A Nanoscale Fano Resonator by Graphene-Gold Dipolar Interference

A nanoscale Fano resonator composed of a hybrid graphene disk-gold ring combination is reported in this letter. The inner narrow dipolar resonance of a discrete state induced by graphene interferes with the outside broad dipolar resonance of a continuum state induced by gold, thus forming an asymmetric Fano transparency within the absorption window. The metastructure exhibits a wide tunable band along with an excellent refractive index sensing capability of 2344 nm/RIU. The geometry adjustment modulates the spectral response giving chances to the equivalent of electromagnetically induce transparency. Moreover, the group index exceeds 760 within the transparency window enabling a potential use in slow light or light storage applications. The analytic analysis is in accordance with the numerical simulation results.     

Subwavelength Micro-Antenna for Achieving Slow Light at Microwave Wavelengths via Electromagnetically Induced Transparency in 2D Metamaterials

In this paper, a tunable slow light 2D metamaterial is presented and investigated. The metamaterial unit cell is composed of three metallic strips as radiative and non-radiative modes. Once introducing asymmetry, a transparency window induced by coupling between the dark and bright modes is observed. The transmission characteristics and the slow light properties of the metamaterial are verified by numerical simulation, which is in a good agreement with theoretical predictions. The impact of asymmetric parameter on transparency window is also investigated. Simulation results show the spectral properties and the group index of the proposed 2D metamaterial can be tunned by adjusting asymmetric structure parameter, temperature and also the metal used in the metamaterial. Furthermore, the electromagnetic field distributions, excited surface currents, induced electric dipole and quadruples, and slow light properties of the metamaterial are investigated in details as well as transmission spectral responses. The outstanding result is that, the 2D-metamaterial is in a high decrease of the group velocity and therefore slow light applications, because in the best state, the group velocity in our structure decreases by a factor of 221 at T=100 K using copper as metal in optimization asymmetric case.     

Tunable Slow Light in Graphene Metamaterial in a Broad Terahertz Range

A graphene-based metamaterial with tunable electromagnetically induced transparency is numerically studied in this paper. The proposed structure consists of a graphene layer composed of H shape between two cut wires, by breaking symmetry can control EIT-like effects and by increasing the asymmetry in the structure has strong coupling between elements. It is important that the peak frequency of transmission window can be dynamically controlled over a broad frequency range by varying the chemical potential of graphene layer. The results show that high refractive index sensitivity and figure of merit can be achieved in asymmetrical structures which is good for sensing applications. We calculated the group delay and the results show we can control the group velocity by varying the S parameter in asymmetrical structure. The characteristics of our system indicate important potential applications in integrated optical circuits such as optical storage, ultrafast plasmonic switches, high performance filters, and slow-light devices.     

High Sensitivity Nanoplasmonic Sensor Based on Plasmon-Induced Transparency in a Graphene Nanoribbon Waveguide Coupled with Detuned Graphene Square-Nanoring Resonators

A novel nanoscale structure for high sensitivity sensing which consists of a graphene nanoribbon waveguide coupled with detuned graphene square-nanoring resonators (GSNR) based on edge mode is investigated in detail. By altering the Fermi energy level of the graphene, the plasmon-induced transparency (PIT) window from the destructive interference between a radiative square-nanoring resonator and a dark square-nanoring resonator can be easily tailored. The coupled mode theory (CMT) is used to show that the theoretical results agree well with the finite difference time domain (FDTD) simulations. This nanosensor yields a ultrahigh sensitivity of ∼2600 nm/refractive index unit (RIU) and a figure of merit (FOM) of ∼54 in the mid-infrared (MIR) spectrum. The revealed results indicate that the Fermi energy level of the graphene and the coupling distance play important roles in optimizing the sensing properties. Our proposed structure exerts a peculiar fascination on the realization of ultra-compact graphene plasmonic nanosensor in the future.

     

Investigating the Characteristics of a Double Circular Ring Resonators Slow Light Device Based on the Plasmonics-Induced Transparency Coupled with Metal-Dielectric-Metal Waveguide System

We have numerically investigated an analog of electromagnetically induced transparency (EIT) in a metal-dielectric-metal (MDM) waveguide bend. The geometry consists of two asymmetrical stubs extending parallel to an arm of a straight MDM waveguide bend. Finite-difference time-domain simulations show that a transparent window is located at 1550 nm, which is the phenomenon of plasmonic-induced transparency (PIT). Signal wavelength is assumed to be 820 nm. The velocity of the plasmonic mode can be largely slowed down while propagating along the MDM bends. Multiple-peak plasmon-induced transparency can be realized by cascading multiple cavities with different lengths and suitable cavity-cavity separations. Large group index up to 73 can be obtained at the PIT window. Our proposed configuration may thus be applied to storing and stopping light in plasmonic waveguide bends. In addition, the relationship between the transmission characteristics and the geometric parameters including the radius of the nano-ring, the coupling distance, and the deviation length between the stub and the nano-ring is studied in a step further. The velocity of the plasmonic mode can be largely slowed down while propagating along the MDM bends. For indirect coupling, formation of transparency window is determined by resonance detuning, but, evolution of transparency is mainly attributed to the change of the coupling distance. Theoretical results may provide a guideline for control of light in highly integrated optical circuits. The characteristics of our plasmonic system indicate a significant potential application in integrated optical circuits such as optical storage, ultrafast plasmonic switch, highly performance filter, and slow light devices.     

New Type Design of the Triple-Band and Five-Band Metamaterial Absorbers at Terahertz Frequency

A new scheme to achieve a simple design of triple-band metamaterial absorber at terahertz frequency is presented. In this scheme, we employ a traditional sandwich structure, which is consisted of a metallic resonator and an appropriate thickness of the dielectric layer backed with an opaque metallic board, as the research object. Three strong but discrete resonance peaks with the narrow bandwidths and high absorptivities are realized. The combination of the dipolar resonance, LC (inductor-capacitor circuit) resonance, and the surface resonance of the metallic resonator determines the triple-band absorption. Numerical results also show that the frequencies of the three absorption bands and the number of the resonance peaks can be effectively tuned by adjusting or changing the geometric parameters of the metallic resonator. Moreover, we present a simple design of five-band terahertz absorber by further optimizing the sizes of the metallic elements in the top layer of the metamaterial. The design of the unit structures will assist in designing innovative absorbing devices for spectroscopy imaging, detection, and sensing.     

Broadband Plasmon-Induced Transparency in Plasmonic Metasurfaces Based on Bright-Dark-Bright Mode Coupling

Plasmon-induced transparency (PIT), an analog of electromagnetically induced transparency, originates from destructive interference of plasmonic resonators with different quality factors and brings about the extreme dispersion within the narrow transparency window, promising remarkable potential for slow light, nonlinear optics and biochemical sensors. However, sometimes a broad transmission frequency band is more desirable for other applications such as bandpass filters. In general, strong coupling between bright and dark plasmon modes in coupled resonant systems leads to wide transparency bandwidth at the PIT resonance. Based on multi-oscillator coupling theory, a metasurface structure consisting of three perpendicularly connected metallic nanobars is purposefully designed and numerically demonstrated to support broadband PIT spectral response. The near-field patterns indicate that the broadening of the transparent band results from the constructive interference of dual excitations of the single non-radiative (dark) resonator by the two radiative (bright) antennas. These results show that this scheme of bright-dark-bright mode coupling is significantly beneficial for designing filters operating over a broad frequency range.

     

Electromagnetically Induced Transparency and Sharp Asymmetric Fano Line Shapes in All-Dielectric Nanodimer

Low-loss electromagnetically induced transparency (EIT) and asymmetric Fano line shapes are investigated in a simple planar silicon dimer resonator. The EIT and Fano effects emerge due to near-field coupling of the modes supported by both the nanoparticles in a dimer structure. Different configurations of the dimer nanostructure are analyzed, which provide distinct EIT and Fano resonances. Furthermore, the tunability of EIT and Fano resonant modes are incorporated by changing the structural parameters. It is also found that the dimer resonator exhibits high Q factor and large electromagnetic field enhancement at Fano resonance and EIT window due to extremely low absorption loss. Such values and narrow resonances are supposed to be useful highly sensitive sensors and slow-light applications.     

Tailoring Active Far-Infrared Resonator with Graphene Metasurface and Its Complementary

Far-infrared part of electromagnetic spectrum and its technological details have been highly sought after due to its myriad applications including imaging, spectroscopy, industry control, and communication. However, lack of efficient components of electronic and photonic sources/detectors working in this particular spectrum has impeded its widespread application. One of the bottlenecks lies in the compact far-infrared polarization-sensitive resonator/modulator in compatible with pixel-detector for far-infrared spectroscopy. In this work, we demonstrate strong electric resonance response in perforated graphene sheet at this particular electromagnetic region. The results demonstrate inherently different natures for the strong electromagnetic response between graphene-based and metallic metamaterials. Unlike the metallic metamaterials relying on the geometrical inductance for magnetic response, the electric resonance caused by localized dipole/multipolar modes is found to be dominated in graphene and thus enabling sub-wavelength confinement of electromagnetic field. The Babinet’s principle is proposed to be applied for broadband far-infrared modulation and resonant filters design of graphene-based metamaterial. The active tunable electric resonance through electrostatic doping on the graphene-based patterns provides efficient route for compact biosensing, far-infrared imaging, and detection.     

The Influence of Element Deformation on Terahertz Mode Interaction in Split-Ring Resonator-Based Meta-Atoms

The interaction between terahertz (THz) resonance modes and element deformation in rectangular split-ring resonator (RSRR)-based meta-atoms (MAs) is investigated experimentally. Two types of RSRR-based MAs are presented: lateral-varied SRR (LV-SRR) and arm-twisted SRR (AT-SRR). When the distances from the gaps to the opposite sides of above meta-atoms increase from 10 to 40 μm, the inductive-capacitive (LC) resonance modes and dipole oscillation modes exhibit redshift behavior. The quality factor (Q factor) of LC resonance decreases while that of dipole oscillation modes increases. The THz mode interaction is subject to the distance between the gap and opposite side. An extension of lateral side contributes much more to the enhancement of Q factor of dipole oscillation mode than the twisted arms. The relationship between the near-field coupling effect and THz modes is revealed by the analysis of surface currents as well as the electric energy density distribution, as is in agreement with the experimental results.     

Multi-layered Graphene Silica-Based Tunable Absorber for Infrared Wavelength Based on Circuit Theory Approach

Graphene can be utilized as a tunable material for a wide range of infrared wavelength regions due to its tunable conductivity property. In this paper, we use Y-shaped silver material resonator placed over the top of multiple graphene silica-layered structures to realize the perfect absorption over the infrared wavelength region. We propose four different designs by placing the graphene sheet over silica. The absorption and reflectance performance of the structures have been explored for 1500- to 1600-nm wavelength range. The proposed design also explores the absorption tunability of the structure for the different values of graphene chemical potential. We have reported the negative impedance for the perfect absorption for proposed metamaterial absorber structures. All the metamaterial absorbers have reported 99% of its absorption peaks in the infrared wavelength region. These designs can be used as a tunable absorber for narrowband and wideband applications. The proposed designs will become the basic building block of large photonics design which will be applicable for polariser, sensor, and solar applications.

     

A Novel Terahertz Semiconductor Metamaterial for Slow Light Device and Dual-Band Modulator Applications

In this paper, we propose a novel planar semiconductor metamaterial which consists of two H-shape structures which are nested together and composed of InSb deposited on a thin quartz substrate. The two H-shape structures serve as the bright modes and are exited strongly by the incident wave and interact with each other. This coupling leads to a powerful plasmonically induced transparency (PIT) effect at terahertz frequencies. This scheme provides a way to achieve slow light, and the corresponding group index can reach a value of 1300. We calculated group velocity dispersion (GVD) and saw this structure was a low group velocity dispersion (LGVD) system. Therefore, the proposed structure will be useful in designing slow-light devices, optical buffers, delay lines, and ultra-sensitive sensors. We also showed that the proposed design is tunable, namely changes in geometric parameters and type of semiconductor can largely change the group index. In addition, we considered another application for our design that is a thermal dual-band terahertz metamaterial modulator and numerically obtained frequency and amplitude modulation depth, tunability bandwidth, and loss for this device. We obtained an amplitude modulator depth of 99.7 % and a frequency modulator depth of 47 % that verified this structure can be used in wireless communication and encode information systems in the THz regime.     

Which Is the Interpretation of Plasmon Resonance in 2D Split-Ring Resonator Structure—Standing Wave or LC Circuit?

We studied the 2D split-ring resonator (SRR) structure of normal transverse magnetic incidence by changing two parameters separately, the length of the axis of the square ring corresponding to inductance (L) and the width of the gap related to the capacitance (C). The magnetic resonance wavelength of the first mode shows the character of the standing wave when L was changed and C was unchanged. However, it only can be interpreted by LC circuit when C was changed and L was unchanged. The standing wave model and the LC circuit are double faces of magnetic resonance, which provide the design rules for engineering resonant properties of SRRs.     

Narrow-band enhanced absorption of monolayer graphene at near-infrared (NIR) sandwiched by dual gratings

One possibility of enhancing light absorption in monolayer graphene at near-infrared (NIR) wavelength region with grating structures is proposed and investigated. It is demonstrated that it is possible to achieve near-perfect absorption when a single monolayer graphene is sandwiched between two gratings with optimized geometric parameters at normal incidence for transverse electric (TE) polarization. By means of the rigorous coupled-wave analysis (RCWA), the effects of technological tolerances on the optical response of the structure by varying geometric parameters and incident angle are studied. The proposed photonic structure could be efficiently exploited as a building block for innovative optical absorbers or photodetectors in combination with active materials.     

Equivalent Circuit Method Analysis of Graphene-Metamaterial (GM) Absorber

This paper presents an effective method to model and analyze graphene-metamaterial (GM) absorbers by using an equivalent circuit model. A reliable and closed formula to describe the absorption mechanism of the GM structure was derived from this approach. With the obtained expressions, the effect of the graphene chemical potential on the absorber’s resonance frequency is able to be predicted. In order to verify this proposed equivalent circuit method, an absorber consists of metamaterial and graphene was simulated and the physical mechanism was well explained. This method provides an effective way to analyze multilayered GM absorbers for the future.     

Spectrally Tunable Optical Transmission of Titanium Nitride Split Ring Resonators

In this paper, the optical properties of titanium nitride split ring resonators as an intermetallic metamaterial nanostructure were studied. Our simulation shows the presence of plasmon and LC resonances in the transmission spectrum of a cell consists of four u-shape split ring resonators. The effect of different parameters of resonator such as size, periodic constant, and the material between arms in addition to the polarization of incident beam was examined on the resonance behavior of the system. Also, the optical properties of a cell consist of four complementary split ring resonators within titanium nitride thin film were investigated. An excited mode was observed at λ = 840 nm that was attributed to the plasmon resonance. Changing the arrangement and configuration of the system from C _1v to C _2v symmetry led to the presence of the LC mode beside the plasmon mode in the transmission spectrum. Also, we explored a connection between the complementary split ring resonators and orderly perforated surface plasmon systems. It was determined that a transition occurred from resonator-type to surface plasmon behavior by increasing the size of resonator above 170 nm.     

A High-Performance Refractive Index Sensor Based on Fano Resonance in Si Split-Ring Metasurface

We present a high-performance refractive index sensor based on Fano resonance with a figure of merit (FOM) about 56.5 in all-dielectric metasurface which consists of a periodically arranged silicon rings with two equal splits dividing them into pairs of arcs of different lengths. A Fano resonance with quality factor ~133 and spectral contrast ratio ~100% arises from destructive interference of two antiphase electric dipoles in the two arcs of the split-ring. We can turn on and/or off the Fano resonance with a modulation depth nearly 100% at the operating wavelength of 1067 nm by rotating the polarization of incident light. We believe that our results will open up avenues for the development of applications using Fano resonance with dynamically controllability such as biochemical sensors, optical switching, and modulator.     

Magnetic Coupling Effect on Nonlinear Properties in Negative Index Metamaterials with a Kerr Nonlinearity

We investigate magnetic coupling effect on nonlinear electromagnetic properties in a three-dimensional negative index metamaterial constituted by arrays of conducting wires and split-ring resonators embedded into a Kerr nonlinear dielectric. Numerical results show that the switches of nonlinear electromagnetic properties between right-handed and left-handed properties depend closely on magnetic coupling strength, which can be divided into several different coupling regions according to the angular frequency of incident light and the nonlinear types (focused or defocused) of the dielectric. These properties may be instructive for designing optimizely composite metamaterials with negative refraction and provide various routes to manipulating light.     

Perfect Absorption of Light by Coherently Induced Plasmon Hybridization in Ultrathin Metamaterial Film

We demonstrated numerically that light can be totally absorbed by an ultrathin metamaterial film through coherently induced plasmon hybridization. Two fundamental modes, namely symmetrical and antisymmetrical modes, are observed in the metal–insulator–metal structure and attributed to the electric and magnetic resonance, respectively. Each kind of resonance is related to a distinct absorption peak for the corresponding coherent inputs. In particular, it is found that the antisymmetrical absorption is almost omnidirectional and suitable for divergent beams with arbitrary polarization and angle of incidence. To interpret the interaction of magnetic and electric fields with the structure, effective material parameters of the metamaterial are also retrieved, showing good agreement with the intuitive discussion. Furthermore, the general condition of coherent perfect absorption in a metamaterial thin film is given, which could be helpful for the design and understanding of such absorbers.     

Multispectral Broadband Light Transparency of a Seamless Metal Film Coated with Plasmonic Crystals

Broadband light transparency of metallic structures has long been pursued due to the potential applications in the optoelectronic communications, flat panel displays, and clean solar energy. Considerable efforts have been made on the multiband electromagnetic wave transparency of plasmonic metamolecules. However, far less work has been focused on the multispectral light transparency of a seamless metal film. Here, we for the first time propose a seamless metal film structure coated by double conventional plasmonic crystals and demonstrate the observed multispectral broadband light transparency behavior. A maximum transmittance larger than 92 % is achieved. The average transmittance of the whole spectral range from 550 to 1,100 nm is exceeding 45.8 %, suggesting the achievement of an ultra-broadband semi-transparent window. Particularly, the transparency features are highly scalable by tuning the structural parameters. Plasmonic resonances and the metallic particle–film plasmonic interactions are responsible for the observed optical transparency properties. These findings and merits make the proposed structure a good candidate for numerous potential applications, including the optoelectronic components, transparent displayers, and light harvesting.