Cut-wire Resonators Research Articles

Terahertz sensor based on plasmon-induced transparency in a carbon nanotube metamaterial

In this paper, the plasmon-induced transparency (PIT) effect based on a carbon nanotube (CNT) resonator structure is achieved. An array of two split ring resonators (SRRs) and a cut wire (CW) resonator are utilized to form the proposed metamaterial. A PIT transparency window is achieved under a TM polarization terahertz light. Results show that the PIT effect is originated from the near-field coupled of the bright mode and dark mode. A coupled harmonic oscillator model is used to describe the near-field coupling between the bright mode and subradiant mode, and the results agree well with the FDTD simulation. The effect of geometrical sizes, like structure period, the radius and the splitting degree of the split ring resonator, the length of the cut wire resonator, and the coupling distance on the PIT window is analyzed in detail. Besides, the sensing and slow light performance of the proposed CNT metamaterial are studied, a maximum sensitivity of 0.74 THz RIU, and a time delay of 0.54 ps are obtained. Therefore, the proposed CNT-based device can be applied to the PIT effect, near-infrared modulators, slow light devices, sensors, and other fields.

Electromagnetically Induced Transparency Analog of Asymmetric Perovskite Metamaterial in the THz Spectral Region

The analogy of electromagnetically induced transparency (EIT) in perovskite metamaterials is characterized by the numerical simulations in finite-difference time-domain (FDTD). The perovskite metamaterials consist of two cut wire resonators (CWRs) and a disk resonator (DR) on a polyimide substrate. The analysis revealed the characteristic dynamics of the electromagnetic field, the near-field couplings of CWRs and DR, and the EIT-like spectral features of perovskite metamaterials as functions of the asymmetry parameter and polarization direction. The strong coupling and destructive interference of bright and bright–dark transitions in perovskite metamaterials displayed EIT-like transparency at 653.5 GHz with a high Q-factor of approximately 1470, a sensitivity of 531 GHz/RIU and a figure of merit of around 780. In addition, perovskite metamaterials exhibited slow light with a group delay of about 106 ps and a group index of approximately 3100. These results may provide an important perspective for understanding the coupling mechanism and applications of perovskite materials in slow-light devices, THz sensors, and tunable switching in THz spectral region.

Analog of electromagnetically induced transparency based on asymmetric nickel-ferrite metamaterials in THz regime

The electromagnetically induced transparency-like characteristics of asymmetric nickel-ferrite metamaterials through near-field coupling are investigated in the terahertz regime. The nickel-ferrite metamaterial consists of a cut wire resonator on the front side and a disk resonator at the back of the quartz substrate. The analog of electromagnetically induced transparency is characterized by employing a finite difference frequency domain technique. By designing the nickel-ferrite metamaterial to overcome Snoek’s limitation of ferrite materials in the THz spectral region, electromagnetically induced transparency-like spectral features with a high quality factor and slow-light effect are achieved. These results may provide an important way to understand the coupling mechanism and ferrite material application in slow-light devices, THz sensors, and tunable switching in THz frequency bands.

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.

High-efficiency multi-band multi-polarization metasurface-based reflective converter with multiple plasmon resonances

A simple design of a triple-band multi-polarization reflective polarization converter is presented. The proposed device consists of a modified split-ring structure and a cut-wire resonator. A linearly polarized (LP) incident electromagnetic (EM) wave can be reflected into an orthogonally directed LP wave in two widebands and a circularly polarized (CP) wave in one wideband. The triple-band multi-polarization conversion performance is achieved by combing multiple plasmon resonances, which can be controlled by adjusting the parameters of the modified split-ring structure and the cut-wire resonator independently. The simulations show that a y/x-polarized incident EM wave can be rotated to an x/y-polarized reflected wave from 5.75–9.40 GHz (a fractional bandwidth of 48.18%) and 14.22–16.28 GHz (a fractional bandwidth of 13.51%) with a polarization conversion ratio over 0.9. Besides, a CP reflection is obtained from 10.58–13.88 GHz (a fractional bandwidth of 26.98%) with an axial ratio of less than 3 dB. The device is prototyped and a good agreement between experimental and simulated results is obtained. The proposed multi-functional polarization converter yields a wideband in each polarization conversion band and can be extended to work in other frequency bands.

Low-power threshold gas discharge by enhanced local electric field in electromagnetically-induced-transparency like metamolecules

To realize efficient nonlinear metamaterials, we investigate a method for enhancing the local electric field in a metamolecule composed of two radiatively coupled cut-wire resonators where resonance of the cut-wire resonators and low-group-velocity propagation of an incident electromagnetic wave simultaneously occurs. Numerical analysis shows that the local electric field in the metamolecule can be enhanced by decreasing the electrode size and the gap of the capacitor structure of the cut-wire resonators while keeping the equivalent electrical circuit parameters of the metamolecule constant. We measure and compare the threshold incident power for a gas discharge in a metamolecule fabricated in our previous study and that in a metamolecule with shrunken capacitor structures. The experiment reveals that shrinking the capacitor structure while keeping the resonance frequency of the metamolecule decreases the threshold incident power for a gas discharge and may increase the gas pressure where the threshold incident power is minimum. Further development of this work will enable us to realize efficient nonlinear metamaterials that have atmospheric-pressure gas as a nonlinear element.

Broadband long-wave infrared metamaterial absorber based on single-sized cut-wire resonators.

Broadband absorption is critical for the applications of metamaterial absorbers. In this work, a broadband long-wave infrared (LWIR) absorber with classical metal-dielectric-metal configuration is numerically demonstrated. The absorber consists of single-sized cut-wire arrays that show broadband and high extinction ratio, attributed to polarization-selective simultaneous excitation of propagated and localized surface plasmon resonances. The average absorption rate of the TM wave reaches 91.7% and 90% of the incident light is absorbed by the resonator in the wavelength range of 7.5-13.25µm so that the average extinction ratio in the resonator layer reaches 125. The polarization insensitive broadband absorption can be obtained by a cross resonator which can be treated as a pair of cut-wires perpendicular to each other. Our metamaterial absorber with single-sized resonators shows spatially concentrated broadband absorption and may have promising applications for hot-electron devices, infrared imaging, and thermal detection.

Multi-Band Electromagnetically-Induced-Transparency Metamaterial Based on the Near-Field Coupling of Asymmetric Split-Ring and Cut-Wire Resonators in the GHz Regime

A metamaterial (MM), mimicking electromagnetically-induced transparency (EIT) in the GHz regime, was demonstrated numerically and experimentally by exploiting the near-field coupling of asymmetric split-ring and cut-wire resonators. By moving the resonators towards each other, the original resonance dip was transformed to a multi-band EIT. The phenomenon was explained clearly through the excitation of bright and dark modes. The dispersion characteristic of the proposed MM was also investigated, which showed a strongly-dispersive behavior, leading to a high group index and a time delay of the MM. Our work is expected to contribute a simple way to develop the potential devices based on the multi-band EIT effect.

Programmable terahertz metamaterial with multiple logic characteristics

We present a design of programmable terahertz (THz) metamaterial (PTM) with multiple logic characteristics. The proposed PTM device is composed of a cut-wire resonator (CWR) sandwiched two face-to-face split-ring resonators (SRRs). By moving CWR along z-axis direction to change vertical distance between CWR and substrate, the programmable coding digits could be switched from four digits to five digits by alternating the polarization of incident THz wave. The electromagnetic responses of PTM device are stable in TE mode and can be tuned by moving vertical distance of CWR in TM mode. These results indicate the potential possibilities of PTM to be used as a stable polarization switch and an effective programmable metamaterial in the THz frequency range.

Polarization-Sensitive Metamaterials with Tunable Multi-Resonance in the Terahertz Frequency Range

We propose two designs of polarization-sensitive metamaterials (PSMs), which are composed of face-to-face spilt-ring resonators (SRRs) and a cut-wire resonator (CWR) sandwiched by two face-to-face SRRs. For convenient description, they are denoted as PSM_1 and PSM_2, respectively. PSM_1 and PSM_2 are fabricated by tailoring Au layers with periodic configurations on silicon-on-insulator (SOI) substrates. By changing the incident polarization light, the electromagnetic responses of PSM_1 can be manipulated between single-resonance and dual-resonance, while those of PSM_2 exhibit switching behavior between single-resonance and triple-resonance. By enlarging the distance between the gap centers of the two face-to-face SRRs along the y-axis direction, the electromagnetic responses of PSM_1 show switching characteristics from single-resonance to triple-resonance at the transverse electric (TE) mode and from dual-resonance to triple-resonance at the transverse magnetic (TM) mode. PSM_2 exhibits switching characteristics from single-resonance to triple-resonance at the TE mode and from dual-resonance to quad-resonance at the TM mode. Furthermore, by changing the width of the CWR under the condition of two face-to-face SRRs with a constant gap distance, PSM_2 exhibits stable electromagnetic responses at the TE mode and tunable resonances at the TM mode, respectively. This work paves the way to the possibility of metamaterial devices with great tunability, switchable bandwidth, and polarization-dependence characteristics.

Numerical and Theoretical Study of Tunable Plasmonically Induced Transparency Effect Based on Bright-Dark Mode Coupling in Graphene Metasurface.

In this paper, we numerically and theoretically study the tunable plasmonically induced transparency (PIT) effect based on the graphene metasurface structure consisting of a graphene cut wire (CW) resonator and double split-ring resonators (SRRs) in the middle infrared region (MIR). Both the theoretical calculations according to the coupled harmonic oscillator model and simulation results indicate that the realization of the PIT effect significantly depends on the coupling distance and the coupling strength between the CW resonator and SRRs. In addition, the geometrical parameters of the CW resonator and the number of the graphene layers can alter the optical response of the graphene structure. Particularly, compared with the metal-based metamaterial, the PIT effect realized in the proposed metasurface can be flexibly modulated without adding other actively controlled materials and reconstructing the structure by taking advantage of the tunable complex surface conductivity of the graphene. These results could find significant applications in ultrafast variable optical attenuators, sensors and slow light devices.

Active formatting modulation of electromagnetically induced transparency in metamaterials

We experimentally demonstrate for the first time an active all-optical ultrafast modulation of electromagnetically induced transparency-like effect in a hybrid device of sapphire/Si/metamaterial. From numerical simulations, it can be deducted that the tuning process is attributed to the coupling between the dark mode existing in split-ring resonators and the bright mode existing in cut wire resonators. The transmission amplitude modulation is accompanied by the slow-light effect. In addition, the ultrafast formation process is measured to be as fast as 2 ps. This work should make an important contribution to novel chip-scale photonic devices and terahertz communications.

Ultra-broadband and high-efficiency reflective polarization rotator based on fractal metasurface with multiple plasmon resonances

A fractal metasurface-based ultra-broadband and high conversion efficiency reflective polarization rotator (PR) is proposed. The linearly y-/x-polarized (LP) incident electromagnetic (EM) wave can be rotated over 90ºafter reflection by the proposed metasurface. The unit cell of the proposed PR is designed based on the fractal cut-wire resonator. Compared with a typical cut-wire resonator exciting three resonances, the fractal one can generate an extra resonant frequency. An ultra-broad bandwidth with a polarization conversion ratio greater than 0.9 (PCR>0.90) can be obtained from 8.00 to 23.00 GHz (a fractional bandwidth of 96.8%) at normal incidence, which is realized by the combination of the four resonances. The physical working mechanism and the polarization conversion performance of the metasurface are analyzed by surface current and E-field distributions, simulations and explained based on numerical calculations and plasmon resonance modes. Good agreement between the experiments and simulations is obtained. Compared with traditional PRs and other published metasurface-based PRs, the proposed metasurface can achieve an ultra-broad band and a high efficiency with a sub-wavelength size of the unit cell. The use of fractal topologies provides a novel and efficient method to further upgrade resonance characteristics in the design of PR, and the scalable geometry of the design shows its potential applications at optical, terahertz, and microwave frequencies.

Active tunable terahertz micro-electromechanical metamaterial absorber

Terahertz (THz) devices and technologies have attracted great interest. One of the most important applications of the THz frequency region is in absorbers. In this paper, we propose and investigate a novel active tunable THz micro-electromechanical metamaterial absorber. The absorber is composed of cut wire resonators and split u-shaped ring resonators. The structure is analyzed by finite integral technique (FIT). A circuit model approach is also used to verify the numerical simulation results. Our proposed circuit model approach is simple and advantageous. The numerical FIT and circuit model results are in good agreement. The proposed absorber and its circuit model could be utilized in micro-electromechanical metamaterial devices and circuits such as switches and detectors in the THz range.

Actively tunable slow light in a terahertz hybrid metal-graphene metamaterial

We theoretically and numerically demonstrate an actively tunable slow light in a hybrid metal-graphene metamaterial in the terahertz (THz) regime. In the unit cell, the near field coupling between the metallic elements including the bright cut wire resonator and the dark double split-ring resonator gives rises to a pronounced transmission peak. By positioning a monolayer graphene under the dark mode resonator, an active modulation of the near field coupling is achieved via shifting the Fermi level of graphene. The physical origin can be attributed to the variation in the damping rate of the dark mode resonator arising from the conductive effect of graphene. Accompanied with the actively tunable near filed coupling effect is the dynamically controllable phase dispersion, allowing for the highly tunable slow light effect. This work offers an alternative way to design compact slow light devices in the THz regime for future optical signal processing applications.

Independently tunable dual-spectral electromagnetically induced transparency in a terahertz metal–graphene metamaterial

We theoretically investigate the interaction between the conductive graphene layer with the dual-spectral electromagnetically induced transparency (EIT) metamaterial and achieve independent amplitude modulation of the transmission peaks in the terahertz (THz) regime. The dual-spectral EIT resonance results from the strong near field coupling effects between the bright cut wire resonator in the middle and two dark double-split ring resonators on the two sides. By integrating monolayer graphene under the dark mode resonators, the two transmission peaks of the EIT resonance can exhibit independent amplitude modulation via the shifting of the Fermi level of the corresponding graphene layer. The physical mechanism of the modulation can be attributed to the variation of damping factors of the dark mode resonators arising from the tunable conductivity of graphene. This work shows great prospects in designing multiple-spectral THz functional devices with highly flexible tunability and implies promising applications in multi-channel selective switching, modulation and slow light.

Dynamic electromagnetically induced transparency based on a metal-graphene hybrid metamaterial

A novel design of a dynamic electromagnetically induced transparency (EIT) metamaterial is proposed. The metamaterial consists of two kinds of cut-wire metal resonators with a graphene strip placed between them. The destructive interference between the two resonators gives rise to a transparency window. By varying the Fermi energy of the graphene through external gating, the EIT effect can be manipulated dynamically and the maximum modulation depth can reach up to 81%. The efficient modulation is controlled only in the EIT window with slight changes in transmission dips, which may avert the additional noises at adjacent frequencies in the modulation process. Moreover, the actively controlled slow-light effect and sensing performances can also be realized as the corresponding EIT window is modulated. This work provides a strategy to achieve a tunable EIT effect in a metal-graphene hybrid structure and exhibits potential applications in designing terahertz modulators, environmental sensors and slow-light devices.

Localized terahertz electromagnetically-induced transparency-like phenomenon in a conductively coupled trimer metamolecule.

We experimentally investigate the terahertz (THz) electromagnetically-induced transparency (EIT)-like phenomenon in a metamolecule (MM) of three-body system. This system involves a couple of geometrically identical split-ring resonators (SRRs) in orthogonal layout conductively coupled by a cut-wire resonator. Such a three-body system exhibits two frequency response properties upon to the polarization of incident THz beam: One is the dark-bright-bright layout to the horizontally polarized THz beam, where there is no EIT-like effect; the other is bright-dark-dark layout to the vertically polarized THz beam, where an EIT-like effect is observable. The transparency window can be tuned from 0.71 THz to 0.74 THz by the displacement of cut-wire inside the trimer MM. A maximum of 7.5 ps group delay of THz wave is found at the transparent window of 0.74 THz. When the cut-wire moved to the mid-point of lateral-side of SRR, the EIT-like phenomenon disappears, this leads to a localized THz slow-light effect. The distribution of surface currents and electric energy reveals that the excited inductive-capacitive (LC) oscillation of bright-SRR dominates the high frequency side-mode, which is isolated to the displacement of cut-wire resonator. However, the low frequency side-mode originates from the constructive hybridization of LC resonance in dark-SRR coupled with a localized S-shaped dipole oscillator, which is tunable by the displacement of cut-wire. As a consequence, the group delay as well as the spectral configuration of transparency window can be manipulated by tuning one side-mode while fixing the other. Such an experimental finding reveal the EIT-like effect in a conductively coupled three-body system and manifests a novel approach to achieve tunable THz slow-light device.

Implementation of selective controlling electromagnetically induced transparency in terahertz graphene metamaterial

A terahertz electromagnetically induced transparency (EIT) metamaterial, consisting of single-layer graphene cut wire resonator arrays with closely placed graphene closed ring resonator arrays, was designed and numerically investigated in this paper. A distinct transparency window resulting from the near field coupling between two resonators can be obtained in the transmission spectrum. More importantly, since two resonator elements of all unit cells connect respectively with the corresponding metallic pads (Pad 1 and Pad 2) by the separated graphene wires, the location and amplitude of the transparency window, and the associated group delay and delay bandwidth product can be actively controlled by the selective doping graphene. Moreover, compared with other separated graphene patterns, a more convenient and fast modulation can be realized by applying gate bias voltage. In addition, a two-particle model was employed to theoretically study EIT behaviors of the graphene metamaterial with different doping states, and the analytic results agree excellently with our numerical results. Therefore, the work could offer a new platform for exploring actively tunable slow light terahertz devices such as modulators, buffers, and optical delays.

Switching and extension of transmission response, based on bending metamaterials

The electromagnetically-induced transparency (EIT)-like effects in planar and non-planar metamaterials (MMs) were investigated for microwave (GHz) frequencies. The specific MMs used in this study consisted of a cut-wire resonator and a ring resonator, where were placed on the top and the bottom layers, respectively. A transmission window was produced, due to the interference between bright- and bright-mode coupling. Using the numerical and the experimental results, we demonstrate that the bending of MM leads to enhanced transmission and bandwidth, as well as an additional EIT-like peak. This provides an effective way of realizing the tunable devices, including the switching sensors.