U-shaped Split Ring Resonators Research Articles

Conductively coupled terahertz metamaterials with dual functions of electromagnetically induced transparent and Fano effects for sensing applications

A terahertz metamaterial structure consisting of two U-shaped split-ring resonators and a horizontal cut-line resonator is designed for realizing the electromagnetically induced transparency (EIT) effect and the Fano resonance effect. The genesis of the EIT and Fano resonance bifunctionality is illustrated by combining the near-field distribution analysis of transparent windows and transmission dips. Interestingly, the bifunctional dual-band transparency effect could transform into a single-functional single-band transparency effect when we change the position of the square metal sheet in the vertical direction of the composited terahertz metamaterial structure. In addition, based on the high-quality factor of Fano resonance response, the proposed bifunctional terahertz metamaterial device has good refractive index sensing sensitivity. These results indicate that our proposed terahertz metamaterial can provide guidance for the design of subsequent multifunctional and integrated metamaterials and optoelectronic devices.

Ultrahigh Q surface lattice resonance supported by a U-shaped resonant ring nanoarray.

We achieved an ultrahigh Q surface lattice resonance (SLR) using a conventional U-shaped split ring resonator (U-SRR) array. Numerical results confirmed by semi-analytical analysis show that with the transmission resonance amplitude up to 0.8, the Q-factor of the SLR can still surpass 104. The physical mechanisms of the ultrahigh Q-factor were also investigated. Besides the radiation suppression provided by conventional SLR, the unique geometry of the U-SRR can further offer dual radiation suppression mechanisms: reduction of the dipole moment and excitation of the in-plane quadrupole. We expect that the proposed ultrahigh Q SLR platform will be explored for more flexible and advanced nanoscale devices.

High-Q transmission characteristics in terahertz guided-mode magnetic resonance system

Introduction: Guided mode resonance is generated by coupling wave diffractions with the waveguided mode. The guided mode resonances provide narrow-linewidth and resonance intensity for high quality factor (Q-factor) optical resonators.Methods: we demonstrate the high-Q guided mode resonances propagating on a low-loss, terahertz guided-mode magnetic resonance system, which are periodic square lattices of U-shaped split ring resonators (SRRs) on quartz substrates.Results: By choosing a judicious array period, two distinct frequency guided mode resonances and a magnetic dipole resonance with high Q-factor are observed. The interaction of the two resonances at similar frequencies produces a total transmission peak.Discussion: The dependences of the magnetic dipole resonance on the lattice period and structural parameters are investigated and discussed. The frequency difference between these two guided mode resonances widens with increasing Lattice period. The sharp spectral feature of each resonance results in the abrupt degradation of the spectral edge transmission. The proposed scheme is promising for efficient THz sensing, THz switching, and slow-light devices.

Tunable Electromagnetically Induced Transparent Window of Terahertz Metamaterials and Its Sensing Performance

The electromagnetically induced transparency effect of terahertz metamaterials exhibits excellent modulation and sensing properties, and it is critical to investigate the modulation effect of the transparent window by optimizing structural parameters. In this work, a unilateral symmetrical metamaterial structure based on the cut-wire resonator and the U-shaped split ring resonator is demonstrated to achieve electromagnetically induced transparency-like (EIT-like) effect. Based on the symmetrical structure, by changing the structural parameters of the split ring, an asymmetric structure metamaterial is also studied to obtain better tuning and sensing characteristics. The parameters for controlling the transparent window of the metamaterial are investigated in both passive and active modulation modes. In addition, the metamaterial structure based on the cut-wire resonator, unilateral symmetric and asymmetric configurations are investigated for high performance refractive index sensing purposes, and it is found that the first two metamaterial structures can achieve sensitivity responses of 63.6 GHz/RIU and 84.4 GHz/RIU, respectively, while the asymmetric metamaterial is up to 102.3 GHz/RIU. The high sensitivity frequency response of the proposed metamaterial structures makes them good candidates for various chemical and biomedical sensing applications.

Mechanically reconfigurable and electrically tunable active terahertz chiral metamaterials

Active and efficient control of polarization states of electromagnetic wave is highly desirable in broad optical and terahertz (THz) devices. Herein, we have proposed a type of mechanically reconfigurable and electrically tunable THz chiral metamaterials that are consisting of periodical U-shaped split ring resonators (USRRs) patterned on the graphene Miura-origami (G-Mori) structures. We demonstrate that their circular dichroism (CD) as high as 0.93 originate from the coupling effect between the proper arranged USRRs and the G-Mori structure, which can be tuned through continuous change of the folding angle and Fermi energy level of graphene origami via mechanical and electrical methods in reversible manner, respectively. Furthermore, the proposed chiral metamaterials possess ideal quarter-wave-plate behavior at the resonance which also can be controlled by combining mechanical and electrical methods. The mechanically reconfigurable and electrically tunable circular dichroism and polarization conversion properties of the chiral metamaterials will be suitable for development of various polarization-related THz devices, such as highly sensitive chiral sensors, active polarization modulators and converters.

Double Narrow Fano Resonances via Diffraction Coupling of Magnetic Plasmon Resonances in Embedded 3D Metamaterials for High-Quality Sensing.

We theoretically demonstrate an approach to generate the double narrow Fano resonances via diffraction coupling of magnetic plasmon (MP) resonances by embedding 3D metamaterials composed of vertical Au U-shaped split-ring resonators (VSRRs) array into a dielectric substrate. Our strategy offers a homogeneous background allowing strong coupling between the MP resonances of VSRRs and the two surface collective optical modes of a periodic array resulting from Wood anomaly, which leads to two narrow hybridized MP modes from the visible to near-infrared regions. In addition, the interaction effects in the VSRRs with various geometric parameters are also systematically studied. Owing to the narrow hybrid MP mode being highly sensitive to small changes in the surrounding media, the sensitivity and the figure of merit (FoM) of the embedded 3D metamaterials with fabrication feasibility were as high as 590 nm/RIU and 104, respectively, which holds practical applications in label-free biosensing, such as the detection of medical diagnoses and sport doping drugs.

The investigation of the electromagnetic coupling effect in terahertz toroidal metasurfaces and metamaterials

We proposed and fabricated toroidal dipole (TD) metasurfaces(MSs) with a metamolecule of two coplanar U-shaped split ring resonators(USRRs) fabricated on polyimide substrate, and TD metamaterials (MMs) were the stacks of two TD MSs layers, whose frequencies, electromagnetic (EM) distributions and Q factor can be efficiently affected by the EM coupling effect in TD MSs/MMs. It was found that the resonances frequencies of TD MMs were shifted to lower frequencies due to the increase of inductance by the stacks of metallic layer. Meanwhile, the high-frequency TD resonance in TD MMs would be tailored by the periodicity. Considering the relation between TD resonances and the scattering power of TD (Ty), the Q factor depended highly on the value of Ty in the same metamolecule structure. The optimization in excitation of TD provide opportunity to further increase the Q-factor of metamaterial and pave a way for potential applications in terahertz sensor and other functional devices.

Wideband and multiband electromagnetically induced transparency in graphene metamaterials

A multiband tunable electromagnetic induced transparency (EIT) effect in metamaterial at microwave frequency range is investigated. The sandwich structure contains silicon dioxide and gold layers. The metamaterial structure has multiband EIT phenomenon due to coupling with U-Shaped split-ring resonators (SRRs) and cut wire (CW). Two different modes can be obtained in CW and a single band EIT effects in SRRs. Results show that the different resonances in the structure lead to multiband EIT. By adding the finding of the graphene layer on top of the structures, EIT window can be changed obviously. It is shown that the graphene can adjust EIT phenomenon. The group index is calculated to exhibit the slow light effect. The demonstrated phenomenon can provide valuable variety of important applications, including microwave communication technology, microwave devices, slow light and switch devices.

Dark mode tailored electromagnetically induced transparency in terahertz metamaterials

In this paper, a novel terahertz metamaterial structure composed of a pair of sub-wavelength reverse U-shaped split ring resonators (RUSRs) and cut wire (CW) resonator is designed to realize electromagnetically induced transparency (EIT) effect in weak coupling region. Theoretical and simulated results show that by modulating the relative coupling distance between CW and SRR or mutual distance between SRR pair, the EIT-like phenomenon can be tailored. Furthermore, by introducing photosensitive silicon (Si) cell into the units of the dark mode resonator, actively optical control of the EIT-like effect is realized through increasing the dark mode damping rate. The present work provides an alternative method to design ultrasensitive sensors, filters and slow-light devices in the THz regime.

Dual Toroidal Dipole Resonance Metamaterials under a Terahertz Domain.

We proposed and fabricated a flexible, planar, U-shape-modified structure metamaterial (MM) that was composed of two metallic pattern layers separated by a polyimide layer, where each metallic pattern layer consists of two U-shaped split ring resonators (USRRs). The coupling effect between the two USRRs in the same metallic layer was vital to the formation of dual toroidal dipole (TD) resonances. The measured and simulated results showed that both low quality factor (Q) (~1.82) and high Q (~10.31) TD resonances were acquired synchronously at two different frequencies in the MMs by adjusting the distance between the two coplanar USRRs. With the interaction of the USRRs, the energy levels of the USRRs were split into inductance-capacitance (LC)-induced TD resonance at low frequency and dipole-induced TD resonance at high frequency. Thus, the electric multipole interaction played an important role in determining the energy level of the TD resonance. The better strength of the high frequency TD resonance can be confined to an electromagnetic field inside a smaller circular region, and thus, a higher Q was obtained. In order to investigate the TD mechanism more in depth, the power of the electric dipole, magnetic dipole, electric circular dipole, and TD were quantitatively calculated. Dual TD MMs on a freestanding substrate will have potential applications in functional terahertz devices for practical applications.

Metal-substrate-enhanced magnetic dipole resonance in metamaterials for high-performance refractive index sensing

In this work, we theoretically investigate metal-substrate-enhanced magnetic dipole resonance in metamaterials for high-performance refractive index sensing. The metamaterials are composed of periodic arrays of vertical U-shaped split-ring resonators, dielectric spacer, and metal substrate. Because the metal substrate blocks the transmission channel of light, the radiative damping of magnetic dipole resonance is nearly completely suppressed and thus its quality factor is increased noticeably. Owing to the narrow bandwidth, nearly-zero reflectance, and huge enhancement of electromagnetic fields at the magnetic dipole resonance, our designed metamaterial sensor has very high sensitivity (S = 1308 nm/RIU, S* = 26/RIU) and figure of merit (FOM = 52, FOM* = 1187). The good sensing capability allows for a much more sensitive detection of small refractive index changes and suggests potential applications in biosensing.

Proximity effect correction in electron-beam lithography based on computation of critical-development time with swarm intelligence

Electron-beam lithography (EBL) is an important technique in manufacturing high-resolution nanopatterns for broad applications. However, the proximity effect in EBL can degrade the pattern quality and, thus, impact the performance of the applications greatly. The conventional proximity effect correction (PEC) methods, which employ computationally intensive cell or path removal method for development simulation, are very computational lengthy, especially for complex and large-area patterns. Here, the authors propose a novel short-range PEC method by transforming the evaluation of pattern feasibility into the shortest path problem based on the concept of critical-development time. The authors combine this evaluation algorithm with the swarm intelligence which mimics the natural collective behavior of animals to optimize the design of electron dose distribution in EBL. The PEC algorithm is applied for pattern fabrication for U-shaped split-ring resonator and produces optimized exposure pattern that shows excellent agreement with the targeted objectives. Our work on the PEC strategy reduces the computational cost significantly and is particularly suitable for the design of complex pattern with various constraints.

Touching Gold Nanoparticle Chain Based Plasmonic Antenna Arrays and Optical Metamaterials

The control of light–material interactions at the nanoscale requires optical elements with sizes much smaller than the wavelength of light. Plasmonic nanostructures and optical metamaterials enable drastic control and manipulation of light at such small scales. However it is quite challenging to further reduce the size of resonant elements using conventional plasmonic nanostructures. In this paper, we propose novel optical resonators that rely on the conducting plasmon mode of touching nanoparticle chains that enable significant size reduction when compared with widely used nanostripe antennas and U-shaped split-ring resonators. We employ full-field electromagnetic simulations to study the resonance mechanisms of nanoparticle chain arrays. In comparison with the nanobar plasmonic antennas, a nanoparticle chain based antenna with similar physical sizes operates at larger wavelengths, opening routes for deep subwavelength plasmonic resonators. Moreover, using nanoshell chain arrays, we demonstrate an optica...

Double Fano resonances due to interplay of electric and magnetic plasmon modes in planar plasmonic structure with high sensing sensitivity

Double Fano resonant characteristics are investigated in planar plasmonic structure by embedding a metallic nanorod in symmetric U-shaped split ring resonators, which are caused by a strong interplay between a broad bright mode and narrow dark modes. The bright mode is resulted from the nanorod electric dipole resonance while the dark modes originate from the magnetic dipole induced by LC resonances. The overlapped dual Fano resonances can be decomposed to two separate ones by adjusting the coupling length between the nanorod and U-shaped split ring resonators. Fano resonances in the designed structure exhibit high refractive-index sensing sensitivity and figure of merit, which have potential applications in single or double-wavelength sensing in the near-infrared region.

Design of plasmonic toroidal metamaterials at optical frequencies

Toroidal multipoles are the subject of growing interest because of their unusual electromagnetic properties different from the electric and magnetic multipoles. In this paper, we present two new related classes of plasmonic metamaterial composed of purposely arranged of four U-shaped split ring resonators (SRRs) that show profound resonant toroidal responses at optical frequencies. The toroidal and magnetic responses were investigated by the finite-element simulations. A phenomenon of reversed toroidal responses at higher and lower resonant frequencies has also been reported between this two related metamaterials which results from the electric and magnetic dipoles interaction. Finally, we propose a physical model based on coupled LC circuits to quantitatively analyze the coupled system of the plasmonic toroidal metamaterials.

Optical magnetic response of upright plasmonic molecules in 3D metamaterial

An array of fabricated U-shaped split-ring resonators shows a profound response to the magnetic field of light incident normal to the sample.

Asymmetric transmission of linearly polarized waves and polarization angle dependent wave rotation using a chiral metamaterial

An electrically thin chiral metamaterial structure composed of four U-shaped split ring resonator pairs is utilized in order to realize polarization rotation that is dependent on the polarization of the incident wave at 6.2 GHz. The structure is optimized such that a plane wave that is linearly polarized at an arbitrary angle is an eigenwave of the system at this frequency. The analytical relation between the incident polarization and the polarization rotation is derived using transmission matrices. Furthermore, the proposed structure exhibits an asymmetric transmission of linearly polarized waves at 6.2 GHz. Plane waves traveling in opposite but perpendicular directions to the material plane are rotated by different angles. On the other hand, four incident polarization angles have been found for the same structure, at which the transmission is symmetric. The experiment results are in good agreement with the numerical results.

Asymmetric chiral metamaterial circular polarizer based on four U-shaped split ring resonators

An asymmetric chiral metamaterial structure is constructed by using four double-layered U-shaped split ring resonators, which are each rotated by 90° with respect to their neighbors. The peculiarity of the suggested design is that the sizes of the electrically and magnetically excited rings are different, which allows for equalizing the orthogonal components of the electric field at the output interface with a 90° phase difference when the periodic structure is illuminated by an x-polarized wave. As a result, left-hand circular polarization and right-hand circular polarization are obtained in transmission at 5.1 GHz and 6.4 GHz, respectively. The experiment results are in good agreement with the numerical results.

Transmission resonances in periodic U-shaped metallic nanostructures

The spectral response of crescent-like metallic nanostructures, a sub-class of U-shaped split-ring resonators, on a glass substrate at normal incidence is studied numerically. Also, the interpretation of transmission resonances arising from periodic conventional standard split-ring resonators with rectangular edges (SSRR) at normal incidence is revisited. In particular, we focus on one specific transmission resonance which is present for nano-crescents (NC) but absent in the case of SSRRs used for metamaterials. It is proposed that for a U-shaped metallic structure of arbitrary geometry, coupling of plasmonic eigen modes at all the surfaces of the three-dimensional structure is essential to be considered. The manner in which the coupling takes place between plasmonic modes at all the surfaces of the three-dimensional structure is what completely characterizes transmission resonances, and it is unique for each given resonance.

The origin of magnetic polarizability in metamaterials at optical frequencies - an electrodynamic approach

We explain the origin of the electric and particular the magnetic polarizabiltiy of metamaterials employing a fully electromagnetic plasmonic picture. As example we study an U-shaped split-ring resonator based metamaterial at optical frequencies. The relevance of the split-ring resonator orientation relative to the illuminating field for obtaining a strong magnetic response is outlined. We reveal higher-order magnetic resonances and explain their origin on the basis of higher-order plasmonic eigenmodes caused by an appropriate current flow in the split-ring resonator. Finally, the conditions required for obtaining a negative index at optical frequencies in a metamaterial consisting of split-ring resonators and wires are investigated.