Strong Coupling between Dark Plasmon and Anapole Modes

Octupole plasmon resonance improves light enhancement by a metal nanodimer

Metal nanoparticles are extensively used in science and technology to resonantly confine and enhance optical fields. Highest enhancement factors are achieved in nanosized gaps of metal dimers. It is commonly assumed that higher-order plasmon resonances, such as electric quadrupole and octupole, are in nanoparticles much weaker than a dipole resonance. Indeed, in the classical multipole expansion that deals with the scattered fields, these "dark" multipoles can be invisible. In this work, we show that an octupole resonance in a metal nanodimer can lead to a substantially larger field enhancement than a dipole resonance. The effect is explained by the fact that the near-field enhancement provided by the excited electric currents can be strong when the excitation is dark. This finding extends the design principles of a plasmonic nanostructure toward higher-order multipoles that, being naturally narrowband, can be useful for a variety of applications, especially in plasmonic sensing and detection.

Relations between near-field enhancements and Purcell factors in hybrid nanostructures of plasmonic antennas and dielectric cavities

Strong near-field enhancements (NFEs) of nanophotonic structures are believed to be closely related to high Purcell factors (FP). Here, we theoretically show that the correlation is partially correct; the extinction cross section (σ) response is also critical in determining FP. The divergence between NFE and FP is especially pronounced in plasmonic-dielectric hybrid systems, where the plasmonic antenna supports dipolar plasmon modes and the dielectric cavity hosts Mie-like resonances. The cavity's enhanced-field environment can boost the antenna's NFEs, but the FP is not increased concurrently due to the larger effective σ that is intrinsic to the FP calculations. Interestingly, the peak FP for the coupled system can be predicted by using the NFE and σ responses. Furthermore, the limits for FP of coupled systems are considered; they are determined by the sum of the FP of a redshifted (or modified, if applicable) antenna and an individual cavity. This contrasts starkly with the behavior of NFE which is closely associated with the multiplicative effects of the NFEs provided by the antenna and the dielectric cavity. The differing behaviors of NFE and FP in hybrid cavities have varied impacts on relevant nanophotonic applications such as fluorescence, Raman scattering and enhanced light-matter interactions.

Strong Optical Coupling of Lattice Resonances in a Top-down Fabricated Hybrid Metal-Dielectric Al/Si/Ge Metasurface

Optical metasurfaces enable the manipulation of the light-matter interaction in ultrathin layers. Compared with their metal or dielectric counterparts, hybrid metasurfaces resulting from the combination of dielectric and metallic nanostructures can offer increased possibilities for interactions between modes present in the system. Here, we investigate the interaction between lattice resonances in a hybrid metal-dielectric metasurface obtained from a single-step nanofabrication process. Finite-difference time domain simulations show the avoided crossing of the modes appearing in the wavelength-dependent absorptance inside the Ge upon variations in a selected geometry parameter as evidence for strong optical coupling. We find good agreement between the measured and simulated absorptance and reflectance spectra. Our metasurface design can be easily incorporated into a top-down optoelectronic device fabrication process with possible applications ranging from on-chip spectroscopy to sensing.

Plasmon-induced magnetic anapole mode assisted strong field enhancement

Optical metamaterials, sensing, nonlinear optics, and surface-enhanced spectroscopies have witnessed the remarkable potential of the anapole mode. While dielectric particles with a high refractive index have garnered significant attention in recent years, the exploration of plasmonic anapole modes with intense localized electric field enhancements in the visible frequency range remains limited. In this study, we present a theoretical investigation on the relationship between the strongest near-field response and magnetic anapole modes, along with their substantial enhancement of Raman signals from probing molecules. These captivating findings arise from the design of a practical metallic oblate spheroid-film plasmonic system that generates magnetic anapole resonances at frequencies within the visible-near-infrared range. This research not only sheds light on the underlying mechanisms in a wide range of plasmon-enhanced spectroscopies but also paves the way for innovative nano-device designs.

Low loss sensitivity of the anapole mode in localized defective nanoparticles

The excitation of a nonradiating anapole in a high-index dielectric nanosphere is an effective pathway for enhancing light absorption. Here, we investigate the effect of localized lossy defects on the nanoparticle based on Mie scattering and multipole expansion theories and find its low sensitivity to absorption loss. The scattering intensity can be switched by tailoring the defect distribution of the nanosphere. For a high-index nanosphere with homogeneous loss distributions, the scattering abilities of all resonant modes reduce rapidly. By introducing loss in the strong field regions of the nanosphere, we achieve independent tuning of other resonant modes without breaking the anapole mode. As the loss increases, the electromagnetic scattering coefficients of the anapole and other resonant modes show opposite trends, along with strongly suppressed corresponding multipole scattering. While regions with strong electric fields are more susceptible to loss, the anapole's inability to emit or absorb light as a dark mode makes it hard to change. Our findings provide new opportunities for the design of multi-wavelength scattering regulation nanophotonic devices via local loss manipulation on dielectric nanoparticles.

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.

Hybrid Graphene-Supported Aluminum Plasmonics

Controlled fabrication of devices for plasmonics on suspended graphene enables obtaining tunable localized surface plasmon resonances (LSPRs), reducing the red-shift of LSPRs, and creating hybrid 3D-2D systems promising for adjustable dipole-dipole coupling and plasmon-mediated catalysis. Here, we apply a low-cost fabrication methodology to produce patterned aluminum nanostructures (bowties and tetramers) on graphene monolayers via electron-beam lithography and trap platinum (Pt) nanoclusters (NCs) within their hotspots by thermal annealing. We reveal the LSPRs of aluminum plasmonics on graphene using electron energy-loss spectroscopy (EELS) and energy-filtered transmission electron microscopy (EFTEM) in a monochromated scanning transmission electron microscope (STEM). The LSPRs of these nanostructures are measured to be between visible and ultraviolet regions of the spectrum and are confirmed by electromagnetic simulations. The antibonding dipole and bonding dipole modes of both structures are tuned by controlling their gap size. The tetramers enable the simultaneous excitation of both antibonding and bonding dipole modes at the poles of nanoprisms, while bowties allow us to excite these modes separately either at the poles or within the hotspot. We further show that the hybrid nanocavity-NC systems are in the intermediate coupling regime providing an enhanced plasmon absorption in the Pt NCs via the energy transfer from the antibonding dipole mode to the Pt NCs. The dipole LSPR of Pt NCs also couples to the bonding-type breathing mode in bowties. Our findings suggest that these hybrid nanocavity-graphene systems are of high application potential for plasmon-mediated catalysis, surface-enhanced fluorescence, and quantum technologies.

Toroidal dipole-modulated dipole-dipole double-resonance in colloidal gold rod-cup nanocrystals for improved SERS and second-harmonic generation

Colloidal metal nanocrystals (NCs) show great potential in plasmon-enhanced spectroscopy owing to their attractive and structure-depended plasmonic properties. Herein, unique Au rod-cup NCs, where Au nanocups are embedded on the one or two ends of Au nanorods (NRs), are successfully prepared for the first time via a controllable wet-chemistry strategy. The Au rod-cup NCs possess multiple plasmon modes including transverse and longitudinal electric dipole (TED and LED), magnetic dipole (MD), and toroidal dipole (TD) modulated LED resonances, producing large extinction cross-section and huge near-field enhancements for plasmon-enhanced spectroscopy. Particularly, Au rod-cup NCs with two embedded cups show excellent surface-enhanced Raman spectroscopy (SERS) performance than Au NRs (75.6-fold enhancement excited at 633 nm) on detecting crystal violet owing to the strong electromagnetic hotspots synergistically induced by MD, LED, and TED-based plasmon coupling between Au cup and rod. Moreover, the strong TD-modulated dipole-dipole double-resonance and MD modes in Au rod-cup NCs bring a 37.3-fold enhancement of second-harmonic generation intensity compared with bare Au NRs, because they can efficiently harvest photoenergy at fundamental frequency and generate large near-field enhancements at second-harmonic wavelength. These findings provide a strategy for designing optical nanoantennas for plasmon-enhanced applications based on multiple plasmon modes.

ELECTRONIC SUPPLEMENTARY MATERIAL

Supplementary material (SEM image of Au rod-one-cup NCs; TEM image of Au/PbS hybrids; SEM image of Au rod-two-cup NCs; low-amplification SEM image of Au rod-two-cup NCs; experimental extinction and calculated electric field distributions of Au NR excited at different wavelengths; calculated absorption and scattering spectra of Au rod-one-cup NCs; schematic illustration of the cut plane and the corresponding magnetic field distribution under L3 excitation; Raman spectra of CV (10^-6 M) adsorbed on Au rod-cup NCs with different cup sizes; calculated magnetic field distribution of Au rodcup NCs excited at 532 and 633 nm; calculated electric field distributions of Au rod-one-cup NC excited at 600 nm along TE and LE; the models of Au rod-cup NCs used in the simulations) is available in the online version of this article at 10.1007/s12274-022-4562-5.

Strong Dipole-Quadrupole-Exciton Coupling Realized in a Gold Nanorod Dimer Placed on a Two-Dimensional Material

Simple systems in which strong coupling of different excitations can be easily realized are highly important, not only for fundamental research but also for practical applications. Here, we proposed a T-shaped gold nanorod (GNR) dimer composed of a long GNR and a short GNR perpendicular to each other and revealed that the dark quadrupole mode of the long GNR can be activated by utilizing the dipole mode excited in the short GNR. It was found that the strong coupling between the dipole and quadrupole modes can be achieved by exciting the T-shaped GNR dimer with a plane wave. Then, we demonstrated the realization of strong dipole-quadrupole-exciton coupling by placing a T-shaped GNR on a tungsten disulfide (WS2) monolayer, which leads to a Rabi splitting as large as ~299 meV. It was confirmed that the simulation results can be well fitted by using a Hamiltonian based on the coupled harmonic oscillator model and the coupling strengths for dipole-quadrupole, dipole-exciton and quadrupole-exciton can be extracted from the fitting results. Our findings open new horizons for realizing strong plasmon-exciton coupling in simple systems and pave the way for constructing novel plasmonic devices for practical applications.

Doubly mirror-induced electric and magnetic anapole modes in metal-dielectric-metal nanoresonators

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 mediated giant photothermal nonlinearity in nanostructured silicon

Featured with a plethora of electric and magnetic Mie resonances, high index dielectric nanostructures offer a versatile platform to concentrate light-matter interactions at the nanoscale. By integrating unique features of far-field scattering control and near-field concentration from radiationless anapole states, here, we demonstrate a giant photothermal nonlinearity in single subwavelength-sized silicon nanodisks. The nanoscale energy concentration and consequent near-field enhancements mediated by the anapole mode yield a reversible nonlinear scattering with a large modulation depth and a broad dynamic range, unveiling a record-high nonlinear index change up to 0.5 at mild incident light intensities on the order of MW/cm². The observed photothermal nonlinearity showcases three orders of magnitude enhancement compared with that of unstructured bulk silicon, as well as nearly one order of magnitude higher than that through the radiative electric dipolar mode. Such nonlinear scattering can empower distinctive point spread functions in confocal reflectance imaging, offering the potential for far-field localization of nanostructured Si with an accuracy approaching 40 nm. Our findings shed new light on active silicon photonics based on optical anapoles.