Double toroidal spoof localized surface plasmon resonance excited by two types of coupling mechanisms

Actively tunable toroidal response in microwave metamaterials

Toroidal dipole moment has attracted much attention in recent years due to their novel electromagnetic response such as non-reciprocal interactions and unusual low-radiating manifestations. However, most of the previously reported toroidal dipole moment are incapable of real-time control of direction and intensity. In this paper, an actively tunable toroidal metamaterials are proposed to achieve programmable toroidal dipole manipulations with electric control. The intensity and direction of toroidal dipole can be sensitively regulated by electrically controlling the loaded diodes. Our proof-of-concept experiments show that the toroidal dipole could be dynamically switched to the electric and magnetic dipole. Meantime, the direction of toroidal dipole also could be controlled. Experimental and numerical results, in good agreement, demonstrate good performance of the proposed toroidal metamaterials, with potential applications in modulators, sensors, and filters.

Incident-angle-insensitive toroidal metamaterial

The incident-angle-insensitive toroidal dipole resonance on an asymmetric double-disk metamaterial is investigated in the near infrared band. Numerical results show that when the incident angle of excitation light varies from 0° to 90°, our metastructure not only always maintains stable toroidal dipole resonance characteristics, but also presents an excellent local field confinement. Under normal incidence, the polarization angle accessible to a dominant toroidal dipole resonance can be expanded to 70° in spite of the weakened electric field amplitude probed in the gap-layer. Moreover, the dependent relationships of toroidal dipole resonance on the radial asymmetry Δr and gap distance are also explored. The local electric field amplitude can also reach a maximum by structural optimization. The works enrich the research of toroidal moment and provide more application potentials in optical devices.

Multipole resonance and Vernier effect in compact and flexible plasmonic structures

Spoof surface plasmons in corrugated metal surfaces allow tight field confinement and guiding even at low frequencies and are promising for compact microwave photonic devices. Here, we use metal-ink printing on flexible substrates to construct compact spoof plasmon resonators. We clearly observe multipole resonances in the microwave frequencies and demonstrate that they are still maintained even under significant bending. Moreover, by combining two resonators of slightly different sizes, we demonstrate spectral filtering via the Vernier effect. We selectively address a target higher-order resonance while suppressing the other modes. Finally, we investigate the index-sensing capability of printed plasmonic resonators. In the Vernier structure, we can control the resonance amplitude and frequency by adjusting a resonance overlap between two coupled resonators. The transmission amplitude can be maximized at a target refractive index, and this can provide more functionalities and increased design flexibility. The metal-ink printing of microwave photonic structures can be applied to various flexible devices. Therefore, we expect that the compact, flexible plasmonic structures demonstrated in this study may be useful for highly functional elements that can enable tight field confinement and manipulation.

Toroidal dipole resonance in an asymmetric double-disk metamaterial

Toroidal dipole response in metamaterials was usually based on a complex structure with special arrangements or symmetries. In this paper, we propose an asymmetric double-disk metamaterial to numerically and experimentally demonstrate the toroidal dipole response in microwave frequency range. When the upper disk has an offset angle θ ranging from 0 to 100 degrees with respect to the lower one, the toroidal dipole resonance always plays the decisive role, which has been proved by calculating the scattered power in terms of the multipole scattering theory. Besides, the dependence of toroidal dipole response on structural parameters has been explored. Our works enrich the research of toroidal moment and, meanwhile, present more application potentials in meta-devices from microwave to optical regime.

Anapole moment of localized spoof plasmon polaritons based on a hybrid coupling mechanism

In the past several years, the anapole mode has received increasing interest and has been used in numerous applications. However, little relevant work exists on localized spoof plasmon polaritons (LSSPs), which are limited by the excitation of the electric dipole in a symmetrical structure. The lack of an electric dipole makes the excitation of the anapole moment difficult. In this study, we experimentally demonstrate that compact planar metadisks can exhibit a radiationless anapole mode of LSSPs at microwave frequencies. By integrating large and small split-ring resonators, the strong interaction between conductive and inductive coupling is shown to excite the electric dipole. The necessary condition for excitation of the electric dipole using the hybrid coupling mechanism is derived by analyzing equivalent LCR circuits. The proposed structure exhibits nearly equal magnitudes of the toroidal and electric dipoles. Moreover, analytical and numerical approaches are developed to explain the physics of the hybrid coupling mechanism accurately. Further experimental measurements confirm the theoretical predictions.

Enhanced toroidal localized spoof surface plasmons in homolateral double-split ring resonators

In this paper, toroidal localized spoof surface plasmons (LSSPs) based on homolateral double-split ring resonators is proposed and experimentally demonstrated at microwave frequencies. By introducing a new split in the conventional single-split ring resonator, the magnetic field in resonator is locally modified. The double-split ring resonator can create the mixed coupling in the structure, leading to the enhancement of magnetic field. Both numerical simulations and experiments are in good agreement. Compared with traditional toroidal LSSPs based on the single-split ring resonators, the imperfection of toroidal LSSPs is resolved, the intensity of toroidal resonance and the figure of merit (FoM) are significantly enhanced. To understand and clarify the enhanced magnetic field phenomena, we analyze the role of the double-split ring resonator. The effect of location of source and spacing between two splits on the resonance intensity are also discussed. A higher intensity of toroidal LSSPs resonance could be achieved by changing the spacing between two splits. Additionally, it is experimentally demonstrated that the enhanced toroidal LSSPs resonance is sensitivity to the background medium. The results of our research provide a new idea for exciting the enhanced toroidal dipole.

Selective amplification of spoof localized surface plasmons

Here, selective amplification of spoof localized surface plasmons (LSPs) has been demonstrated, where two coupling stubs gathering energies from the spoof LSPs resonator are designed on the back of the corrugated metal-insulator-metal ring resonator. The quadrupole mode is selected and amplified through the coupling stubs and the incorporated amplifier chip, and the measured transmission intensity has been increased from $ - {6.46}\;{\rm dB}$-6.46dB to 10.74 dB by adjusting the bias voltage. The amplification mechanism is fully investigated by using the circuit simulation and the full-wave simulation. The numerical simulations and experimental measurement agree well with each other. The active amplified resonator can be widely used in chemical and biological sensing in microwave frequency.

Propagation of toroidal localized spoof surface plasmons using conductive coupling

Here, on a platform of two split-ring resonator (SRR) disks in the microwave regime, we have numerically and experimentally investigated the coupling of toroidal localized spoof surface plasmons (LSSPs). The coupling effect is investigated both theoretically and experimentally. We observe that magnetic dipole coupling exists in the toroidal LSSPs coupling and causes a rearrangement of the toroidal LSSPs, which suppresses the propagation of toroidal LSSPs. To realize the propagation of toroidal LSSPs, we introduce conductive coupling into the SRR disks. The conductive coupling can correct magnetic dipole coupling and enhance toroidal LSSPs coupling. Both numerical simulations and experiments are in good agreement. The toroidal LSSPs can be effectively propagated, even in the three right-angle-bent SRR disks. This study paves the way toward a better understanding of toroidal LSSPs coupling and finds many applications in the transfer of electromagnetic energy using toroidal moments.