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.

     

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.

     

Actively Tunable Fano Resonance Based on a T-Shaped Graphene Nanodimer

We present the strength modulation and frequency tuning of Fano resonance by employing a graphene nanodimer formed by two coplanar perpendicular nanostrips with different dimensions. The Fano resonance is induced by destructive interference between the bright dipole mode of a short nanostrip and the dark quadrupole mode of a long nanostrip. The strength, line width, and resonance frequency of the Fano resonance can be actively modulated by changing the spatial separation of those two graphene nanostrips and the Fermi energy of the graphene nanodimer, respectively, without re-fabricating the nanostructures. The tuning of the strength and resonance frequency can be attributed to the coupling strength and optical properties of graphene, respectively. Importantly, a figure of merit value as high as 39 is achieved in the proposed nanostructures. Our results may provide potential applications in optical switching and bio-chemical sensing.     

Dynamically Tunable by Kerr Effect Multichannel Filter Based on Plasmon Induced Transparencies at Optical Communication Range

Dynamically tunable multichannel filter based on plasmon-induced transparencies (PITs) is proposed in a plasmonic waveguide side-coupled to slot and rectangle resonators system at optical communication range. The slot and rectangle resonators in this system can be regarded as radiative or dark resonators as same as the radiative or dark elements in the metamaterial structure with the help of the evanescent coupling. The multiple PIT responses which can enable the realization of nanoscale filter with four channels are originated from the direct near-field coupling and indirect phase couple through a plasmonic waveguide simultaneously. Moreover, the magnitudes and bandwidths of the filter can be efficiently tuned by controlling of the geometric parameters such as the coupling distances and the pump light-induced refractive index change of the Kerr material which is embedded into the metal-dielectric-metal waveguide between the radiative resonators.     

Plasmonically Induced Absorption and Transparency Based on Stub Waveguide with Nanodisk and Fabry-Perot Resonator

A novel metal-insulator-metal (MIM) plasmonic waveguides structure, which is composed by stub waveguide with nanodisk and Fabry-Perot (F-P) resonator, has been proposed and numerically simulated with the finite-difference time-domain (FDTD). Based on the three-level system, the extreme destructive interference between bright and dark resonators gives rise to the distinct plasmonically induced absorption (PIA) response with the abnormal dispersion and novel fast-light feature. Simultaneously, the dramatic double plasmonically induced transparency (PIT) effect with slow-light characteristic can also be achieved in the system. The relationship between the transmission characteristics and the geometric parameters is studied in detail. By optimum design, the modulation depth of the PIA transmission spectrum of 90 % with 0.145 and 0.14 ps fast-light effect can be gained simultaneously, and the peak transmissivity of the double PIT system of 75.2 and 72.8 % with ?0.38 ps slow light-effect can be achieved. The simulated transmission features are in agreement with the temporal-coupled mode theory (CMT). The characteristics of the system indicate an important potential application in integrated optical circuits such as slow-light and fast-light devices, high-performance filter, and optical storage.     

Multispectral Plasmon-Induced Transparency Based on Asymmetric Metallic Nanoslices Array Metasurface

We propose a 3D metasurface structure with unsymmetrical metallic slices array. The tunable plasmon-induced transparency (PIT) effects and different electric field mode distributions could be realized by modulating the structure parameters and angle of incidence. The radiative and dark elements of the asymmetric metallic slices unit cell structure are analyzed. The transmission spectra and the electric fields distributions are studied by the finite element method (FEM). We demonstrate that PIT phenomena based on those metasurface array structures may have applications as tunable sensors and filters in nanophotonics and integrated optics.     

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.     

Manipulating Magnetoinductive Coupling with Graphene-Based Plasmonic Metamaterials in THz Region

Coupling between physical modes can trigger some new physical phenomena such as frequency shift, new mode, and Rabi splitting. Resonant modes in graphene-based metamaterials provide a new platform for the research of coupling. In this work, we demonstrate that the plasmonic coupling of split ring resonator (SRR) dimer in graphene-based metamaterials can be easily manipulated. This magnetoinductive coupling can switch on/off the dark modes easily, which is usually done by symmetry-breaking structure previously. Furthermore, the dark mode can also be activated by Fermi energy as well as carrier concentration changing with either physical or chemical methods conveniently. In addition, different graphene-based SRR dimer configurations present different coupling strengths, which benefits the designing and optimizing of graphene-based metamaterials. The demonstration could enhance the versatility of both coupling studies in terahertz (THz) region and graphene-based metamaterials for THz devices.     

Theoretical Analysis of Plasmon-Induced Transparency in Ring-resonators Coupled Channel Drop Filter Systems

The plasmon-induced transparency (PIT) in ring-resonators coupled channel drop filter (CDF) systems is investigated theoretically and numerically in this paper. A coupled mode theory-based transfer matrix method (CMT-TMM) is introduced owning to the symmetric and evanescent coupling, which is confirmed by the finite-difference time-domain (FDTD) simulation results. The drop waveguide provides the necessary optical feedback for the interference effect in realizing the PIT, and a new way for adjusting PIT effect in a fixed structure is also given. Finally, the phase and the group dispersion in the transparency window are discussed for investigating the slow light effect in our systems, and a group index of ~22 is obtained. The proposed plasmonic systems possess both the slow light and the dropping properties and may have potential and flexible applications in fundamental research of integrated plasmonic devices.     

Dynamically Tunable Terahertz Plasmon-Induced Transparency Analogy Based on Asymmetric Graphene Resonator Arrays

Plasmonics - A plasmon-induced transparency (PIT) effect based on an asymmetric graphene loop structure has been proposed and investigated in this paper. The microstructure consists of a pair of...     

Tunable Plasmon-Induced Transparency Effect in MIM Side-Coupled Isosceles Trapezoid Cavities System

We propose a plasmonic structure based on the metal-insulator-metal waveguide with the side-coupled isosceles trapezoid cavities. Both of the structures based on the side-coupled trapezoid cavities separated or connected with waveguides can realize the plasmon-induced transparency (PIT). By adjusting the structure parameters, the off-to-on PIT response can be tunably achieved. The coupled mode theory (CMT) method is used to study the PIT phenomenon and explain the transmission characteristics. This work may provide a potential way for designing highly integrated photonic devices.

     

Tunable Fano Resonance Based Mode Interference in Waveguide-Cavity-Graphene Hybrid Structure

In this paper, a graphene strip is introduced into a metal-insulator-metal (MIM)-integrated square cavity hybrid structure; the transmission spectra are theoretically investigated by the finite different time domain (FDTD) methods. An asymmetric Fano resonance dip that has high figure of merit (FOM) value appears in the transmission band. According to the multimode interference coupled mode theory (MICMT) analytical method, the Fano resonance originates from the coherent coupling between TM₁₀ cavity magnetic mode and graphene plasmonic resonance electric mode. The center wavelength, full width at half maximum (FWHM), and FOM value of the Fano resonance can be tuned dynamically by altering the Fermi level of the graphene. Through breaking the symmetry of the hybrid structure or introducing double graphene strips with different Fermi level into hybrid structure, double Fano resonance are realized. This study can provide some theoretical basis and design reference for designing ultrahigh sensitivity plasmonic sensor.

     

Dynamically Switchable Multispectral Plasmon-Induced Transparency in Stretchable Metamaterials

We propose dynamically switchable multispectral plasmon-induced transparency (PIT) with high modulation depth in a three-dimensional metamaterial standing on a flexible substrate. The proposed metamaterial is composed of a pair of metal–insulator–metal (MIM) nano-cut-wires and a pair of insulator–metal–insulator (IMI) nano-cut-wires. Results show that two PIT windows can be achieved because of the near-field coupling between the dipole supported by the IMI nano-cut-wire and two quadrupoles supported by the MIM structures. These two PIT windows can be blue-shifted or even flipped over by stretching the substrate along one direction, or be switched off by stretching along the other direction. A classical coupled oscillator model is developed to quantitatively describe and explain these results. We expect this work will find promising applications in multispectral sensors, slow light devices and nonlinear optical devices.

     

Tunable plasmon-induced transparency based on bright-bright mode coupling between two parallel graphene nanostrips

Plasmonics - Tunable plasmon-induced transparency (PIT) is realized for the mid-infrared region only by using two parallel graphene nanostrips. The weak hybridization between the two bright modes...     

Analysis of Mid-Infrared Surface Plasmon Modes in a Graphene-Based Cylindrical Hybrid Waveguide

A graphene-based cylindrical hybrid surface plasmon polariton waveguide, composed of a silicon nanowire core surrounded by a silica layer and then a graphene layer, is investigated using the finite-difference time-domain method. The analytical solutions and the numerical simulation show that an ultra-small mode area and a large propagation length can be achieved with this waveguide. Utilizing the perturbation theory of coupled mode, we demonstrate that the six lowest-order coupling modes originate from the coupling of the three lowest-order single-waveguide modes, and the m?=?1 order yy-coupling mode possesses the maximum coupling length and the minimum crosstalk. This waveguide can be used for photonic integrated circuits in the mid-infrared range.     

Design Optimization and Fabrication of Graphene/J-Aggregate Kretschmann-Raether Devices for Refractive Index Sensing Using Plasmon-Induced Transparency Phenomena

Plasmonics - Here, a novel plasmon-induced transparency (PIT) sensing platform based on a Kretschmann–Raether configuration with graphene/J-aggregate materials is proposed. The J-aggregate...     

Surface Plasmon Polariton Waveguide by Bottom and Top of Graphene

Semiconductor surface plasmon polariton (SPP) waveguide has unique optical properties and compatibility with existing integrated circuit manufacturing technology; thus, SPP devices of semiconductor materials have wide application potential. In this study, a new integrated graphene SPP waveguide is designed using the bottom and top roles of graphene. Moreover, a T waveguide structure is designed by InGaAs of semiconductor gain, with rectangular GaAs material on both sides. The structure adopts light to stimulate the SPP, where its local area is enhanced by the interaction between two interface layers and a semiconductor gain and where its frequency can be adjusted by the thickness of the graphene. Characteristic analysis reveals the coupling between the T semiconductor gain and the SPP mode. The propagation distance of the waveguide can reach 75 cm, the effective mode field is approximately 0.0951λ ², the minimum of gain threshold is approximately 2992.7 cm^?1, and the quality factor (FOM) can reach 180. The waveguide structure which provides stronger localization can be compatible with several optical and electronic nanoscale components. That means, it can provide light for surface plasmon circuit and also can provide a great development in the low-threshold nanolaser.     

Magnetic-Based Fano Resonance by a Trimer with Y-shaped Gap

We introduce a Y-shaped gap into a silver disk to break the structure symmetry which can be looked as a loop-linked structure. Magnetic resonances are excited by incident light when incident electric field is parallel to the trimer plane. Fano resonance is generated by the coupling between bright electric mode and dark magnetic mode. These resonances can be adjusted by tuning the gap size, the radius of trimer, and the position of Y-shaped gap. The extinction cross section of the structure is calculated with the finite element method (FEM). The maximum figure of merit (FOM) is 37.8. Both the magnetic and electric field are greatly enhanced at the Fano dip and the magnetic resonance peak.     

Deep Subwavelength Confinement of Mid-infrared Plasmon Modes by Coupling Graphene-Coated Nanowire with a Dielectric Substrate

In this paper, we report the realization of highly confined mid-infrared plasmon modes by coupling freestanding graphene-coated nanowire with a dielectric substrate. It is shown that the presence of a nearby dielectric substrate not only breaks the azimuthal symmetry of the plasmon modes but also has a strong impact on the coupling and hybridization of these modes. The degree of interactions with the substrate depends on the permittivity of the substrate, the key structural parameters of the nanowire, the operation frequency and chemical potential of graphene, as well as the gap distance between the nanowire and supporting substrate. It is found that compared to freestanding case, using a high-index substrate and adjusting the gap distance can result in the following benefits: (i) an ultra-small mode area and a long propagation length can be realized simultaneously, (ii) a stronger field enhancement in the low-index gap region and improved figure of merit can be achieved, and (iii) a huge reduction in the crosstalk can be made which is crucial for the realization of high-density integrated nanophotonic devices.     

Tunable,Mid-Infrared Ultra-Narrowband Filtering Effect Induced by Two Coplanar Graphene Strips

The two coplanar graphene strips coupling system supported on substrates is proposed and constructed on a monolayer graphene by spatially varying gate voltages. It is investigated numerically by using the finite-difference time-domain method. Simulation results reveal that despite of no traditional ring, disk, and rectangular geometry resonators used usually in metallic plasmonic filters, this structure based on the edge mode propagation exhibits an original, ultra-narrowband band-stop filtering effect in the mid-infrared region. This filtering effect results from the novel side-coupled resonator formed by the parallel graphene strips. The transmission spectrum is tuned and modified not only by engineering the locations of gate voltages without re-fabricating structures but also via changing substrates. Simulation results are consistent with the theoretical analysis. Our studies hence support the fabrication of ultra-compact planar plasmonic devices in nano-integrated circuits.