Anapole States Research Articles

Engineered pseudo and hybrid anapole states in a silicon nanoresonator metasurface

Significant attention has been devoted to realizing non-radiating states (popularly known as anapole states) in several systems due to the strong localization of electromagnetic fields, which can be attained through destructive interference of various dipole moments, yielding fundamental or higher-order anapole states. Recently, it has been shown that the interference of higher-order excitation also permits light manipulation at the nanoscale and provides additional benefits such as efficient power transfer and enhancement in nonlinearities. In this work, we report discrete electric anapole (pseudo and hybrid) by careful design of an all-dielectric metasurface consisting of a silicon nanodisk such that a higher-order toroidal electric dipole (TD) and quadrupole interfere destructively, producing a hybrid anapole at 815 nm, whereas the pseudo anapole is generated when TD and second-order TD minima occur at the same wavelength of 522 nm. The phase plots confirm the findings of these radiationless states. Such dual and distinct non-radiating current configurations may find applications in spectroscopy, sensing, switching, optical nonlinearity, and optomechanics.

Selective superinvisibility effect via compound anapole

We introduce a general concept and consider a characteristic approach to obtain the narrow-band suppression of total electromagnetic scattering independent of the irradiation conditions for the compound dielectric structures supporting unique anapole states. To emphasize this independence from the irradiation conditions, we call the selective superinvisibility effect. We show that the realization of this concept allows us to reach simultaneously several goals significantly suppress the scattering (at a certain wavelength); provide the scattering suppression for any polarization and direction of incidence waves; accumulate electromagnetic energy in the near-field zone and inside of the scatterer. The combination of these physical properties in a compound structure makes it possible to consider them as building blocks for two-dimensional or three-dimensional metamaterials with the selective transparent property practically independent of the irradiation conditions. Our study includes theoretical modeling based on a multipole approach and a corresponding experimental verification.

Reconfigurable dielectric resonators with imbedded impedance surfaces—From enhanced and directional to suppressed scattering

Resonant elements play a vital role in tailoring of the radiation and scattering properties of devices, such as antennas and functional material platforms. We presently demonstrate a simple resonator that supports a multitude of scattering states. The resonator is a hybrid structure consisting of a finite-height dielectric cylinder integrated with a concentric impedance surface. Given its simple configuration, we apply the classical Lorentz–Mie theory to analyze its scattering properties analytically. Through a careful tuning of its geometry, the resonator is found to support enhanced and directive scattering states as well as the suppressed scattering states also known as anapole states. A prototype of the resonator has been built and tested at microwave frequencies. It utilizes water as the dielectric and a metallic tube with periodic slits as the impedance surface. Exploiting the flexibility of water, the design is easily reconfigured for different scattering responses: fully filled, the resonator is found to scatter predominantly in the forward direction, whereas an anapole state emerges with significant reduction of scattering when the resonator is partially filled with water. Consequently, the proposed resonator may be of great interest within the broad area of antenna design and functional material platforms, encompassing not only the obvious microwave frequencies but also the THz- and optical domain using high-permittivity dielectrics and graphene/nano-particle surfaces.

Finite-Element Method Simulations of the Tunable Magnetic Anapole State in an All-Graphene Metasurface: Implications for Near-Field Sensing, Nanoscale Optical Trapping, and Cloaking

In this article, we have proposed an all-graphene metasurface where the magnetic anapole state can be dynamically tuned. The structure is composed of two layers of graphene separated by a dielectric spacer on an oxide substrate, forming a gap surface plasmons resonator structure in the long-wavelength infrared region. The top graphene layer is patterned into interconnected disks, while the interconnection is for electrical tuning purposes only. By multipolar analysis using the finite-element method (FEM) simulation, we have shown that at the particular dimension, the dominant electric dipole scattering can be minimized. This happens at the resonant point where along with the electric dipole suppression, the magnetic and magnetic toroidal dipoles destructively interfere, which gives rise to a magnetic anapole state. This resonant point can be tuned in two ways, either passively, which is the lithographical variation of structure parameters, or actively, by harnessing the chemical potential of the graphene. We have shown here both mechanisms whereby actively tuning we can shift the magnetic anapole to the far-infrared region. Through the interplay of multipole analysis, we have shown that by varying the angle of incidence, we can control the radiating channels, which can be termed an anapole switch. For the active tuning cases, by voltage mapping of the chemical potential, we have shown the effect of chemical potential on the components of the multipolar families. Our device concept and such implementations hold promise for the application in near-field sensing, nanoscale optical trapping, cloaking, etc.

Light Guidance Aided by the Toroidal Dipole and the Magnetic Quadrupole in Silicon Slotted-Disk Chains.

Far-field scattering of high-index nanoparticles can be hugely reduced via interference of multipolar moments giving rise to the so-called anapole states. It has been suggested that this reduced scattering can contribute to efficient transmission along periodic chains of such nanoparticles. In this work, we analyze via numerical simulation and experiments the transmission of light along chains of regular and slotted silicon disks in the frequency region over the light cone. We do not observe transmission at wavelengths corresponding to the excitation of the first electric anapole for regular disks. However, large transmission along straight and curved chains is observed for slotted disks due to the simultaneous excitation of the toroidal dipole and magnetic quadrupole modes in the disks. Photonic band calculations unveil that such large transmission can be ascribed to leaky resonances, though bound states in the continuum do not appear in the structures under analysis. Experiments at telecom wavelengths using silicon disk chains confirm the numerical results for straight and bent chains. Our results provide new insights into the role of radiationless states in light guidance along nanoparticle chains and offer new avenues to utilize Mie resonances of simple nanophotonic structures for on-chip dielectric photonics.

Optically transparent and flexible-assembled metasurface rasorber for infrared-microwave camouflage based on a hybrid anapole state.

Hybrid anapole state, originating from the destructive interference of more than one basic electromagnetic multipole moments with their toroidal counterparts, enables the simultaneous suppression of multiple leading scattering channels, thereby demonstrates promising applications in perfect absorption and electromagnetic camouflage. However, the formation of hybrid anapoles is challenging because a careful overlap of electromagnetic multipoles with their toroidal counterparts is required. In this study, we propose and experimentally demonstrate a transparent and flexible assembled metasurface rasorber supporting hybrid anapole states for infrared and microwave camouflage, which not only supports low IR emissivity in the range of 8-14 μm but also exhibits an absorption-transmission-absorption response in the microwave band. In addition, the conformal and tunable performances of the fabricated metasurface rasorber are experimentally demonstrated. Our study provides a new strategy for designing multispectral camouflage metasurfaces.

Unraveling Dipolar Regime and Kerker Conditions in Mid‐Index Mesoscale Dielectric Materials

AbstractNanophotonic phenomena, such as zero optical back scattering, nonradiating anapole states, etc. are related to the excitation of single dipolar modes—hence so far, they have only been observed within a few relatively high‐index dielectric materials (refractive index, n > 3.5) in the nanoscale regime at the optical frequencies. Here, dipolar regime is unraveled, close‐to‐zero backscattering is demonstrated, and optical anapoles are excited in mid‐index dielectric spheres (titanium di‐oxide, TiO2; n ≈ 2.6) at the mesoscale regime (particle diameter, d ≈ incident wavelength, λ) under illumination with tightly focused Gaussian beams (TFGBs). Successive scattering minima associated with dipolar excitation are observed satisfying the first Kerker condition in the scattering spectra of single TiO2 spheres with diameters in the micrometer range. Moreover, at specific wavelengths, the electric and magnetic dipolar scattering amplitudes of the dielectric microspheres simultaneously go close‐to‐zero, leading to the excitation of hybrid optical anapoles with a total scattering intensity ≈ 5 times weaker for TFGB illumination with numerical aperture, NA ≈ 0.95 compared to NA ≈ 0.1. The result pushes the boundary of the observation of nanophotonic phenomena to a new regime with regards to type and size of the materials.

Possibility of Anapole State in Dielectric Nanohole Array Metasurfaces with Different Hole Shapes

The optical anapole resonances in nanostructures display strong field confinement and substantially suppressed scattering. In this study using three-dimensional finite-difference time-domain simulations, it is shown that high refractive index dielectric nanohole array metasurfaces having different profiles of the holes can possess the anapole state. The multipole decomposition including the dipole electric toroidal moment for the lattice elements of the arrays is provided. The anapole state in the lattice elements is illustrated by time-averaged distributions of the energy. As a result, high-index freestanding metasurfaces with anapole state can be designed.

Toroidal anapole with point magnetic dipoles

The role of multipolar expansion of the localized current is significant in a broad range of disciplines in the physics of nowadays. It is actively used in such problems as the design of nanoantennas in photonics or the description of exotic states of matter. The toroidal multipoles are the third group of multipoles which complements the electric and magnetic multipolar families. The investigation of toroidal multipoles is important because of their role in the formation of so-called anapole states, non-radiating sources. The further study showed that there are also exist the higher-order toroidal dipoles or mean-square radii, which are equivalent. Here we suggest the structure which consists of point magnetic dipoles and supports the toroidal anapole state, which is obtained with the destructive interference of electric toroidal dipole with the first mean-square radius of the electric toroidal dipole. We also present an analytical condition for toroidal anapole with this system of point magnetic dipoles placed in two circles.

Experimental observation of higher-order anapoles in individual silicon disks under in-plane illumination

Anapole states—characterized by a strong suppression of far-field scattering—naturally arise in high-index nanoparticles as a result of the interference between certain multipolar moments. Recently, the first-order electric anapole, resulting from the interference between the electric and toroidal dipoles, was characterized under in-plane illumination as required in on-chip photonics. Here, we go a step further and report on the observation of higher-order (magnetic and second-order electric) anapole states in individual silicon disks under in-plane illumination. To do so, we increase the disk dimensions (radius and thickness) so that such anapoles occur at telecom wavelengths. Experiments show dips in the far-field scattering perpendicular to the disk plane at the expected wavelengths and the selected polarizations, which we interpret as a signature of high-order anapoles. Some differences between normal and in-plane excitation are discussed, in particular, the non-cancelation of the sum of the Cartesian electric and toroidal moments for in-plane incidence. Our results pave the way toward the use of different anapole states in photonic integrated circuits either on silicon or other high-index dielectric materials.

Manipulating light scattering and optical confinement in vertically stacked Mie resonators

Abstract High index dielectric nanoresonators have gained prominence in nanophotonics due to lower losses compared to plasmonic systems and their ability to sustain both electric and magnetic resonances. The resonances can be engineered to create new types of optical states, such as bound-states in a continuum (BIC) and anapoles. In this work, we report on the optical properties of vertically stacked AlGaAs nanodisk Mie resonators. The nanodisks are designed to support an anapole state in the visible wavelength region (400–700 nm). The vertically stacked nanodisk resonators are fabricated from AlGaAs/GaAs multilayer samples with a fast and scalable patterning method using charged sphere colloidal lithography. Both measurements and finite difference time domain (FDTD) simulations of two and three stacked resonators show a sharp dip in the reflectance spectra at the anapole wavelength. For the 2 and 3 disk stacks the reflectance dip contrast at the anapole wavelength becomes very pronounced in the specular reflectance and is attributed to increased directional scattering due to an antenna effect. FDTD simulations show there is enhanced field confinement in all the disks at the anapole wavelength and the confined energy within the individual disks in the stack is at least 2–5 times greater compared to an isolated single nanodisk of the same dimension. Furthermore, the field confinement consistently increases with adding more disks in the stack. These vertically stacked AlGaAs nanodisk resonators can be a very exciting platform to engineer light matter interactions for linear and non-linear optical applications. The general principles of the fabrication method can be adapted to other wavelength ranges and can also be adapted for other III–V material combinations as well as for Si/SiO2.

Anapole states and scattering deflection effects in anisotropic van der Waals nanoparticles

Transition metal dichalcogenides (TMDCs), belonging to the class of van der Waals materials, are promising materials for optoelectronics and photonics. In particular, their giant optical anisotropy may enable important optical effects when employed in nanostructures with finite thickness. In this paper, we theoretically and numerically study light scattering behavior from anisotropic MoS2 nanocylinders, and highlight its distinct features advantageous over the response of conventional silicon particles of the same shape. We establish two remarkable phenomena, appearing in the same MoS2 particle with optimized geometry. The first one is a pure magnetic dipole scattering associated with the excitation of the electric-dipole anapole states. Previously reported in core-shell hybrid (metal/dielectric) systems only, it is now demonstrated in an all-dielectric particle. The second phenomenon is the super-deflection in the far field: the maximum scattering may occur over a wide range of directions, including forward-, backward- and side-scattering depending on the mutual orientation of the MoS2 nanocylinder and the incident wave. In contrast to the well-known Kerker and anti-Kerker effects, which appear in nanoparticles at different frequencies, the super-deflection can be achieved by rotating the particle at a constant frequency of incident light. Our results facilitate the development of functional optical devices incorporating nanostructured anisotropic TMDCs and may encourage further research in meta-optics based on highly anisotropic materials.

Strong optical coupling in metallo-dielectric hybrid metasurfaces.

Metasurfaces consisting of hybrid metal/dielectric nanostructures carry advantages of both material platforms. The hybrid structures can not only confine electromagnetic fields in subwavelength regions, but they may also lower the absorption losses. Such optical characteristics are difficult to realize in metamaterials with only metal or dielectric structures. Hybrid designs also expand the scope of material choices and the types of optical modes that can be excited in a metasurface, thereby allowing novel light matter interactions. Here, we present a metallo-dielectric hybrid metasurface design consisting of a high-index dielectric (silicon) nanodisk array on top of a metal layer (aluminum) separated by a buffer oxide (silica) layer. The dimensions of Si nanodisks are tuned to support anapole states and the period of the nanodisk array is designed to excite surface plasmon polariton (SPP) at the metal-buffer oxide interface. The physical dimensions of the Si nanodisk and the array periods are optimized to excite the anapole and the SPP at normal incidence of light in the visible-NIR (400-900 nm) wavelength range. Finite difference time domain (FDTD) simulations show that, when the nanodisk grating is placed at a specific height (∼200 nm) from the metal surface, the two modes strongly couple at zero detuning of the resonances. The strong coupling is evident from the avoided crossing of the modes observed in the reflectance spectra and in the spectral profile of light absorption inside the Si nanodisk. A vacuum Rabi splitting of up to ∼ 129 meV is achievable by optimizing the diameters of Si nanodisk and the nanodisk array grating period. The proposed metasurface design is promising to realize open cavity strongly coupled optical systems operating at room temperatures.

Deterministic Excitation of Polarization-Sensitive Extrinsic Anapole State in Si Nanodisk Clusters.

Nanoparticle clusters provide new degrees of freedom for light control due to their mutual interaction compared with an individual one. Here, the authors demonstrate theoretically and experimentally a type of optical anapole (a nonradiating state) termed as extrinsic anapole, with mode field spreading across Si nanodisk dimers unlike the intrinsic one that is confined within individual nanodisks. The extrinsic anapole is sensitive to the polarized excitation. When the electric vector E of excitation is perpendicular to the dimer axis, the coupled toroidal dipole (TD) mode is largely enhanced and broadened to be spectrally overlapped with the electric dipole (ED) mode. The destructive interference of these two modes results in the generation of the extrinsic anapole. However, it vanishes when E is parallel to the dimer axis. Such polarization dependence can be relieved with the participation of the third nanodisk. Scattering spectra of Si nanodisk trimers stay almost unchanged under different polarized excitations, although the near-field distributions are quite different. Finally, enhanced white-light emission is observed in Si nanodisk clusters, which can be attributed to the near-infrared absorption enhancement induced by extrinsic anapole states. The findings manifest that high-index all-dielectric nanodisk clusters are promising for light manipulation based on mode interference.

Tailoring magnetic dipole emission by coupling to magnetic plasmonic anapole states

The interaction between magnetic quantum emitters and the local electromagnetic environment is a promising method to manipulate the spontaneous emission. However, it is severely limited by the weak interactions between the magnetic component of light and natural materials. Herein, we demonstrate that the special type of anapole states associated with the “onefold” electric toroidal dipole moment can be excited by efficient interaction between magnetic dipole emitters and silver oligomers. Based on magnetic anapole states, the radiative power is effectively suppressed with significant coupling between the emitter and the silver nonamer, physically providing an ideal playground for the study of non-radiative transitions. These findings not only introduce magnetic anapoles to plasmonics but also open a door for the development of new high-performance magnetic-dipole-based optoelectronic devices.

Exploring Mie Resonances, Anapole States, and Anapole-Exciton Polaritons in Nanopatterned TMD Materials Using STEM EELS

Exploring Mie Resonances, Anapole States, and Anapole-Exciton Polaritons in Nanopatterned TMD Materials Using STEM EELS

Anapole States in Gap-Surface Plasmon Resonators.

Anapole states associated with the destructive interference between dipole and toroidal moments result in suppressed scattering accompanied by strongly enhanced near fields. In this work, we comprehensively examine the anapole state formation in metal-insulator-metal configurations supporting gap surface-plasmon (GSP) resonances that are widely used in plasmonics. Using multipole decomposition, we show that in contrast to the common case of dielectric particles with out-of-phase superposition of electric and toroidal dipoles anapole states in GSP resonators are formed due to the compensation of magnetic dipole moments. Unlike anapole states in dielectric particles, magnetic anapole states in GSP resonator does not provide a pronounced suppression of scattering, but it features huge electric field enhancement, which we verify by numerical simulations and two-photon luminescence measurements. This makes the GSP resonator configuration very promising for use in a wide range of applications, ranging from nonlinear harmonic generation to absorption enhancement and sensing.

Achieving maximum scattering circular dichroism through the excitation of anapole states within chiral Mie nanospheres

The chirality-dependent asymmetric light-matter interaction can be enhanced using various artificial photonic structures as well as engineered incident field. Compared with chiral photonic structures, the engineered optical field is less explored and recently recognized as a new regime in tailoring light-matter interaction. In this work, we demonstrate that weakly chiral spheres can exhibit the maximum scattering circular dichroism (CD) by tailoring the incident field to construct chirality-sensitive anapole states. By considering the chiral terms radiated from Mie nanospheres as perturbation, a multiscattering model is established to predict the chiroptical Mie scattering response of these nanospheres. This model provides a specific guideline to tailor the incident field to realize the upper limit of the achievable scattering CD.

Mass Fabrication of WS2 Nanodisks and their Scattering Properties

AbstractHigh‐index all‐dielectric resonators have been developed into an important platform for light manipulation at the nanoscale over the past decade. Although they are widely used as 2D materials, transition metal dichalcogenides (TMDCs), as an emerging all‐dielectric material, have also been used to fabricate optical nanoantennas that support multipolar Mie resonances. However, their fabrication depends heavily on electron‐beam lithography (EBL) or focused ion beam (FIB), which is expensive and time‐consuming for practical applications. To address this issue, here, a fast low‐cost method is put forward which combines polystyrene (PS) nanospheres with physical vapor deposition by electron‐beam evaporation and magnetron sputtering to fabricate WS2 nanodisks in a mass‐production manner. After annealing, the A‐ and B‐exciton features as well as anapole states are observed in the scattering spectra of WS2 nanodisks. The light scattering anisotropy of individual WS2 nanodisks and spectral tunability of the anapole are studied. In addition, absorption enhancement due to the strong field localization of anapole states in hexagonal WS2 nanodisk arrays is numerically demonstrated. This work manifests that this etching‐free method is promising for fabrication of scalable TMDC nanodisks suitable for practical applications.

Optical Kerr nonlinearity of dielectric nanohole array metasurface in proximity to anapole state.

Metasurfaces have attracted a great deal of attention from researchers due to their prominent optical properties. In particular, metasurfaces may consist of structures possessing optical anapole resonances with strong field confinement and substantially suppressed scattering. As a result, such nanostructures display enhanced nonlinear optical properties. In this paper by means of three-dimensional finite-difference time-domain simulations, the ability of anapole modes in high-index dielectric metasurfaces with circular nanopores is shown. In the vicinity of the anapole state, the effective optical Kerr nonlinearity increases by orders of magnitude. Simultaneously, the optical transmission of the metasurface can reach high values up to unity.