Vertical Split-ring Resonators Research Articles

Manipulation on radiation angles via spatially organized multipoles with vertical split-ring resonators

Abstract Herein, the radiation patterns of single-split ring resonators (SSRRs) and double-split ring resonators (DSRRs) in the vertical direction are tailored by reconfiguring the resonator geometries. To design unequal arm lengths for controlling the floating split angle of the resonators and changing their electromagnetic multipole compositions, vertical metamaterials were fabricated using the metal-stress-driven self-folding method. The simulation results well agree with the experimental transmittance and reflectance results and demonstrate the geometry-dependent angle variation of the far-field radiation. Symmetric SSRRs and DSRRs radiate in the vertical and horizontal directions, respectively. With increasing pad shift, the radiation angle of the asymmetric SSRR completely rotates toward the horizontal direction along the ring plane, but the DSRRs can rotate only from 0° to 45° to the horizontal plane. Furthermore, by decomposing the multipoles into their constituents, we show that the directional scattering performance can be verified by manipulating the horizontal and vertical components of the electric dipoles. This novel combination of SSRRs and DSRRs can effectively and efficiently reconfigure the radiation direction in the infrared (IR) region, paving the way for color routers, metasurfaces, and directive IR emitters in compact optical metadevices.

Anapole state revealed by cloaking metallic cylinders with split ring resonators

This paper proposes the experimental demonstration of an anapole-based cylindrical electromagnetic cloaking scheme. An anapole state is excited by arranging around a cylindrical metallic target vertical split-ring resonators, forming an equivalent surface admittance boundary condition able to suppress the scattering. Using Mie formalism and multipole scattering theory, we identify the actual reason behind the cloaking operation, characterizing the anapole condition by the scattering contributions from toroidal and electric dipole moments. Numerical results are verified using full-wave simulation softwares and subsequently validated with back-scattering measurements inside an anechoic chamber.

Dual ultrahigh-Q Fano Resonances of 3D gap metamaterials for slow light from ultraviolet to visible range

We theoretically report an approach to support multispectral ultranarrow Fano resonances (FRs) from ultraviolet (UV) to visible region in a 3D metamaterial composed of aluminium vertical split-ring resonators with a nanogap (Al VSRRG). The plasmonic hybridization between the 1st order broad localized surface plasmon (LSP) resonance of the Al VSRRG and the 1st order narrow optical diffraction mode propagated in the dielectric substrate caused by the Wood anomaly, and the plasmonic hybridization between the 2nd order LSP resonance and the 2nd order optical diffraction mode produce the dual ultranarrow FRs with quality (Q) factors of 118 and 437 in the visible and UV regions, respectively. The simulated dual FRs phenomenon of the proposed Al VSRRG array is well explained by the coupled oscillator theory. In comparison with the Al VSRR without a gap, both the Q-factors of the double FR peaks and the localized electromagnetic field intensities of the Al VSRRG are further increased. The positions and line shapes of the ultrahigh-Q double FRs tuned by different geometric parameters are also investigated. Our designed simple 3D gap metamaterial has large group indexes of 210 and 240 in visible and UV regions, suggesting promising applications in slow light nanodevices.

Design of a Bandpass Filter with Mixed Electromagnetic Coupling Paths Using Vertical Split-Ring Resonators

We propose bandpass filters (BPFs) with mixed electromagnetic coupling paths (MEMCPs) that comprise two coupled vertical split-ring resonators (VSRRs) and present the equivalent circuit models of the coupled VSRRs. We demonstrate that the dominant coupling modes for the top and bottom layers of the VSRRs are magnetic (M) and electric (E), respectively, and that M-dominant coupling is required for high-selectivity BPFs. BPFs with narrow and wide bandwidths were designed based on the generalized coupling matrix. The proposed BPFs were fabricated and measured, and it is verified that the proposed BPFs have high selectivity due to the transmission zeros and a small circuit footprint due to the vertical structure of resonators. The fabricated narrow- and wide-band BPFs have the fractional bandwidths of 3.62% and 5.81%, respectively, with the overall size of 0.29 λ g × 0.043 λ g .

Extremely high Q-factor terahertz metasurface using reconstructive coherent mode resonance.

High Q-factor resonance has a pivotal role in wide applications for manipulating electromagnetic waves. However, high Q-factor resonance, especially in the terahertz (THz) regime, has been a challenge faced by plasmonic metamaterials due to the inherent ohmic and radiation losses. Here, we theoretically present a unique metasurface scheme to produce extremely high Q-factor Fano resonance of the reconstructive coherent mode in the THz regime. The THz metasurface is composed of periodically arranged vertical symmetric split ring resonators (SRRs), which can produce perfect reconstructive coherent coupling effect in the sense that dipole radiation is destructively suppressed. Under the polarized electric field perpendicular to SRR gap, the surface currents are out of phase for an individual SRR, leading to the cancellation of net dipole moment. The reconstructive coherent mode resonance can occur between each SRR and its neighboring SRRs, accompanied by destructive interference of the scattered fields of each SRR. This is due to the coupling between the localized resonance of individual particles and the Rayleigh anomaly of the array. The proposed metasurface can significantly suppress far-field radiation and perform an extremely high Q-factor beyond 104 level with large modulation depth in the THz region, which pushes the advancement of THz high Q-factor resonance. The extremely high Q-factor of reconstructive coherent mode is tunable by adjusting the geometry parameters. The design strategy is useful to develop ultra-sensitive sensors, narrow-band filters and strong interaction of field-matter in the THz regime.

High Sensing Properties of Magnetic Plasmon Resonance by Strong Coupling in Three-Dimensional Metamaterials

We present a method to enhance the sensing properties of a 3D metamaterial sensor. The concept is based on the combination of magnetic plasmon (MP) resonance in 3D metamaterials and a dielectric waveguide, in which a narrow-band hybrid MP resonance can be formed by the strong coupling of the MP resonance in the vertical split ring resonators (VSRRs) with a propagating optical waveguide mode in the dielectric waveguide. Because the hybrid MP resonance of the VSRRs are extremely sensitive to the changes in the refractive index of the material surrounding the VSRRs, the designed 3D metamaterial sensor has very high sensitivity (S = 300 nm/RIU, S <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">*</sup> = 16/RIU) and figure of merit (FOM = 30, FOM <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">*</sup> = 390), which could find potential applications in label-free biosensing.

3D cross-bended metasurfaces with polarization insensitivity and high-Q resonances

Metasurfaces with high-quality (HQ) resonances are always desired for their potential applications in biomedicine and photonic devices. However, the realization of HQ near-infrared metasurfaces with polarization insensitivity has faced problems with metallic loss, large feature size and limited configurations. In order to build up HQ metasurfaces working at the near-infrared range, we propose a 3D bended metasurface configuration consisting of a metallic supported frame and cross vertical split ring resonators, which are fabricated by a focused ion beam defined origami method. The relationship between Q factor/modulation depth of the resonances and the geometric morphology of the meta-atoms are established, and its polarization insensitivity is verified. Two HQ resonances with 22%/26% modulation depth and the quality factor of 24/30 at 1.75/2.5 μm are experimentally obtained in the optimized metasurface. In addition, a polarization insensitive Fano resonance with large modulation depth can also be observed at the low frequency band, which can be modulated by tuning the bending angle. This HQ 3D cross-bended metasurface will create a more flexible approach to design versatile micro/nano-photonic devices and biological components.

Ultraminiature Circularly Polarized RFID Antenna Inspired by Crossed Split-Ring Resonator

This article presents a novel ultraminiature circularly polarized (CP) antenna inspired by crossed split-ring resonators (SRRs), and its derivative forms with frequency and polarization re-configurability for radio frequency identification (RFID) applications. A novel resonator structure with two metal–insulator–metal (MIM) capacitors that is inspired by vertical SRR (VSRR) is first proposed, which could be easily miniaturized while achieving a reasonable high radiation gain. Two crossed dual-capacitance ring resonators (C-DRRs), which can be equivalent to two crossed magnetic dipoles, are then arranged perpendicularly and excited simultaneously with an SMA connector. Only an aluminum ground sheet, metallic screws, and a thin low-cost dielectric PCB board are used, where the dielectric loss is substantially minimized. This SRR-inspired CP antenna, as well as its frequency and polarization reconfigurable derivative by introducing a mechanically tunable inductor or using electrically controllable components, are analyzed and experimentally demonstrated, respectively. The developed antenna achieves an electrically small size of 0.135 $\lambda _{0}\,\,\times $ 0.135 $\lambda _{0}\,\,\times $ 0.045 $\lambda _{0}$ , and its size is considerably smaller than other reported compact RFID reader antennas. The measured −10 dB bandwidth, peak gain, and radiation efficiency are 3.1% (900–928 MHz), 5.51 dBic, and 86.5%, respectively, which agree perfectly with the simulation. They demonstrate superior advantages in terms of ultracompact size, low-cost, wide axial ratio (AR) beam-width, high efficiency, and easy fabrication/assembling for RFID-related applications.

Highly sensitive 3D metamaterial sensor based on diffraction coupling of magnetic plasmon resonances

We theoretically investigate the diffraction coupling of magnetic plasmon (MP) resonances in three-dimensional (3D) metamaterials comprising a rectangular array of metallic vertical split-ring resonators (VSRRs). By suspending 3D metamaterials into air to reduce the substrate effect, the interaction between MP resonances of VSRRs and a collective surface mode signaled by Wood anomaly in the periodic array can cause a narrow-band mixed mode with greatly enhanced magnetic fields. Due to the interaction, an anti-crossing phenomenon similar to Rabi splitting in atomic physics, is also observed and explained satisfactorily with the help of a coupled oscillator model. For the narrow-band mixed mode, the greatly enhanced magnetic fields in VSRR gaps are lifted off from the substrate to be fully exposed to the sensing medium. Therefore, the 3D metamaterials have good sensing performance factors (S = 900 nm/RIU, FOM = 38, and FOM* = 125), which are promising for label-free biochemical sensors.

Symmetry Breaking and Resonances Hybridization in Vertical Split Ring Resonator Metamaterials and the Excellent Sensing Potential

The resonances with narrow linewidths generated in metamaterials are valuable for numerous applications ranging from bio-sensing to narrow-band filter. And breaking structural symmetry of metamaterials is one of the common and efficient approaches to achieve such sharp resonances. In this paper, we present symmetry breaking introduced in vertical split ring resonators (VSRR) metamaterials to excite narrow resonance. Different from the previously reported planar asymmetric metamaterials, the three-dimensional metamaterials are dominantly excited by the magnetic field component of the terahertz illumination together with the electric component. In this case, sharp resonances with characteristic magnetic field distributions are excited. And the sharpest resonance shows up with an ultra-narrow line-width (full width at half maximum, FWHM is 5.90 GHz) and high Q of 327 at 1.93 THz. Through systematic theoretical analysis and numerical mode implementation, we reveal the sharp resonance dips are produced by resonance hybridization between a resonance mode induced by the destruction of the structural symmetry and another special resonance inherent in the symmetric VSRR metamaterials. In addition, the fabrication robustness and sensing potential of the asymmetric VSRR metamaterials are explored in this paper. The results show that the AVSRR design is insensitive to the errors in the fabrication process, like rough metal surface and smooth metal corner, but highly sensitive to the approaching substance, which prove the excellent sensing performance.

Solar panel integrated compact magneto-dielectric loaded sequentially rotated navigation antenna

In this paper we present a novel solar panel integrated navigation antenna for solar powered vehicles and devices. The radiator is a compact sequentially rotated patch array loaded with a magneto-dielectric (MD) layer to achieve miniaturization for ease of integration with solar panels. The magneto-dielectric substrate of the proposed antenna consists of MD unit cells created by vertical split ring resonators (SRR). The unit cells are arranged in 11 × 1 fashion beneath each radiator to reduce the dimension of the same to 70% with equivalent performance compared with the respective original patch radiators. An array of such radiators is formed and fed with a sequential feed network comprising T-junction power dividers and delay lines. The proposed antenna is then integrated and tuned with a solar panel and measured. The measured results show that the antenna operates in the L1 GPS band with an impedance bandwidth of 20 MHz centered at 1.575 GHz. The antenna is Right Handed Circularly Polarized (RHCP) with axial ratio bandwidth (ARBW) of 100% due to the sequential rotation. The peak Axial Ratio (AR) values are <3 dB with a peak gain of 7.89 dBic. The proposed solar panel integrated navigation antenna is developed with the view of using it as a multipurpose candidate in a compact shared dimension in solar powered vehicles and devices to provide power to the systems as well as global positioning and navigation of the same.

Second Harmonic Light Manipulation with Vertical Split Ring Resonators.

The second harmonic generation (SHG) of vertical and planar split-ring resonators (SRRs) that are broken centro-symmetry configurations at the interface of metal surface and air is investigated. Strong interactions, better electromagnetic field confinements, and less leakage into the substrate for vertical SRRs are found. Experimental results show a 2.6-fold enhancement of SHG nonlinearity, which is in good agreement with simulations and calculations. Demonstrations of 3D metastructures and vertical SRRs with strong SHG nonlinearity majorly result from magnetic dipole and electric quadrupole clearly provides potential applications for photonics and sensing.

Enhancing the Magnetic Plasmon Resonance of Three-Dimensional Optical Metamaterials via Strong Coupling for High-Sensitivity Sensing

We report an effective method to enhance and modify the magnetic plasmon (MP) resonance in three-dimensional (3D) optical metamaterials consisting of periodic arrays of silver vertical split-ring resonators (VSRRs) for high-sensitivity sensing. By positioning the 3D metamaterials above a thick silver film separated by a silica dielectric spacer layer, the strong coupling between the MP resonance in the VSRRs and the surface plasmons polaritons (SPPs) propagating on the silver film can be realized and gives rise to an ultra-narrowband hybrid MP mode with a huge enhancement of magnetic fields. For the coupling to happen, the magnetic field direction of the SPPs should be parallel to the magnetic moment induced in the VSRRs. More importantly, because the ultra-narrowband hybrid MP mode is extremely sensitive to the surrounding media, the sensitivity and the figure of merit (FOM) of the 3D metamaterials can reach as high as 700 nm/RIU and 170, respectively, suggesting that the proposed 3D metamaterials hold potential applications in label-free biomedical sensing.

Ultrasensitive terahertz metamaterial sensor based on vertical split ring resonators

An ultrasensitive metamaterial sensor based on double-slot vertical split ring resonators (DVSRRs) is designed and numerically calculated in the terahertz frequency. This DVSRR design produces a fundament LC resonance with a quality factor of about 20 when the incidence magnetic field component normal to the DVSRR array. The resonant characteristics and sensing performance of the DVSRR array design are systematically analyzed employing a contrast method among three similar vertical split ring resonator (SRRs) structures. The research results show that the elimination of bianisotropy, induced by the structural symmetry of the DVSRR design, helps to achieve LC resonance of a high quality factor. Lifting the SRRs up from the substrate sharply reduces the dielectric loss introduced by the substrate. All these factors jointly result in superior sensitivity of the DVSRR to the attributes of analytes. The maximum refractive index sensitivity is 788 GHz/RIU or 1.04×105 nm/RIU. Also, the DVSRR sensor maintains its superior sensing performance for fabrication tolerance ranging from −4% to 4% and wide range incidence angles up to 50° under both TE and TM illuminations.

Bandpass filter design based on vertical split‐ring resonators

A novel bandpass filter (BPF) design based on vertical split-ring resonators (VSRRs) was developed for single- and dual-band applications. The filter is composed of VSRRs, transmission lines, and apertures. The characteristics of single- and dual-band BPFs using a single VSRR and dual VSRRs, respectively, were investigated. The bandpass characteristics arising from VSSR resonance can be controlled by the VSRR dimensions, and the independent bandpass characteristics of dual VSRRs can be used to implement a dual-band BPF. A full-wave simulation and experimental measurements were used to measure their performance. The simulated results for the single- and dual-band BPFs were in good agreement with the experimental results.

Vertical split-ring resonator perfect absorber metamaterial for IR frequencies realized via femtosecond direct laser writing

Perfect electromagnetic metamaterial absorbers based on three-dimensional (3D) vertical split-ring resonators for an infrared spectral range were fabricated using a combination of high-resolution direct laser write lithography and a simple metalization by sputtering. In accordance with theoretical predictions, the fabricated samples exhibit perfect absorption resonances tunable in the wavelength range of 4.5–9.2 µm by changing the dimensions and spacing of the resonators. The structures exhibit polarization and incidence angle-invariant operation with absorbance in excess of 0.85 for incidence angles up to 30°. In the future, they may find applications as narrow-band thermal emitters and for signal enhancement in mid-IR photodetectors.

Three dimensional metafilms with dual channel unit cells

Three-dimensional (3D) metafilms composed of periodic arrays of silicon unit cells containing single and multiple micrometer-scale vertical split ring resonators (SRRs) per unit cell were fabricated. In contrast to planar and stacked planar structures, these 3D metafilms have a thickness t ∼ λd/4, allowing for classical thin film effects in the long wavelength limit. The infrared specular far-field scattering response was measured for metafilms containing one and two resonators per unit cell and compared to numerical simulations. Excellent agreement in the frequency region below the onset of diffractive scattering was obtained. For dense arrays of unit cells containing single SRRs, normally incident linearly polarized plane waves which do not excite a resonant response result in thin film interference fringes in the reflected spectra and are virtually indistinguishable from the scattering response of an undecorated array of unit cells. For the resonant linear polarization, the specular reflection for arrays is highly dependent on the SRR orientation on the vertical face for gap-up, gap-down, and gap-right orientations. For dense arrays of unit cells containing two SRRs per unit cell positioned on adjacent faces, the specular reflection spectra are slightly modified due to near-field coupling between the orthogonally oriented SRRs but otherwise exhibit reflection spectra largely representative of the corresponding single-SRR unit cell structures. The ability to pack the unit cell with multiple inclusions which can be independently excited by choice of incident polarization suggests the construction of dual-channel films where the scattering response is selected by altering the incident polarization.

3D conductive coupling for efficient generation of prominent Fano resonances in metamaterials.

We demonstrate a 3D conductive coupling mechanism for the efficient generation of prominent and robust Fano resonances in 3D metamaterials (MMs) formed by integrating vertical U-shape split-ring resonators (SRRs) or vertical rectangular plates along a planar metallic hole array with extraordinary optical transmission (EOT). In such a configuration, intensified vertical E-field is induced along the metallic holes and naturally excites the electric resonances of the vertical structures, which form non-radiative “dark” modes. These 3D conductive “dark” modes strongly interfere with the “bright” resonance mode of the EOT structure, generating significant Fano resonances with both prominent destructive and constructive interferences. The demonstrated 3D conductive coupling mechanism is highly universal in that both 3D MMs with vertical SRRs and vertical plates exhibit the same prominent Fano resonances despite their dramatic structural difference, which is conceptually different from conventional capacitive and inductive coupling mechanisms that degraded drastically upon small structural deviations.

Vertical split-ring resonator based anomalous beam steering with high extinction ratio.

Metasurfaces created artificially with metal nanostructures that are patterned on surfaces of different media have shown to possess “unusual” abilities to manipulate light. Limited by nanofabrication difficulties, so far most reported works have been based on 2D metal structures. We have recently developed an advanced e-beam process that allowed for the deposition of 3D nanostructures, namely vertical split-ring resonators (VSRRs), which opens up another degree of freedom in the metasurface design. Here we explore the functionality of beam steering with phase modulation by tuning only the vertical dimension of the VSRRs and show that anomalous steering reflection of a wide range of angles can be accomplished with high extinction ratio using the finite-difference-time-domain simulation. We also demonstrate that metasurfaces made of 3D VSRRs can be made with roughly half of the footprint compared to that of 2D nano-rods, enabling high density integration of metal nanostructures.

Plasmon coupling in vertical split-ring resonator metamolecules.

The past decade has seen a number of interesting designs proposed and implemented to generate artificial magnetism at optical frequencies using plasmonic metamaterials, but owing to the planar configurations of typically fabricated metamolecules that make up the metamaterials, the magnetic response is mainly driven by the electric field of the incident electromagnetic wave. We recently fabricated vertical split-ring resonators (VSRRs) which behave as magnetic metamolecules sensitive to both incident electric and magnetic fields with stronger induced magnetic dipole moment upon excitation in comparison to planar SRRs. The fabrication technique enabled us to study the plasmon coupling between VSRRs that stand up side by side where the coupling strength can be precisely controlled by varying the gap in between. The resulting wide tuning range of these resonance modes offers the possibility of developing frequency selective functional devices such as sensors and filters based on plasmon coupling with high sensitivity.