Excitation of anapole mode and near-field localization enhancement based on graphene-assembled film flexible metasurfaces.

Anapole mode is a nonradiative resonance originating from the destructive interference between co-excited Cartesian electric dipole and toroidal dipole moments. With at least two symmetric circulating currents, the anapole mode in all-dielectric nanoresonators provides the opportunity to operate the double perfect electric conductor (PEC) mirror effects. In this work, unlike the conventional metal-dielectric-metal (MDM) nanostructure generating a plasmonic magnetic resonance, two metal components are employed to produce the fictitious images of the middle dielectric, and the whole system can thus excite the doubly mirror-induced anapole mode. Electric anapole mode and its magnetic counterpart are, respectively, investigated in two types of MDM configurations according to their own symmetric characteristics. Benefiting from the double PEC mirror effects, the doubly mirror-induced electric and magnetic anapole modes possess the larger average electric-field enhancement factors (9 and 56.9 folds compared with those of the conventional ones, respectively), as well as the narrower line widths. This work will pave a new way for tailoring and boosting anapole modes in metal-dielectric hybrid nanoresonators and open up new opportunities for many significant applications in nonlinear and quantum nanophotonics.

Anapole modes of all-dielectric nanostructures hold great promise for many nanophotonic applications. However, anapole modes can hardly couple to other modes through far-field interactions, and their near-field enhancements are dispersed widely inside the nanostructures. These facts bring challenges to the further increasing of the response of an anapole mode. Here, we theoretically show that an anapole mode response in a dielectric nanostructure can be boosted through electromagnetic interactions with the coupling distance of a wavelength scale, which is beyond both the near-field and far-field limits. The all-dielectric nanostructure consists of a disk holding an anapole mode and a ring. Both analytical calculations and numerical simulations are carried out to investigate the electromagnetic interactions in the system. It is found that the electric dipoles associated with the fields of the anapole mode on the disk undergo retardation-related interactions with the electric dipoles associated with the ring, leading to the efficiently enhanced response of the anapole mode. The corresponding near field enhancement on the disk can reaches more than 90 times for a slotted silicon disk-ring nanostructure, where the width of the slot is 10 nm. This enhancement is about 5 times larger than that of an individual slotted disk. Our results reveal the greatly enhanced anapole mode through electromagnetic couplings in all-dielectric nanostructures, and the corresponding large field enhancement could find important applications for enhanced nonlinear photonics, near-field enhanced spectroscopies, and strong photon–exciton couplings.

We reveal that an isotropic, homogeneous, subwavelength particle with high refractive index can produce ultra-small total scattering. This effect, which follows from the inhibition of the electric dipole radiation, can be identified as a Fano resonance in the scattering efficiency and is associated with the excitation of an anapole mode in the particle. This anapole mode is non-radiative and emerges from the destructive interference of electric and toroidal dipoles. The invisibility effect could be useful for the design of highly transparent optical materials.This article is part of the themed issue 'New horizons for nanophotonics'.

Since the observations of the dynamic toroidal dipole and non-radiating anapole, the field of toroidal electrodynamics has grown rapidly paving the route to novel physical phenomena including new types of optical activity, Aharonov-Bohm effects, invisibility, and Lorentz reciprocity violations with potential applications in spectroscopy, sensing, telecommunications, and light emission. However, despite significant advances in our understanding of toroidal light-matter interactions’ the excitation of toroidal and anapole modes in matter is based on heuristic approaches, which typically require complex patterning and/or illumination conditions resulting in narrowband operation and a strong background of unwanted multipole excitations. Detecting anapoles and toroidal dipoles poses an even greater challenge owing to their weak coupling to free-space radiation. Recently, Flying Doughnuts, single-cycle toroidal pulses, have been put forward as ideal probes for toroidal and anapole excitations in matter. In this talk, we will present our recent work on the light-matter interactions of Flying Doughnut pulses with simple dielectric and plasmonic systems, such as particles and films, demonstrating broadband toroidal excitations and discuss strategies for their unambiguous detection.

Due to their ability to produce high electric field enhancements in relatively large nanoscale volumes with minimum absorption and nonradiating properties, anapole modes excited in high index dielectric nanostructures have attracted considerable attentions in these years. We propose a design strategy to simultaneously excite the anapole mode efficiently and maintain its resonant wavelength, which has been remained as a challenge in the conventional dielectric nanostructures. Based on analyzing the relationship between the field enhancement factor and scattering intensity of the electric and toroidal dipoles, we introduce two and four nanocuboids into the nil field intensity areas in the silicon disk system, respectively. The geometric volume of the system can be increased effectively and the electric field enhancement is boosted to be 190% and 250% while the resonant wavelength of the anapole mode is almost maintained constant. The systems combined with a slot in the strongest field intensity area also follow the same law, revealing that the design strategy can be easily extended to other geometric, material and frequency systems. Different from the design strategy to add new components into the areas with strong field intensity, the incorporations occurring at the minimum intensity area is another design scheme to engineer the properties of the resonant systems and can find broad applications in nano-device designs.

We theoretically study the near-field couplings of two stacked all-dielectric nanodisks, where each disk has an electric anapole mode consisting of an electric dipole mode and an electric toroidal dipole (ETD) mode. Strong bonding and anti-bonding hybridizations of the ETD modes of the two disks occur. The bonding hybridized ETD can interfere with the dimer’s electric dipole mode and induce a new electric anapole mode. The anti-bonding hybridization of the ETD modes can induce a magnetic toroidal dipole (MTD) response in the disk dimer. The MTD and magnetic dipole resonances of the dimer form a magnetic anapole mode. Thus, two dips associated with the hybridized modes appear on the scattering spectrum of the dimer. Furthermore, the MTD mode is also accompanied by an electric toroidal quadrupole mode. The hybridizations of the ETD and the induced higher-order modes can be adjusted by varying the geometries of the disks. The strong anapole mode couplings and the corresponding rich higher-order mode responses in simple all-dielectric nanostructures can provide new opportunities for nanoscale optical manipulations.

The toroidal dipole is a localized electromagnetic excitation independent from the familiar magnetic and electric dipoles. It corresponds to currents flowing along minor loops of a torus. Interference of radiating induced toroidal and electric dipoles leads to anapole, a nonradiating charge-current configuration. Interactions of induced toroidal dipoles with electromagnetic waves have recently been observed in artificial media at microwave, terahertz, and optical frequencies. Here, we demonstrate a quasi-planar plasmonic metamaterial, a combination of dumbbell aperture and vertical split-ring resonator, that exhibits transverse toroidal moment and resonant anapole behavior in the optical part of the spectrum upon excitation with a normally incident electromagnetic wave. Our results prove experimentally that toroidal modes and anapole modes can provide distinct and physically significant contributions to the absorption and dispersion of slabs of matter in the optical part of the spectrum in conventional transmission and reflection experiments.

Recently, localized spoof surface plasmon polaritons (LSSPPs) have emerged as a new research frontier due to their unique properties and potential applications in integrated circuits. The toroidal dipole is a localized electromagnetic response different from electric and magnetic dipoles. Anapole is the nonradiating source produced by the interference between identical radiation patterns of toroidal and electric dipoles in the far field. This paper proposes a novel high Q‐factor compact planar resonator consisting of split‐ring resonators to support the LSSPPs anapole at microwave frequency, which shows an apparent nonradiating charge–current configuration. In simulation and measurement, we observe the nonradiating property of the anapole mode. Further, we demonstrate that the LSSPPs anapole resonance has the tunable property, which is very sensitive to structural changes. These might lead to potential applications in the design of plasmonic deformation sensors, cloaking, and absorbers.

Multi-resonant metasurfaces are of great significance in the applications of multi-band nanophotonics. Here, we propose a novel metasurface design scheme for simultaneously supporting quasi-bound states in continuum (QBIC) and other resonant modes, in which QBIC resonance is generated by mirror or rotational symmetry breaking in oligomers while other resonant modes can be simultaneously excited by rationally designing the shapes of meta-atoms within oligomers. As an example, the simultaneous excitation of QBIC and anapole modes are demonstrated in a dimer metasurface composed of asymmetric dumbbell-shaped apertures. Based on the far-field multipole decomposition and near-field electromagnetic field distributions, the origin mechanisms of QBIC and anapole mode are elucidated. The symmetry breaking of dumbbell-shaped dimer results in QBIC. Within a certain asymmetric variation range, the contributions of toroidal dipole moment and electric dipole moment with approximately equal magnitudes remain dominant, which allows the anapole mode to always present. The effectiveness of the proposed design scheme is further confirmed by the experimental results identical with the evolutions of numerical simulation. In terahertz biosensing experiments, the anapole mode exhibits a higher sensitivity of 271.3 GHz (nmol/μl)-1, whereas the QBIC can achieve a lower detection limit of 0.015 nmol/μl and expands the detection range by almost an order of magnitude. Our findings are beneficial to designing multi-resonant metasurfaces with different resonance modes and promote the corresponding applications in the fields of biosensing, lasers, filtering, and nonlinearity.

Metal–dielectric nanostructures in the optical anapole modes are essential for light–matter interactions due to the low material loss and high near-field enhancement. Herein, a hybrid metal–dielectric nanoantenna composed of six wedge-shaped gold (Au) nanoblocks as well as silica (SiO2) and silicon (Si) nanodiscs is designed and analyzed by the finite element method (FEM). The nanoantenna exhibits flexibility in excitation and manipulation of the anapole mode through the strong coupling between the metal and dielectrics, consequently improving the near-field enhancement at the gap. By systematically optimizing the structural parameters, the electric field enhancement factors at wavelengths corresponding to the anapole modes (AM1 and AM2) can be increased to 518 and 1482, respectively. Moreover, the nanoantenna delivers great performance in optical sensing such as a sensitivity of 550 nm/RIU. The results provide guidance and insights into enhancing the coupling between metals and dielectrics for applications such as surface-enhanced Raman scattering and optical sensing.

In this paper, we demonstrate numerically that T1O2 microparticles can exhibit a destructive mode interference in the range 1–3 THz. Resonantly strong destructive interference of the toroidal and electric dipole contributions to the scattered field was achieved by a geometry tuning. An anapole mode can be observed as a composition of electric and toroidal dipole moments.

Enhanced anapole and coupled toroidal dipole moments sustaining the synthesized terahertz (THz) metamaterial innovatively promise for absorption‐transmission‐absorption (ATA) metamaterial rasorber (MMR). The anapole mode, due to the strong interaction of electric and magnetic dipole moments, and Fabry–Pérot resonance are supported by the dumbbell‐shaped apertures topologically distributed in a double‐layer silver plate, thus forming the transmission window with high frequency‐selectivity. Meanwhile, the coupled toroidal dipole moments depending on the toroidal structure can cause destructive and constructive interference in far and near fields, respectively, thus causing much electromagnetic energy loss in a wide band. Combining anapole with toroidal dipole moments, the ATA MMR depending on the THz metamaterial can attain high absorption (over 0.88) at 1.46–2.29 THz with relative absorption bandwidth (RABW) of 44.27%, and at 3.51–4.3 THz the RABW of 20.26%, and simultaneously the transmission (over 0.7) window covering 2.9–3.21 THz, with relative transmission bandwidth (RTBW) of 10.3%. These superior properties of the MMR promise a great potential for practical application in radar and electromagnetic stealth fields. Furthermore, the innovative combination of anapole and toroidal dipole moments in the MMR assists in enriching the theoretical framework and strategically opening up state‐of‐the‐art pathways of multipole electrodynamics.

Nonradiating current configurations attract attention of physicists for many years as possible models of stable atoms. One intriguing example of such a nonradiating source is known as ‘anapole'. An anapole mode can be viewed as a composition of electric and toroidal dipole moments, resulting in destructive interference of the radiation fields due to similarity of their far-field scattering patterns. Here we demonstrate experimentally that dielectric nanoparticles can exhibit a radiationless anapole mode in visible. We achieve the spectral overlap of the toroidal and electric dipole modes through a geometry tuning, and observe a highly pronounced dip in the far-field scattering accompanied by the specific near-field distribution associated with the anapole mode. The anapole physics provides a unique playground for the study of electromagnetic properties of nontrivial excitations of complex fields, reciprocity violation and Aharonov–Bohm like phenomena at optical frequencies.

Natural toroidal molecules, such as biomolecules [1] and proteins [2], possess toroidal dipole moments that are hard to be detected, which leads to extensive studies of artificial toroidal materials. Metamaterials [3-4] are sub-wavelength artificial structures that can be specifically designed to manipulate the intensity of induced electromagnetic multidipoles. Recently, toroidal metamaterials [5-6] have been widely investigated to enhance toroidal dipole moments while the other multipoles are eliminated due to the spacial symmetry. However, to effectively excite a toroidal dipole, a specific excitation method is necessary since a closed-loop of induced magnetic dipoles in a toroidal metamaterial weakly interact with the external wave. This is a key issue that has to be carefully taken into account in existing toroidal experiments. Moreover, most of generated toroidal dipole moments are either aligned vertically to the substrate surface or embedded in a dielectric, leading to another constraint for further applications. In this paper, we present a novel design for a toroidal metamaterial with multilayered structures, which composed of a gold dumbbell-shaped aperture and a vertical split-ring resonator (VSRR). The induced toroidal dipoles show several advantages like free-standing and vertically oscillating configuration that are distinguishable from previously reported works. It is worth to mention that the non-radiating from the destructive interference from the toroidal and electric dipoles can also be generated in our proposed structures.

Background: Dielectric nanoantennas have recently emerged as an alternative solution to plasmonics for nonlinear light manipulation at the nanoscale, thanks to the magnetic and electric resonances, the strong nonlinearities, and the low ohmic losses characterizing high refractive-index materials in the visible/near-infrared (NIR) region of the spectrum. In this frame, AlGaAs nanoantennas demonstrated to be extremely efficient sources of second harmonic radiation. In particular, the nonlinear polarization of an optical system pumped at the anapole mode can be potentially boosted, due to both the strong dip in the scattering spectrum and the near-field enhancement, which are characteristic of this mode. Plasmonic nanostructures, on the other hand, remain the most promising solution to achieve strong local field confinement, especially in the NIR, where metals such as gold display relatively low losses.Results: We present a nonlinear hybrid antenna based on an AlGaAs nanopillar surrounded by a gold ring, which merges in a single platform the strong field confinement typically produced by plasmonic antennas with the high nonlinearity and low loss characteristics of dielectric nanoantennas. This platform allows enhancing the coupling of light to the nanopillar at coincidence with the anapole mode, hence boosting both second- and third-harmonic generation conversion efficiencies. More than one order of magnitude enhancement factors are measured for both processes with respect to the isolated structure.Conclusion: The present results reveal the possibility to achieve tuneable metamixers and higher resolution in nonlinear sensing and spectroscopy, by means of improved both pump coupling and emission efficiency due to the excitation of the anapole mode enhanced by the plasmonic nanoantenna.