Resonantly enhanced second-and third-harmonic generation in dielectric nonlinear metasurfaces

Nonlinear dielectric metasurfaces provide a promising approach to control and manipulate frequency conversion optical processes at the nanoscale,thus facilitating both advances in fundamental research and the development of new practical applications in photonics,lasing,and sensing.Here,we employ symmetry-broken metasurfaces made of centrosymmetric amorphous silicon for resonantly enhanced second-and third-order nonlinear optical response.Exploiting the rich physics of optical quasi-bound states in the continuum and guided mode resonances,we comprehensively study through rigorous numerical calculations the relative contribution of surface and bulk effects to second-harmonic generation(SHG)and the bulk contribution to third-harmonic generation(THG) from the meta-atoms.Next,we experimentally achieve optical resonances with high quality factors,which greatly boosts light-matter interaction,resulting in about 550 times SHG enhancement and nearly 5000-fold increase of THG.A good agreement between theoretical predictions and experimental measurements is observed.To gain deeper insights into the physics of the investigated nonlinear optical processes,we further numerically study the relation between nonlinear emission and the structural asymmetry of the metasurface and reveal that the generated harmonic signals arising from linear sharp resonances are highly dependent on the asymmetry of the meta-atoms.Our work suggests a fruitful strategy to enhance the harmonic generation and effectively control different orders of harmonics in all-dielectric metasurfaces,enabling the development of efficient active photonic nanodevices.

High-Q resonant terahertz metasurfaces

In photonics, the quest for high-quality (high Q) resonances driven by the physics of bound states in the continuum (BIC)1,2has motivated researchers to explore innovative avenues for realizing groundbreaking applications in lasing3, sensing4and nonlinear photonics5. A conventional strategy to harness the properties of BICs involves breaking the symmetry of resonators in a uniform lattice, allowing uncoupled modes to interact with free space that opens a leaky channel in the form of socalled (quasi) q BIC6modes.

Intelligent metaphotonics empowered by machine learning

In the recent years,a dramatic boost of the research is observed at the junction of photonics,machine learning and artifi-cial intelligence.A new methodology can be applied to the description of a variety of photonic systems including optical waveguides,nanoantennas,and metasurfaces.These novel approaches underpin the fundamental principles of light-matter interaction developed for a smart design of intelligent photonic devices.Artificial intelligence and machine learn-ing 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 overview the evaluation of meta-photonics induced by artificial intelligence and present a summary of the concepts of machine learning with some specif-ic examples developed and demonstrated for metasystems and metasurfaces.

Control of light scattering by nanoparticles with optically-induced magnetic responses

Conventional approaches to control and shape the scattering pattems 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.

Nonlinear chiral metaphotonics:a perspective

We review the physics and some applications of photonic structures designed for the realization of strong nonlinear chiroptical response.We pay much attention to the recent strategy of utilizing different types of optical resonances in metallic and dielectric subwavelength structures and metasurfaces,including surface plasmon resonances,Mie resonances,lattice-guided modes,and bound states in the continuum.We summarize earlier results and discuss more recent developments for achieving large circular dichroism combined with the high efficiency of nonlinear harmonic generation.

Nonlinear photonics with metasurfaces

Nonlinear optics is a well-established field of research that traditionally relies on the interaction of light with macroscopic nonlinear media over distances significantly greater than the wavelength of light. However, the recently emerged field of optical metasurfaces provides a novel platform for studying nonlinear phenomena in planar geometries. Nonlinear optical metasurfaces introduce new functionalities to the field of nonlinear optics extending them beyond perturbative regimes of harmonic generation and parametric frequency conversion,being driven by mode-matching, resonances, and relaxed phase-matching conditions. Here we review the very recent advances in the rapidly developing field of nonlinear metasurface photonics, emphasizing multi-frequency and cascading effects, asymmetric and chiral frequency conversion, nonperturbative nonlinear regimes, and nonlinear quantum photonics, empowered by the physics of Mie resonances and optical bound states in the continuum.

Electrically-pumped compact topological bulk lasers driven by band-inverted bound states in the continuum

One of the most exciting breakthroughs in physics is the concept of topology that was recently introduced to photonics,achieving robust functionalities,as manifested in the recently demonstrated topological lasers.However,so far almost all attention was focused on lasing from topological edge states.Bulk bands that reflect the topological bulk-edge correspondence have been largely missed.Here,we demonstrate an electrically pumped topological bulk quantum cascade laser(QCL)operating in the terahertz(THz)frequency range.In addition to the band-inversion induced in-plane reflection due to topological nontrivial cavity surrounded by a trivial domain,we further illustrate the band edges of such topological bulk lasers are recognized as the bound states in the continuum(BiCs)due to their nonradiative characteristics and robust topological polarization charges in the momentum space.Therefore,the lasing modes show both in-plane and out-of-plane tight confinements in a compact laser cavity(lateral size~3λ_(laser)).Experimentally,we realize a miniaturized THz QCL that shows single-mode lasing with a side-mode suppression ratio(SMSR)around 20 dB.We also observe a cylindrical vector beam for the far-field emission,which is evidence for topological bulk BIC lasers.Our demonstration on miniaturization of single-mode beam-engineered THz lasers is promising for many applications including imaging,sensing,and communications.

自旋调控的有源几何超表面

Metasurfaces,composed of an array of subwavelength optical scatterers on a surface,have demonstrated unprecedented capabilities of manipulating the properties of incoming light(e.g.,amplitude,phase,polarization)[1-3].Among the large family of metasurfaces,geometric metasurfaces have attracted great attention due to superior phase control[4,5].

Bound states in the continuum and Fano resonances in the strong mode coupling regime

The study of resonant dielectric nanostructures with a high refractive index is a new research direction in the nanoscale optics and metamaterial-inspired nanophotonics.Because of the unique optically induced electric and magnetic Mie resonances,high-index nanoscale structures are expected to complement or even replace different plasmonic components in a range of potential applications.We study a strong coupling between modes of a single subwavelength high-index dielectric resonator and analyze the mode transformation and Fano resonances when the resonator’s aspect ratio varies.We demonstrate that strong mode coupling results in resonances with high-quality factors,which are related to the physics of bound states in the continuum when the radiative losses are almost suppressed due to the Friedrich–Wintgen scenario of destructive interference.We explain the physics of these states in terms of multipole decomposition,and show that their appearance is accompanied by a drastic change in the far-field radiation pattern.We reveal a fundamental link between the formation of the high-quality resonances and peculiarities of the Fano parameter in the scattering cross-section spectra.Our theoretical findings are confirmed by microwave experiments for the scattering of high-index cylindrical resonators with a tunable aspect ratio.The proposed mechanism of the strong mode coupling in single subwavelength high-index resonators accompanied by resonances with high-quality factors helps to extend substantially functionalities of all-dielectric nanophotonics,which opens horizons for active and passive nanoscale metadevices.

Room-temperature lasing from nanophotonic topological cavities

The study of topological phases of light underpins a promising paradigm for engineering disorder-immune compact photonic devices with unusual properties.Combined with an optical gain,topological photonic structures provide a novel platform for micro-and nanoscale lasers,which could benefit from nontrivial band topology and spatially localized gap states.Here,we propose and demonstrate experimentally active nanophotonic topological cavities incorporating Ⅲ-Ⅴ semiconductor quantum wells as a gain medium in the structure.We observe room-temperature lasing with a narrow spectrum,high coherence,and threshold behaviour.The emitted beam hosts a singularity encoded by a triade cavity mode that resides in the bandgap of two interfaced valley-Hall periodic photonic lattices with opposite parity breaking.Our findings make a step towards topologically controlled ultrasmall light sources with nontrivial radiation characteristics.

Seeing is believing

Bound states in the continuum are realized in many optical systems as“dark states”,and their presence can be detected in the regime of leaky modes via resonances in far-fields.Here the authors reveal previously unseen structure of bound states in the continuum by exploring strong near-field localization in dielectric metasurfaces.