Multipole Decomposition Research Articles

Ultra-narrow-band circular dichroism by surface lattice resonances in an asymmetric dimer-on-mirror metasurface.

Narrow-linewidth circular dichroism (CD) spectroscopy is a promising candidate to push the limits of molecular handedness detection toward a monolayer or even to a single molecule level. Here, we designed a hybrid metasurface consisting of a periodic array of symmetry-breaking dielectric dimers on a gold substrate, which can generate strong CD of 0.44 with an extremely-narrow linewidth of 0.40 nm in the near-infrared. We found that two surface lattice resonance modes can be excited in the designed metasurface, which can be superimposed in the crossing spectral region, enabling a remarkable differential absorption with a high Q-factor for circular polarizations. The multipole decomposition of the resonance modes shows that the magnetic dipole component contributes most to the CD. Our simulation results also show that the CD response of the chiral structure can be engineered by modulating the structural parameters to reach the optimal CD performance. Ultra-narrow-linewidth CD response offered by the proposed metasurface with dissymmetry provides new possibilities towards design of the high-sensitive polarization detecting, chiral sensing and efficient chiral light emitting devices.

Controlling Topology and Polarization State of Lasing Photonic Bound States in Continuum

AbstractRecently, optical bound states in continuum (BICs) incorporated with optical gain have been reported to exhibit lasing. Here, it is shown that each of the four BICs supported by C symmetric photonic crystal slab can be made to lase, allowing the control over the topological charge of the resulting laser beam. The type of each BIC and their topological charges are identified by imaging the far‐field polarization vortices of the lasing signal. Results are compared with experimentally obtained dispersions, finite element method simulations, and multipole decomposition method based on the microscopic polarization currents in the photonic crystal plane. A demonstration of multimode lasing of two non‐degenerate BICs with opposite topological charges is presented. The momentum space overlap of the BICs results in a unique polarization pattern. The study provides a generalizable example for engineering the topological properties of coherent light.

Polarization-independent near-infrared superabsorption in transition metal dichalcogenide Huygens metasurfaces by degenerate critical coupling

Strong near-infrared absorption of light in semiconducting transition metal dichalcogenides (TMDs) is essential for improving the photocarrier extraction efficiency in optoelectronic devices. Here, we numerically demonstrate that an original TMD Huygens metasurface is specifically designed to overcome the 50% absorptance limit of a subwavelength thin film. The unique metasurface comprising a TMD nanodisk array shows evidence of characteristic Mie resonances including electric and magnetic dipoles. By carefully optimizing the aspect ratio of nanodisks, the extraordinary spectral overlapping of orthogonal electric and magnetic dipole resonances is successfully realized and, thus, enables the super-high absorptance up to 87% in a subwavelength-thin ($\ensuremath{\approx}0.19{\ensuremath{\lambda}}_{0}$) semiconductor structure by degenerate critical coupling at the wavelength of ${\ensuremath{\lambda}}_{0}=903\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$. All numerical results are confirmed by the temporal coupled-mode theory and again via the multipole decomposition method. Importantly, the near-unity absorptance in this TMD Huygens metasurface by degenerate critical coupling not only is tailored throughout the entire near-infrared region but also is both polarization independent and angle insensitive. The absorptance maintaining >80% is observed with the incident angle up to \ifmmode\pm\else\textpm\fi{}10 \ifmmode^\circ\else\textdegree\fi{} at least. Our findings are not only of fundamental interest but could also offer a platform for TMD-based high-speed photodetecting, energy harvesting, and thermal emission.

Collective topological corner modes in all-dielectric photonic crystal supercell arrays.

In this Letter, we propose collective topological corner modes in all-dielectric photonic crystal (PhC) supercell arrays, where each supercell is a second-order topological insulator. We show that coupled multipole corner modes are embedded in surrounding bulk modes at the Γ point even without the band gap, and individual or superposed dipole corner modes are selectively excited with collective behaviors by incident plane waves. These collective modes possess high-quality factors with an optimized thickness of the slab, and multipole decomposition reveals they are dominated by toroidal dipole and magnetic quadrupoles. Finally, we shrink the nontrivial region in each supercell to one unit-cell limit, where we show that collective corner modes still exist. Potential large-area topological applications are also discussed.

Applicability of multipole decomposition to plasmonic- and dielectric-lattice resonances.

Periodic nanoparticle arrays have attracted considerable interest recently since the lattice effect can lead to spectrally narrow resonances and tune the resonance position in a broad range. Multipole decomposition is widely used to analyze the role of the multipoles in the resonance excitations, radiation, and scattering of electromagnetic waves. However, previous studies have not addressed the validity and accuracy of the multipole decomposition around the lattice resonance. The applicability of the exact multipole decomposition based on spherical harmonics expansion has not been demonstrated around the lattice resonance with the strong multipole coupling. This work studies the two-dimensional periodic arrays of both plasmonic and dielectric nanospheres and compares the multipole decomposition results with the analytic ones around their lattice resonances. We study both the effective polarizabilities of multipoles and the scattering spectra of the structures. The analytical results are calculated from the coupled dipole-quadrupole model. This study demonstrates that the exact multipole decomposition agrees well with the numerical simulation around lattice resonances. Only a small number of multipoles are required to represent the results accurately.

Bound states in the continuum from a symmetric mode with a dominant toroidal dipole resonance

Bound states in the continuum (BICs), both the symmetry-protected and accidental ones, can be realized in the band of antisymmetric mode in a nanoparticle chain as long as the particles radiate exactly out of phase pairwisely. However, for the symmetric mode, BICs cannot form in this way. Here, we show that accidental BICs can be generated from the symmetric mode, as exemplified by a toroidal dipole mode-induced BIC in a trimerized system. This type of BIC is fully studied by an eigenmode analysis as well as multipole decomposition. It is also demonstrated that the BICs correspond to certain phase singularity of the system, indicating the robust existence of the BICs ensured by their topological nature. The combination of BICs and toroidal dipole resonance may find applications in lasing and subwavelength waveguiding.

Electromagnetically induced transparency based on magnetic toroidal mode of dielectric reverse-symmetric spiral metasurfaces

The intriguing properties of the toroidal mode (TM) resonance can potentially promote a low-loss light–matter interaction. This study proposes an electromagnetically induced transparency (EIT) resonance with a high quality factor, which can reach 7798, and low mode volume can reach 0.009 μm3, high contrast ratio can reach nearly 100%, in the near-infrared region, which is generated by the magnetic TM in a reverse-symmetric coupling spiral metasurface. A two-oscillator model can only explain the influence of near-field coupling at the EIT point for weak coupling. Moreover, a multipole decomposition method shows that the excitation mechanism of EIT resonances originates from the destructive interference between the subradiant modes (magnetic toroidal dipole-electric quadrupole) and magnetic dipole resonance. Consequently, a new general extinction spectrum interference model is applied to fit all coupling conditions for both weak and strong coupling results that perfectly correspond to the multipole decomposition method. The results of this study could be useful in the analysis and understanding of the electromagnetic coupling characteristics of nanoparticles and provide a design approach for novel metasurfaces for low-loss optical applications.

Broadband unidirectional transverse light scattering in a V-shaped silicon nanoantenna

The efficient manipulation of light-matter interactions in subwavelength all-dielectric nanostructures offers a unique opportunity for the design of novel low-loss visible- and telecom-range nanoantennas for light routing applications. Several studies have achieved longitudinal and transverse light scattering with a proper amplitude and phase balance among the multipole moments excited in dielectric nanoantennas. However, they only involve the interaction between electric dipole, magnetic dipole, and up to the electric quadrupole. Here, we extend and demonstrate a unidirectional transverse light scattering in a V-shaped silicon nanoantenna that involves the balance up to the magnetic quadrupole moment. Based on the long-wavelength approximation and exact multipole decomposition analysis, we find the interference conditions needed for near-unity unidirectional transverse light scattering along with near-zero scattering in the opposite direction. These interference conditions involve relative amplitude and phases of the electromagnetic dipoles and quadrupoles supported by the silicon nanoantenna. The conditions can be applied for the development of either polarization- or wavelength- dependent light routing on a V-shaped silicon and plasmonic nanoantennas.

Excitations of Multiple Fano Resonances Based on Permittivity-Asymmetric Dielectric Meta-Surfaces for Nano-Sensors

We have designed an all-dielectric device based on permittivity-asymmetric rectangular blocks on meta-surfaces, yielding multiple Fano resonances with high sensitivity and high figure of merit (FOM) in the near-infrared regime. By introducing different materials to break the permittivity-symmetry, three sharp Fano peaks are generated with high Q-factors arising from the interference between the sub-radiant modes and the magnetic dipole resonance modes. Combining the field distributions and the multipole decomposition in Cartesian coordinates, the resonance modes are analyzed to be magnetic quadrupoles (MQs) and magnetic dipoles (MDs). Furthermore, the dependence on materials and geometric parameters has been studied. The maximal sensitivity, FOM and Q-factor reach 394 nm/RIU, 4925 and 14437, respectively. This proposed structure provides a good alternative to geometry-asymmetric meta-surface structures and may be used for multichannel sensing, nonlinear optical devices, and laser.

Highly electric field enhancement induced by anapole modes coupling in the hybrid dielectric–metal nanoantenna

Plasmonic nanoantenna has received tremendous attention in various optoelectronic applications owing to their unique optical characteristics with strong localized electric field enhancement. However, the highly intrinsic dissipation losses of metallic nanoantennas limits its application development. Coupling anapole modes of dielectric nanostructure with plasmonic mode of metallic nanoantenna seems to provide a promising alternative to further boost the electric field intensity with low dissipation losses. Herein, we demonstrate that the electric field intensity from a metallic nanoantenna can be remarkably heightened through introducing a dielectric nanostructure to build hybrid dielectric–metal nanoantenna, where the hybrid nanoantenna is composed of a gold nanorod dimer and a slotted silicon nanodisk. Through rationally optimizing the structural parameters, the hybrid nanoantenna exhibits an intensified resonant electric field intensity (> 3700 folds) at the gap of the dimer nanoantenna, which exceeds 30 times higher than that of the individual dimer nanoantenna. The far-field scattering characteristics and the near-field electromagnetic field distributions are systematically investigated to elucidate the field enhancement mechanism via utilizing numerical simulations and multipole decomposition analysis. It can be confirmed that the highly electric field intensity can be primarily attributed to the mode coupling between plasmonic resonances of Au nanoantenna and the radiationless anapole modes supported by the slotted Si nanodisk. The proposed hybrid dielectric–metal nanoantenna can serve as an effective platform to amplify the radiative decay rate and Raman scattering intensity, which paves a prospective avenue in the field of single-molecule surface enhanced spectroscopy.

Ultra-high Q-factor toroidal dipole resonance and magnetic dipole quasi-bound state in the continuum in an all-dielectric hollow metasurface

Fano resonance with high Q-factor plays a pivotal role in the field of optoelectronic applications, especially refractive index sensing. However, ultra-high Q-factor for metallic metasurfaces has been a challenge due to their great ohmic losses. Herein, we propose and numerically analyze an all-dielectric hollow metasurface in the near infrared region which consists of silicon cylinders with two asymmetric rectangular hollows. A sharp Fano resonance with modulation depth close to 100% excited by quasi-bound state in continuum, whose Q-factor can reach 8428 when = 40 nm. With the Cartesian multipole decomposition technique, the two excited Fano resonances can be characterized by toroidal dipole response and magnetic dipole (MD) response, respectively. It is worth noting that the Q-factor of MD mode can reach 17106. Moreover, the dependence of the transmission spectra on different geometric parameters is investigated as well. Due to their narrow linewidths and strong near-field confinement, the proposed structure can be applied to a refractive index sensor, yielding a maximum sensitivity (S) of 160 nm RIU−1 and a maximum figure of merit of 575 RIU−1. It is believed that the proposed structure can offer an excellent prospect for biomedical, agriculture, and chemical sensing applications.

Millimeter-Wave Measurements and Applications of Dipole Modes in Spherical Alumina Dimer

Dielectric dimers composed of spherical Alumina ceramic resonators are studied. The electromagnetic scattering of dimer configurations is analyzed through spherical multipole decomposition. Different configurations of dimer operation are addressed with respect to propagation direction and polarization of the incident wave. In context to forward scattering caused by dipole interaction, measurements are performed in V-band (50–75 GHz) to quantify the corresponding electromagnetic signature of the dielectric dimer. Based on the fundamental dipole modes excited in the subwavelength free-space gap along the dimer axis, electric and magnetic hotspots are proposed, which can be exploited for near-field enhancement-based applications at millimeter-wave frequency. A dedicated frequency-scaled measurement setup is designed to measure the field-enhancement factor between 25 and 30 GHz in the free-space dimer gap, corresponding well to simulated results.

Chirality-selective all-dielectric metasurface structural color display

The polarization dependent switchable structural coloration has shown a prominence for its unnecessity of changing the structure itself to achieve tunable color displays. Nevertheless, a chirality-selective structural color display has been rarely elucidated. Here, we suggest a chirality-selective reflective structural color display under perpendicularly incident light based on all-dielectric metasurfaces. We first investigate a chiral response of a subwavelength thickness two-dimensional (2D) amorphous silicon (Si) structure. The multipole decomposition followed by the electromagnetic field distribution analysis explained the chirality-selective response of the metasurface with the chirality-selective excitation of magnetic dipole (MD) and electric quadrupole (EQ). We then analyzed the structural dependence of MD and EQ, finding a group of metasurfaces which can span the entire visible spectrum under left circularly polarized (LCP) light and show dark, faded colors under right circularly polarized (RCP) light. Our result provides design criteria for chirality-selective all-dielectric structural color displays, applicable to energy and time efficient real-time color switching displays.

Light focusing by silicon nanosphere structures under conditions of magnetic dipole and quadrupole resonances

Metalens is a planar device for light focusing. In this work, we design and optimize c-Si nanosphere metalenses working on the magnetic dipole and quadrupole resonances of the c-Si nanoparticle. Resonant optical response of c-Si nanostructures is simulated by the multipole decomposition method along with the zero-order Born approximation. Limitations of this approach are investigated. The obtained results of optimization are verified by simulation via the T-matrix method.

Strong coupling between excitons in a two-dimensional atomic crystal and quasibound states in the continuum in a two-dimensional all-dielectric asymmetric metasurface

We investigate the interaction between excitons in a two-dimensional atomic crystal and quasibound states in the continuum (q-BIC) in a two-dimensional all-dielectric asymmetric metasurface. By introducing coherent and incoherent coupling terms in a coupled oscillator model, we demonstrate the coexistence of coherent and incoherent coupling processes in the strongly coupled system and the resultant sub/superradiant polariton states. Based on the multipole decomposition method and near-field analysis of full wave simulations, we study the microscopic excitation of multipole components in q-BIC and their coupling to excitons. We reveal that not only the magnetic dipole but also the interference between electric dipole and its toroidal counterpart dominate in the exciton-BIC strong coupling regime. The fractions of the magnetic and electric dipole ingredients are modulated by the topology of the two-dimensional all-dielectric metasurface, exhibiting distinctly different features in the high- and low-energy hybrid modes in the strong coupling system. Our findings are expected to be of importance for both fundamental research in TMD-based light-matter interactions and practical applications in the design of novel, tunable exciton-polariton devices with high compactibility.

Broadband near infrared all-dielectric metasurface absorber

Here we present a broadband all-dielectric metasurface absorber, where elliptical nanohole arrays are entirely embedded into the doped device layer of silicon-on-insulator wafer. From the intraband carrier transitions, the device exhibits enhanced near infrared absorption with a 3-dB bandwidth ∼ 35.3 nm. By the method of detailed multipole decomposition, we find that magnetic dipole resonances predominantly contribute to the absorption enhancement; in addition, the large bandwidth is attributed to the superposition of multiple absorption peaks from the resonances. The absorber designed in this simple structure and compatible with the CMOS techniques is promising for the broadband photodetectors of Si and even Ge. Differently, for the Ge-based photodetectors, the photocarrier excitation originates from interband transitions.

Tunable Toroidal Response in a Reconfigurable Terahertz Metamaterial

AbstractCompared with the traditional electric and magnetic multipoles, the existence of a dynamic toroidal moment has received increasing interest in recent years. This is due to its novel electromagnetic response, including dynamic non‐radiating charge‐current configurations and non‐reciprocal interactions. Reconfigurable terahertz metamaterials where artificial toroidal metamolecules and traditional microelectromechanical systems bi‐material cantilever structures are integrated within the same unit cell are presented. Through modification of the bending angle by thermal actuation, the toroidal dipole intensity increases by five orders of magnitude in the out‐of‐plane direction with an overall increase in the toroidal intensity of nearly an order of magnitude. Terahertz time‐domain spectroscopy is used to determine the evolution of the transmission as a function of the bending angle. This enables numerical confirmation of the toroidal response using multipole decomposition with additional confirmation provided by phase analysis. The results demonstrate the use of bi‐material cantilevers to realize a tunable toroidal moment with potential applications in sensing and next‐generation communication technologies.

Actively Controlled Frequency-Agile Fano-Resonant Metasurface for Broadband and Unity Modulation

The active control to the local resonant mode of metasurface is a promising route for improving the operation bandwidth limitation of metasurface. Here, we propose and experimentally demonstrated the active tunabilities in a frequency-agile Fano-resonant metasurface. The metasurface with a pair of asymmetric split ring resonators is integrated with double varactor diodes for active control of the sharp Fano resonance. It is found that the sharp Fano-type spectrum appears due to the near-field interferences between the collective electric and magnetic dipole modes. The physical insight is revealed through local field analysis, multipole decomposition and temporal coupled-mode theory. It is also found that the metasurface can be employed as a broadband and unity modulator. Hopefully, our results could inspire sophisticated electrically controlled photonic devices with novel functions.

Polarization-independent anapole response of a trimer-based dielectric metasurface

Abstract The phenomenon of anapole has attracted considerable attention in the field of metamaterials as a possible realization of radiationless objects. We comprehensively study this phenomenon in the cluster-based systems of dielectric particles by considering conditions of anapole manifestation in both single trimers of disk-shaped particles and metamaterial composed on such trimers. Our analytical approach is based on the multipole decomposition method and the secondary multipole decomposition technique. They allow us to associate the anapole with the multipole moments of the trimer and the separate multipole moments of its constitutive particles. The manifestation of anapole in a two-dimensional metamaterial (metasurface) is confirmed by checking the resonant states in the reflected field as well as from the electromagnetic near-field patterns obtained from the full-wave numerical simulation. It is demonstrated that the anapole excitation in trimers results in the polarization-independent suppression of reflection with the resonant enhancement of local electromagnetic fields in the metasurface. Finally, experimental verification of the theoretical results is presented and discussed.

Polarization conversion in anisotropic dielectric metasurfaces originating from bound states in the continuum.

We use the semianalytical Cartesian multipole method to investigate the light transmission and reflection spectra of anisotropic dielectric metasurfaces, and extend the multipole decompositions method to account for cross-polarization conversion effects. We observe sharp high-Q resonances arising from a distortion of symmetry-protected bound states in the continuum in asymmetric dielectric metasurfaces, i.e., quasibound states in the continuum. In addition, by further introducing in-plane symmetric breaking perturbation, the polarization conversions of linearly polarized light can be achieved through quasibound states in the continuum. With the aid of temporal coupled-mode theory, we can obtain the limit of cross-conversion under single high-Q resonance, i.e., 0.25. Our work will help to design dielectric metasurfaces to control the polarization states of light.