in absorption and scattering can be engineered by combining Fano resonances with planar chiral nanoparticle clusters. [ABSTRACT FROM AUTHOR]

The ability to control heat flux at the nanoscale opens up numerous exciting possibilities in modern electronics and the field of information processing. In this research, we propose a design with the focus on achieving efficient thermal rectification at moderate gap and relatively low temperature. This study centers on near-field thermal radiation between temperature dependent indium antimonide (InSb) and silicon carbide (3C-SiC) coated with bismuth selenide (Bi 2 Se 3). Our investigation sheds light on the critical role played by the Bi 2 Se 3 layer in enhancing various key parameters, including the net radiative flux, and thermal rectification efficiency (η). We achieved a substantial improvement in the η of a near-field radiative thermal rectifier (NFRTR) due to the presence of the Bi 2 Se 3 sheet. This enhancement is contingent on factors such as the Fermi energy (E f) of Bi 2 Se 3 , emitter temperature, and the vacuum gap (d). Our study culminated in the identification of an optimal design, achieving an impressive η of 75% at an emitter temperature (T H) of 350 K, with vacuum gap (d) set to 20 nm. Furthermore, increasing T H to 500 K resulted in even more promising outcomes, with the highest η reaching 93%. The need for operating the optimized device at moderate temperatures is to strike a balance between efficiency, safety, cost-effectiveness, and material compatibility. These findings represent a significant step forward in the development of efficient Bi 2 Se 3 -based NFRTRs, paving the way for future applications in thermal management, energy conversion systems, and thermal logic gates. [Display omitted] • The use of materials with tailored EM response is important for the design of an efficient thermal rectifier. • The optical parameters of the materials constituting a thermal rectifier should be temperature dependent: an important prerequisite. • The need to explore new materials that can function as efficient thermal rectifiers with moderate gap and low temperature is a subject of active research. • We introduce a matrix method for deriving SPP relationship for a multilayer systems • We proposed the use of bismuth selenide (Bi 2 Se 3) by leveraging on its thermal tunability which is a key factor in mediating thermal radiation. • Combining Bi 2 Se 3 with temperature dependent materials leads to an efficient thermal rectifier. [ABSTRACT FROM AUTHOR]

Spin Hall (SH) shift of light from nanoparticles refers to the transverse displacement of scattered light due to spin–orbit interaction (SOI). Generally, the SH shift is small and suffers from inherent low scattering efficiency. In this paper, the SH shift and its corresponding scattering intensity are simultaneously enhanced for dual nanoparticles. Based on the generalized Lorentz‐Mie theory, the morphology of the nanoparticle is optimized to increase the scattering intensity while preserving the wavelength‐scale SH shift. The simultaneous enhancements are realized by manipulating the electric and magnetic dipolar modes to overlap at their resonances rather than the tails of the scattering. Meanwhile, a new pattern is identified on orbital momentum streamlines, revealing the near field's SOI. In addition, an experiment is proposed to detect the SH shifts in the far‐field observation plane. The scattering intensities from oblate spheroids are enhanced by one order of magnitude compared to the spherical counterparts. The findings may shed new light on the mechanism of light–matter interaction and facilitate the experimental detection of the SH effect of light. [ABSTRACT FROM AUTHOR]

The ultrafast response and broadband absorption of all-optical graphene switches are highly desirable features for on-chip photonic switching. However, because graphene is an atomically thin material, its absorption of guided optical modes is relatively low, resulting in high saturation thresholds and switching energies for these devices. To boost the absorption of graphene, we present a practical design of an electrically-biased all-optical graphene switch that is integrated into silicon slot waveguides, which strongly confine the optical mode in the slotted region and enhance its interaction with graphene. Moreover, the design incorporates a silicon slab layer and a hafnia dielectric layer to electrically tune the saturation threshold and the switching energy of the device by applying direct-current voltages of < 0.5 V. Using this device, a high extinction ratio of 10.3 dB, a low insertion loss of < 0.7 dB, and an ultra-efficient switching energy of 79 fJ/bit at 0.23 V bias are attainable for a 40 µ m long switch. The reported performance metrics for this device are highly promising and are expected to serve the needs of next-generation photonic computing systems. [ABSTRACT FROM AUTHOR]

The scattering of nanoparticles plays a profound role in the recently flourishing fields of plasmonics and metamaterials. However, current investigations into nanoparticle scattering are based on the electric and magnetic resonances only, where their toroidal counterparts are usually not considered. The inclusion of toroidal terms can render new explanations for some fundamental scattering properties and thus may stimulate further breakthroughs in both scattering-related basic researches and applications. Here we revisit the most fundamental problem of Mie scattering by individual spherical nanoparticles and show that compared to conventional interpretations in terms of electric and magnetic responses, the roles played by their toroidal counterparts are indispensable. Based on the demonstration of efficient toroidal dipole excitation in homogeneous dielectric particles, we reveal that the extensively studied scattering transparencies of core-shell nanoparticles can actually be classified into two categories: (i) the trivial transparency with no effective multipole excitations and (ii) the non-trivial transparency induced by the destructive interferences of excited electric and toroidal multipoles. The incorporation of toroidal multipoles offers new insights into the study of nanoparticle scattering in both near and far fields, which may shed new light on many applications, such as biosensing, imaging, nanoantennas, photovoltaic devices, and so on. [ABSTRACT FROM AUTHOR]

We suggest a novel type of photonic topological edge states in zigzag arrays of dielectric nanoparticles based on optically induced magnetic Mie resonances. We verify our general concept by the proof-ofprinciple microwave experiments with dielectric spherical particles, and demonstrate, experimentally, the ability to control the subwavelength topologically protected electromagnetic edge modes by changing the polarization of the incident wave. [ABSTRACT FROM AUTHOR]

Acoustic resonant cavities play a vital role in modern acoustical systems. The ultrahigh quality‐factor resonances are highly desired for some applications such as high‐resolution acoustic sensors and acoustic lasers. Here, a class of supercavity resonances is theoretically proposed and experimentally demonstrated in a coupled acoustic resonator system, arising from the merged bound states in the continuum (BICs) in geometry space. Their topological origin is demonstrated by explicitly calculating their topological charges before and after BIC merging, accompanied by charges annihilation. Compared with other types of BICs, they are robust to the perturbation brought by fabrication imperfection. Moreover, it is found that such supercavity modes can be linked with the Friedrich–Wintgen BICs supported by an entire rectangular (cuboid) resonator sandwiched between two rectangular (or circular) waveguides and thus more supercavity modes are constructed. Then, these coupled resonators are fabricated and such a unique phenomenon—moving, merging, and vanishing of BICs—is experimentally confirmed by measuring their reflection spectra, which show good agreement with the numerical simulation and theoretical prediction of mode evolution. The results may find exciting applications in acoustic and photonics, such as enhanced acoustic emission, filtering, and sensing. [ABSTRACT FROM AUTHOR]