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.     

Tunable Plasmon-Induced Transparency of Double Continuous Metal Films Sandwiched with a Plasmonic Array

Making a continuous metal film with near-unity transparency has received more and more attention in recent years because of its potential applications for various optoelectronic devices. Here, we theoretically show that a high tunable plasmon-induced transparency metal film structure can be performed by double continuous metal films inserted with a two-dimensional hexagonal lattice array of plasmonic nanopariticles. The proposed structure shows near-unity anti-reflection and intensively enhanced transmission via the cooperative effects of strong resonant near-field light input and output coupling by the plasmonic array and the excitation of surface electromagnetic waves of the metal films. The optical response can be efficiently mediated by varying the sizes of nanoparticles and the separated distance between the metal array and the metal films. With the merits of high transparency, sub-wavelength sizes and wholly retained metal characteristics including high conductivity via using the pure metallic materials, the structure proposed here suggests various potential applications in optoelectronic integrated circuits.     

Coupled Mode Demonstration of Slow-Light Plasmonic Sensor Based on Metasurface at Near-Infrared Region

In this paper, we theoretically and numerically reported a dual plasmon-induced transparency and the relevant sensing property in a multi-cross metasurface by the coupled mode analysis. A phase coupling model was established to characterize the optical response of this plasmonic sensor. It was found that the transparency windows were sensitive to the resonance mode of each metal strip, which was well demonstrated by the theoretical model. Both the sensing property and the slow light in this structure were discussed. A high figure of merit of 223 and sensitivity of 850 nm/RIU were achieved. In addition, the 1170-nm near-infrared light can be slowed down by nearly two order of magnitude with group delay of 0.45 ps in this sensor. These results may provide guidance for light-matter interaction-enhanced slow-light sensor and integrated optical circuit design.

     

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.     

A Three-Dimensional Plasmonic Nanostructure with Extraordinary Optical Transmission

We report a 3D plasmonic nanostructure having an extraordinary optical transmission due to localized surface plasmon (LSP) coupling between nanoholes and nanodisks. The nanostructure contains a free-standing gold nanohole array (NHA) film above a cavity and an array of nanodisks at the bottom of the cavity that is aligned with the NHA. For the device, the LSP-mediated resonance position was dependent on the hole and nanodisk diameter as well as the separation distance. Also, the effect of LSP coupling between each hole and corresponding nanodisk became negligible for cavities deeper than 200 nm as observed as a disappearance of the LSP resonance. The greatest LSP resonance transmission and the highest electric field intensity were observed for the structure with the shallowest cavity. In addition, the structure had high surface plasmon resonance sensitivity and may have potential for surface-enhanced Raman spectroscopy and optical trapping applications.     

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.     

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.     

Plasmon-Induced Transparency and Refractive Index Sensing Based on a Trapezoid Cavity Coupled with a Hexagonal Resonator

Plasmonics - Tunable plasmon-induced transparency (PIT) effect is studied in a plasmonic device on the basis of the metal-insulator-metal (MIM) structure which consists of trapezoid and hexagonal...     

Utilizing the Metallic Nano-Rods in Hexagonal Configuration to Enhance Sensitivity of the Plasmonic Racetrack Resonator in Sensing Application

A high sensitive plasmonic refractive index sensor based on metal-insulator-metal (MIM) waveguides with embedding metallic nano-rods in racetrack resonator has been proposed. The refractive index changes of the dielectric material inside the resonator together with temperature changes can be acquired from the detection of the resonance wavelength, based on their linear relationship. With optimum design and considering a tradeoff among detected power, structure size, and sensitivity, the finite difference time domain simulations show that the refractive index and temperature sensitivity values can be obtained as high as 2610 nm per refractive index unit (RIU) and 1.03 nm/°C, respectively. In addition, resonance wavelengths of resonator are obtained experimentally by using the resonant conditions. The effects of nano-rods radius and refractive index of racetrack resonator are studied on the sensing spectra, as well. The proposed structure with such high sensitivity will be useful in optical communications that can provide a new possibility for designing compact and high-performance plasmonic devices.     

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.

     

A High Performance Plasmonic Sensor Based on Metal-Insulator-Metal Waveguide Coupled with a Double-Cavity Structure

A high performance plasmonic sensor based on a metal-insulator-metal (MIM) waveguide coupled with a double-cavity structure consisting of a side-coupled rectangular cavity and a disk cavity is proposed. The transmission characteristics of the rectangular cavity and disk cavity are analyzed theoretically and the improvements of performance for the double-cavity structure compared with a single cavity are studied. The influence of structural parameters on the transmission spectra and sensing performance are investigated in detail. A sensitivity of 1136 nm/RIU with a high figure of merit of 51,275 can be achieved at the resonant wavelength of 1148.5 nm. Due to the high performance and easy fabrication, the proposed structure may be applied in integrated optical circuits and on-chip nanosensors.     

Fano Resonances Induced by Strong Interactions Between Dipole and Multipole Plasmons in T-Shaped Nanorod Dimer

A simple T-shaped plasmonic nanostructure composed of two perpendicular coupled nanorods is proposed to produce strong Fano resonances. By the near-field coupling between the “bright” dipole and “dark” quadrupole plasmons of the nanorods, a deep Fano dip is formed in the extinction spectrum, which can be well fitted by the Fano interference model. The effects of the geometry parameters including nanorod length, coupling gap size, and coupling location to the Fano resonances are analyzed in detail, and a very high refractive index sensitivity is achieved by the Fano resonance. Also by adjusting the incident polarization direction, double Fano resonances can be formed by the interferences of the dipole, quadrupole, and hexapole plasmons. The proposed nanorod dimer structure is agile, and a trimer which supports double Fano resonances can be easily formed by introducing a third perpendicular coupled nanorod. The proposed T-shaped nanorod dimer structure may have applications in the fields of biological sensing and plasmon-induced transparency.     

A Tunable Slow Light Device with Multiple Channels Based on Plasmon-Induced Transparency

Slow light devices with buffering capability play a critical role in all-optical signal processing. In this paper, multiple slow light phenomena are implemented based on plasmon-induced transparency (PIT) in our device. The device mainly consists of dual tooth cavities coupled with stub resonators, respectively. Temporal coupled-mode theory model illustrates that the triple PIT phenomena can be achieved based on different formation mechanisms. The simulation results calculated by the finite-difference time-domain method reveal that significant slow light response occurs at two wavelength regions. In addition, the parameters of structure have an important influence on PIT response and slow light characteristics. Moreover, the separate manipulation of wavelength, transmission and group index at transparency peak can be achieved in different slow light channels by adjusting the structural parameters. This plasmonic device is of great significance for the design of optical networks on chips.

     

Portable Capillary Sensor Integrated with Plasmonic Platform for Monitoring Water Pollutants

In this work, a label-free and inexpensive method for the monitoring of water pollutants is demonstrated. We introduce a localized surface plasmon resonance (LSPR) based plasmonic capillary optical biosensor to detect microalgae cells. Here, the plasmonic capillary biosensor was prepared by decorating the inner walls of a glass capillary with gold nanoparticles that were employed for investigations. Since the gold nanoparticle has the potential to sense pollutants in water rapidly with high sensitivity and they are expected to perform a significant role in environmental monitoring. Our proposed plasmonic capillary sensor has a detection limit of 25 algal cells (Chlorella sp. CB4). Furthermore, the plasmonic capillary sensing platform significantly simplifies sensor fabrication and reduces the cost of the device. We believe that the presented plasmonic sensor could stand as a potential candidate for developing a cost-effective, label-free, and rapid sensing platform to detect microalgae pollutants present in the water at very low concentrations.

     

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.

     

Design of Plasmonic Comb-Like Filters Using Loop-Based Resonators

A subwavelength plasmonic comb-like filter is proposed by using dual symmetric slot cavities which are placed between two parallel metal–insulator–metal (MIM) structure waveguides. The structure can be considered as a resonance loop which consists of slot cavity resonators and MIM waveguide resonators. The reflective wavelength range and channel spacing are determined by the lengths of slot cavities and MIM waveguides, respectively. Three, four, and five reflective channels with high reflection are achieved in a small wavelength range. Higher channel count can be available by increasing the length or the real part of effective index of MIM waveguides. Such a device can find applications in various optical systems such as wavelength demultiplexing components.     

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.

     

Large-Area Two-Dimensional Plasmonic Meta-Glasses and Meta-Crystals: a Comparative Study

The geometrical arrangement of metallic nanoparticles plays a crucial role on the optical response of nanoplasmonic samples due to particle-particle interactions. In this work, large-area, two-dimensional meta-glasses (random arrangements) and meta-crystals (periodic arrangements) made of identical metallic nanoparticles are investigated for three different particle densities of 5, 10, and 15 discs/μm². A direct comparison between random and periodically ordered arrays is presented. The comparison clearly shows that the particle density has the largest influence on the extinction spectra for both periodic and random samples, and that for equal densities, the optical response away from diffraction effects is strikingly similar in both cases. The role of the radial density function and minimum particle distance is also determined. This study elucidates the role of the particle-particle interactions on the response of plasmonic nanoparticles and indicates how to control position and shape of the plasmonic resonance.     

Plasmonic Optical Properties and Applications of Metal Nanostructures

In this review article, we provide an overview of recent research activities in the study of plasmonic optical properties of metal nanostructures with emphasis on understanding the relation between surface plasmon absorption and structure. Both experimental results and theoretical calculations have indicated that the plasmonic absorption strongly depends on the detailed structure of the nanomaterials. Examples discussed include spherical nanoparticles, nanorods, nanowires, hollow nanospheres, aggregates, and nanocages. Plasmon–phonon coupling measured from dynamic studies as a function of particle size, shape, and aggregation state is also reviewed. The fascinating optical properties of metal nanostructures find important applications in a number of technological areas including surface plasmon resonance, surface-enhanced Raman scattering, and photothermal imaging and therapy. Their novel optical properties and emerging applications are illustrated using specific examples from recent literature. The case of hollow nanosphere structures is highlighted to illustrate their unique features and advantages for some of these applications.