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We introduce a novel physical mechanism for achieving giant quality factors ($Q$-factors) in finite-length periodic arrays of subwavelength optical resonators. The underlying physics is based on interference between the band-edge mode and another standing mode in the array, and the formation of spatially localized states with dramatically suppressed radiative losses. We demonstrate this concept for an array of $N$ dipoles with simultaneous cancellation of multipoles up to $N$-th order and the $Q$ factor growing as $Q \propto N^{\alpha}$, where $\alpha \gtrsim 6.88$. Based on this finding, we propose a realistic array of Mie-resonant nanoparticles ($N \lesssim 29$) with a dramatic enhancement of the Purcell factor (up to $\sim $3400) achieved by tuning of the array parameters.

Multiphoton absorption and luminescence are fundamentally important nonlinear processes for utilizing efficient light-matter interaction. Resonant enhancement of nonlinear processes has been demonstrated for many nanostructures; however, it is believed that all higher-order processes are always much weaker than their corresponding linear processes. Here, we study multiphoton luminescence from structured surfaces and, combining multiple advantages of perovskites with the concept of metasurfaces, we demonstrate that the efficiency of nonlinear multiphoton processes can become comparable to the efficiency of the linear process. We reveal that the perovskite metasurface can enhance substantially two-photon stimulated emission with the threshold being comparable with that of the one-photon process. Our modeling of free-carrier dynamics and exciton recombination upon nonlinear photoexcitation uncovers that this effect can be attributed to the local field enhancement in structured media, a substantial increase of the mode overlap, and the selection rules of two-photon absorption in perovskites.

Historically, nanophotonics deals with a control of light at the nanoscale being closely connected with the rapid advances in plasmonics – the physics of surface plasmon polaritons supported by metal–dielectric interfaces. Properly engineered nanostructures allow the subwavelength propagation of light and its strong confinement in nanowaveguides and nanocavities, making possible the field enhancement and lasing. Spaser was suggested as a special type of nanolaser with a very small footprint that can be modulated quickly thus becoming a good candidate for on-chip optical data processing. However, recent developments in the physics of high-index dielectric nanoparticles and resonant dielectric metasurfaces allowed to advance the field of nanophotonics and introduce novel nonplasmonic nanostructures and nanolasers empowered by topology and interference effects. Here we present first some examples of experimentally realized spasers, and then discuss the recent developments in the cutting-edge high-index dielectric nanostructures employed for nonplasmonic nanolasers based on Mie resonances, anapole states, bound states in the continuum, and the physics of topological phases.

We study active dielectric metasurfaces composed of two-dimensional arrays of split-nanodisk resonators fabricated in InGaAsP membranes with embedded quantum wells. Depending on the geometric parameters, such split-nanodisk resonators can operate in the optical anapole regime originating from an overlap of the electric dipole and toroidal dipole Mie-resonant optical modes, thus supporting strongly localized fields and high-Q resonances. We demonstrate room-temperature lasing from the anapole lattices of engineered active metasurfaces with low threshold and high coherence.

Wavelength-scale lasers provide promising applications through low power consumption requiring for optical cavities with increased quality factors. Cavity radiative losses can be suppressed strongly in the regime of optical bound states in the continuum; however, a finite size of the resonator limits the performance of bound states in the continuum as cavity modes for active nanophotonic devices. Here, we employ the concept of a supercavity mode created by merging symmetry-protected and accidental bound states in the continuum in the momentum space, and realize an efficient laser based on a finite-size cavity with a small footprint. We trace the evolution of lasing properties before and after the merging point by varying the lattice spacing, and we reveal this laser demonstrates the significantly reduced threshold, substantially increased quality factor, and shrunken far-field images. Our results provide a route for nanolasers with reduced out-of-plane losses in finite-size active nanodevices and improved lasing characteristics., Though laser action has been reported for optical bound states in the continuum (BIC) cavities with high quality factors, these BIC lasers lacked practical applicability. Here, the authors report an ultralow-threshold super-BIC laser featuring merged symmetry-protected and accidental BICs.

Photons with spin angular momentum possess intrinsic chirality which underpins many phenomena including nonlinear optics1, quantum optics2, topological photonics3 and chiroptics4. Intrinsic chirality is weak in natural materials, and recent theoretical proposals5-7 aimed to enlarge circular dichroism by resonant metasurfaces supporting bound states in the continuum that enhance substantially chiral light-matter interaction. Those insightful works resort to three-dimensional sophisticated geometries, which are too challenging to be realized for optical frequencies8. Therefore, most of the experimental attempts9-11 showing strong circular dichroism rely on false/extrinsic chirality by employing either oblique incidence9, 10 or structural anisotropy11. Here, we report on the experimental realization of true/intrinsic chiral response with resonant metasurfaces where the engineered slant geometry breaks both in-plane and out-of-plane symmetries. Our result marks the first observation of intrinsic chiral bound states in the continuum with near-unity chiral dichroism of 0.93 and record-high quality factor exceeding 2663 for visible frequencies. Our chiral metasurfaces promise a plethora of applications in chiral light sources and detectors, chiral sensing, valleytronics and asymmetric photocatalysis.

We suggest a new strategy for creating stimuli-responsive bio-integrated optical nanostructures based on Mie-resonant silicon nanoparticles covered by an ensemble of similarity negatively charged polyelectrolytes (heparin and sodium polystyrene sulfonate). The dynamic tuning of the nanostructures' optical response is due to light-induced heating of the nanoparticles and swelling of the polyelectrolyte shell. The resulting hydrophilic/hydrophobic transitions significantly change the shell thickness and reversible shift of the scattering spectra for individual nanoparticles up to 60 nm. Our findings bring novel opportunities for the application of smart nanomaterials in nanomedicine and bio-integrated nanophotonics.

We cordially thank all the authors for their valuable contributions to this special issue, and also extend a special thank to Tara Dorrian and Dennis Couwenberg for suggesting and supporting this special issue.

Polarization singularities of vectorial electromagnetic fields locate at the positions (such as points, lines, or surfaces) where properties of polarization ellipses are not defined. They are manifested as circular and linear polarization, for which respectively the semi-major axes and normal vectors of polarization ellipses become indefinite. First observed in conical diffraction in the 1830s, the field of polarization singularities has been systematically reshaped and deepened by many pioneers of wave optics. Together with other exotic phenomena such as non-Hermiticity and topology, polarization singularities have been introduced into the vibrant field of nanophotonics, rendering unprecedented flexibilities for manipulations of light-matter interactions at the nanoscale. Here we review the recent results on the generation and observation of polarization singularities in metaphotonics. We start with the discussion of polarization singularities in the Mie theory, where both electric and magnetic multipoles are explored from perspectives of local and global polarization properties. We then proceed with the discussion of various photonic-crystal structures, for which both near- and far-field patterns manifest diverse polarization singularities characterized by the integer Poincare or more general half-integer Hopf indices (topological charges). Next, we review the most recent studies of conversions from polarization to phase singularities in scalar wave optics, demonstrating how bound states in the continuum can be exploited to generate directly optical vortices of various charges. Throughout our paper, we discuss and highlight several fundamental concepts and demonstrate their close connections and special links to metaphotonics. We believe polarization singularities can provide novel perspectives for light-matter manipulation for both fundamental studies and their practical applications., 17 pages, 7 figures

The coupling of two-dimensional materials with optical metasurfaces is a promising avenue to enhance the advantageous properties of both platforms. Here we integrate an ultrathin monolayer of the t...

Coupling of light from free space to optical fibers is essential for many applications, while commonly used step-index optical fibers provide insufficient coupling efficiencies especially at large ...

Resonant dielectric structures have emerged recently as a new platform for subwavelength nonplasmonic photonics. It was suggested and demonstrated that magnetic and electric Mie resonances can enhance substantially many effects at the nanoscale including spontaneous Raman scattering. Here, we demonstrate stimulated Raman scattering (SRS) for isolated crystalline silicon (c-Si) nanoparticles and observe experimentally a transition from spontaneous to stimulated scattering manifested in a nonlinear growth of the signal intensity above a certain pump threshold. At the Mie resonance, the light gets confined into a low volume of the resonant mode with enhanced electromagnetic fields inside the c-Si nanoparticle due to its high refractive index, which leads to an overall strong SRS signal at low pump intensities. Our finding paves the way for the development of efficient Raman nanolasers for multifunctional photonic metadevices.

Atomically thin monolayers of transition metal dichalcogenides (TMDs) have emerged as a promising class of novel materials for optoelectronics and nonlinear optics. However, the intrinsic nonlinearity of TMD monolayers is weak, limiting their functionalities for nonlinear optical processes such as frequency conversion. Here we boost the effective nonlinear susceptibility of a TMD monolayer by integrating it with a resonant dielectric metasurface that supports pronounced optical resonances with high quality factors: bound states in the continuum (BICs). We demonstrate that a WS2 monolayer combined with a silicon metasurface hosting BICs exhibits enhanced second-harmonic intensity by more than 3 orders of magnitude relative to a WS2 monolayer on top of a flat silicon film of the same thickness. Our work suggests a pathway to employ high-index dielectric metasurfaces as hybrid structures for enhancement of TMD nonlinearities with applications in nonlinear microscopy, optoelectronics, and signal processing.

The field of nonlinear optics is a well-established discipline that relies on macroscopic media and employs propagation distances longer than a wavelength of light. Recent progress with electromagnetic metamaterials has allowed for the expansion of this field into new directions of new phenomena and novel functionalities. In particular, nonlinear effects in thin, artificially structured materials such as metasurfaces do not rely on phase-matching conditions and symmetry-related selection rules of natural materials; they may be substantially enhanced by strong local and collective resonances of fields inside the metasurface nanostructures. Consequently, nonlinear processes may extend beyond simple harmonic generation and spectral broadening due to electronic nonlinearities. This article provides a brief review of basic concepts and recent results in the field of nonlinear optical metasurfaces.

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.

The study of thermal transport in low-dimensional materials has attracted a lot of attention recently after discovery of high thermal conductivity of graphene. Here we study numerically phonon transport in low-dimensional carbon structures being interested in the hydrodynamic regime revealed through the observation of second sound. We demonstrate that correct numerical modeling of such two-dimensional systems requires semi-classical molecular dynamics simulations of temperature waves that take into account quantum statistics of thermalized phonons. We reveal that second sound can be attributed to the maximum group velocity of bending optical oscillations of carbon structures, and the hydrodynamic effects disappear for $T>200$K, being replaced by diffusive dynamics of thermal waves. Our numerical results suggest that the velocity of second sound in such low-dimensional structures is about 6 km/s, and the hydrodynamic effects are manifested stronger in carbon nanotubes rather than in carbon nanoribbons., 13 pages, 15 figures

Controlling the flow of broadband electromagnetic energy at the nanoscale remains a critical challenge in optoelectronics. Surface plasmon polaritons (or plasmons) provide subwavelength localization of light, but are affected by significant losses. On the contrary, dielectrics lack a sufficiently robust response in the visible to trap photons similar to metallic structures. Overcoming these limitations appears elusive, as it implies devising a path to circumvent causality in the quantum-mechanical form of matter. Here we demonstrate that addressing this problem is possible if we employ a novel approach based on suitably deformed reflective metaphotonic structures. The complex geometrical shape engineered in these reflectors emulates nondispersive index responses, which can be inverse-designed following arbitrary form factors. We discuss the realization of essential components such as resonators with an ultra-high refractive index of $n=100$ in diverse profiles. These structures support localization of light in the form of bound states in the continuum (BIC), fully localized in air, in a platform in which all refractive index regions are physically accessible. We discuss our approach to sensing applications, designing a class of sensors where the analyte directly contacts areas of ultra-high refractive index. Leveraging this feature, we report differential sensitivities up to $350$~nm/RIU in structures with footprints of approximately one micron. These performances are two times better than the closest competitor with a similar form factor. Inversely designed reflective metaphotonics offers a flexible technology for controlling broadband light, supporting optoelectronics' integration with large bandwidths in circuitry with miniaturized footprints., 24 pages, 6 figures, submitted

We demonstrate free-standing metasurfaces made by a new wafer-scale and CMOS compatible nanofabrication method and produce nanostructures over large-area transparent oxide membranes. Applications include narrow-band spectral filtering, efficient light focusing, polarization control and optofluidic biosensors.

We demonstrate a novel approach to control photonic bound states in the continuum induced by a symmetry-breaking with tailored resonator heights leading to more precisely engineered system asymmetries and new metasurface functionalities.

We propose and verify experimentally a novel mechanism for achieving superscattering from subwavelength dielectric resonators empowered by the physics of bound states in the continuum.

International audience; High-index dielectric metasurfaces can support sharp optical resonances enabled by the physics of bound states in the continuum (BICs) often manifested in experiments as quasi-BIC resonances. They provide a way to enhance light–matter interaction at the subwavelength scale bringing novel opportunities for nonlinear nanophotonics. Strong narrow-band field enhancement in quasi-BIC metasurfaces leads to an extreme sensitivity to a change of the refractive index that may limit nonlinear functionalities for the pump intensities beyond the perturbative regime. Here we study ultrafast self-action effects observed in quasi-BIC silicon metasurfaces and demonstrate how they alter the power dependence of the third-harmonic generation efficiency. We study experimentally a transition from the subcubic to supercubic regimes for the generated third-harmonic power driven by a blue-shift of the quasi-BIC in the multiphoton absorption regime. Our results suggest a way to implement ultrafast nonlinear dynamics in high-index resonant dielectric metasurfaces for nonlinear meta-optics beyond the perturbative regime.

Rapid progress in miniaturizing vortex devices is driven by their integration with optical sensing, micromanipulation, and optical communications in both classical and quantum realms. Many such efforts are usually associated with on-chip micro- or nanoscale structures in real space and possess a static orbital angular momentum. Recently, a new branch of singular optics has emerged that seeks phase singularities in multiple dimensions, realizing vortex beams with compact nanodevices. Here, we review the topological phase singularities in real space, momentum space, and the spatiotemporal domain for generating vortex beams; discuss recent developments in theoretical and experimental research for generation, detection, and transmission of vortex beams; and provide an outlook for future opportunities in this area, ranging from fundamental research to practical applications.

This invited talk aims to present recent advances in the physics of dielectric metastructures for efficient spatial and temporal control of light by employing multipolar electric and magnetic Mie resonances and bound states in the continuum with applications of these concepts to nanolasers, topological photonics, and sensing.

Plasmonic metasurfaces supporting collective lattice resonances have attracted increasing interest due to their exciting properties of strong spatial coherence and enhanced light-matter interaction. Although the focusing of light by high-numerical-aperture (NA) objectives provides an essential way to boost the field intensities, it remains challenging to excite high-quality resonances by using high-NA objectives due to strong angular dispersion. Here, we address this challenge by employing the physics of bound states in the continuum (BICs). We design a novel anisotropic plasmonic metasurface combining a two-dimensional lattice of high-aspect-ratio pillars with a one-dimensional plasmonic grating, fabricated by a two-photon polymerization technique and gold sputtering. We demonstrate experimentally multiple resonances with absorption amplitudes exceeding 80% at mid-IR using an NA = 0.4 reflective objective. This is enabled by the weak angular dispersion of quasi-BIC resonances in such hybrid plasmonic metasurfaces. Our results suggest novel strategies for designing photonic devices that manipulate focused light with a strong field concentration.

The study of topological phases of light suggests novel opportunities for creating robust optical structures and on-chip photonic devices which are immune against scattering losses and structural disorder. However, many recent demonstrations of topological effects in optics employ structures with relatively large scales. Here we discuss the physics and realisation of topological photonics on small scales, with the dimensions often smaller or comparable with the wavelength of light. We highlight the recent experimental demonstrations of small-scale topological states based on arrays of resonant nanoparticles and discuss a novel photonic platform employing higher-order topological effects for creating subwavelength highly efficient topologically protected optical cavities. We pay a special attention to the recent progress on topological polaritonic structures and summarize with our vision on the future directions of nanoscale topological photonics and its impact on other fields., 32 pages, 5 figures

We review the physics of photonic bound states in the continuum (BICs) and their applications to metadevices, including enhancement of nonlinear response, light-matter interaction, and development of active nanophotonic devices. In particular, we discuss how BIC-empowered dielectric metastructures can be used to generate efficiently high-order optical harmonics from bulk and to boost the intrinsic nonlinearity of transition metal dichalcogenide (TMD) flakes. We explore TMD resonators composed of structured dielectric arrays and individual nanoparticles for strong light-matter coupling phenomena. We discuss the extension of metasurface functionalities for biosensing applications in biomarker detection and quantum information processing with entangled photons. Finally, we demonstrate how tunability of BICs in the momentum space can be used to realize a novel type of efficient lasing based on a finite-size cavity with a small footprint.

Future technologies underpinning high-performance optical communications, ultrafast computations and compact biosensing will rely on densely packed reconfigurable optical circuitry based on nanophotonics. For many years, plasmonics was considered as the only available platform for subwavelength optics, but the recently emerged field of Mie resonant metaphotonics provides more practical alternatives for nanoscale optics by employing resonances in high-index dielectric nanoparticles and their structures such as metasurfaces. In this talk, I hope to discuss both recent advances and future emerging directions in the physics of dielectric Mie-resonant nanostructures with high quality factors (Q factors) for efficient spatial and temporal control of light by employing multipolar Mie resonances and bound states in the continuum, with applications of these concepts to nonlinear optics, active photonics, and topological lasers.

Topological states of light represent counterintuitive optical modes localized at boundaries of finite-size optical structures that originate from the properties of the bulk. Being defined by bulk properties, such boundary states are insensitive to certain types of perturbations, thus naturally enhancing robustness of photonic circuitries. Conventionally, the N-dimensional bulk modes correspond to (N-1)-dimensional boundary states. The higher-order bulk-boundary correspondence relates N-dimensional bulk to boundary states with dimensionality reduced by more than 1. A special interest lies in miniaturization of such higher-order topological states to the nanoscale. Here, we realize nanoscale topological corner states in metasurfaces with C6-symmetric honeycomb lattices. We directly observe nanoscale topology-empowered edge and corner localizations of light and enhancement of light-matter interactions via a nonlinear imaging technique. Control of light at the nanoscale empowered by topology may facilitate miniaturization and on-chip integration of classical and quantum photonic devices., arXiv admin note: text overlap with arXiv:2011.10164

We discuss three examples of different physical systems where the energy localization occurs at the edges due to nonlinearity, topology, or curvature. In many of such systems, the existence of localized edge modes and their properties can be verified experimentally, as for the case of arrays of weakly coupled optical waveguides and topological photonic modes.We discuss three examples of different physical systems where the energy localization occurs at the edges due to nonlinearity, topology, or curvature. In many of such systems, the existence of localized edge modes and their properties can be verified experimentally, as for the case of arrays of weakly coupled optical waveguides and topological photonic modes.

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.