Magnetically Controlled Nanofocusing of a Graphene Plasmonic Lens

In this paper, we propose a method to tailor the nanofocusing of plasmons on graphene plasmonic lens, which is composed of graphene and circular dielectric gratings of magneto-optical material beneath it. With an external magnetic field parallel to graphene surface, the magneto-optical effect of substrate leads to the difference in modal indices of graphene plasmons, which also introduces an additional relative phase difference between these two plasmons during excitation and propagation. Together, these two effects enable us to tailor the position of focal points through external magnetic field, which has been described by an analytical approach based on phase matching and verified by numerical simulations. With an operation wavelength of 8500 nm and an external magnetic field from B = ?1 T to B = 1 T, a shift distance over one and a half times of plasmons wavelength for focal points or donut-shaped field profiles can be obtained under linearly or circularly polarized light. The proposed scheme has potentials in diverse applications, such as the tunable nanofocusing and particle manipulation.     

Low Energy Intraband Plasmons and Electron Energy Loss Spectra of Single and Multilayered Graphene

We present a theory for the calculation of the low energy intraband plasmon frequencies and the electron energy loss (EEL) spectra of single layer and multilayer graphene sheets. Our calculation shows that the number of plasmons that can be excited is equal to the number of graphene layers in the sample. One of these is the dominant in-phase plasmon having a square root dependence on the wave number at low wave vectors, whereas the others are out-of-phase plasmons having near linear dependences on the wave number. The EEL spectra of a single layer graphene shows a single peak at the plasmon frequency, which has been observed experimentally. The EEL spectra of all multilayer graphenes have two peaks, one corresponding to the dominant in-phase plasmon and the other due to the out of phase plasmons. We predict that careful measurement of the EEL of multilayer graphene will show both peaks due to the low energy intraband plasmons.

     

Fishbone-Like High-Efficiency Low-Pass Plasmonic Filter Based on Double-Layered Conformal Surface Plasmons

In this work, we report a fishbone-like high-efficiency low-pass plasmonic filter based on a double-layered conformal surface plasmon waveguide (CSPW) which consists of double-layered symmetrical metal gratings (SMGs) of fishbone shape. Efficient mode conversion between the quasi-transverse electromagnetic (TEM) waves in the microstrip line and the conformal surface plasmons (CSPs) on the double-layered CSPW is realized by using gradient double-layered SMGs and impedance matching technique. Experimental results of the transmission and reflection coefficients of the straight sample show excellent loss-pass performance and agree well with the numerical simulations. The curved samples exhibit low radiation loss when the double-layered CSPW is conformal or even bent thanks to the high confinement of CSPs. The proposed structure can find potential applications in integrating conventional circuits with CSPs devices at microwave and terahertz frequencies.

     

Ultra-compact Spatial Terahertz Switch Based on Graphene Plasmonic-Coupled Waveguide

We are proposing graphene (G)-based multilayered plasmonic spatial switch, operating at 10 THz. It is composed of hBN/Ag/hBN/G/hBN/G/hBN/SiO₂/p⁺-Si multilayers. When a 10-THz transverse magnetic (TM)-polarized signal is normally incident upon the structure top surface, the nanoaperture devised in the Ag nanolayer, acting as a grating, excites surface plasmons at the top graphene micro-ribbons/hBN interface. These surface plasmons depending on the graphenes chemical potentials can be coupled to the lower-right or left graphene micro-ribbons and continue to propagate laterally towards the corresponding output port. Numerical simulations show that a change of ∆V_G ≈ ± 2.7 V in the voltage, applied to the gated micro-ribbons, can modulate their chemical potentials sufficiently to switch the right (left) output port from ON (OFF) to OFF(ON) and vice versa. Besides its low power consumption, the switch ultra-small dimensions make it a potential spatial router suitable for THz-integrated circuit applications.

     

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.

     

Tunnel Light Through a Continuous Optically Thick Metal Film Utilizing Higher Order Magnetic Plasmon Resonance

In this letter, we investigate the extraordinary optical transmission behavior of a flat continuous metal film sandwiched by magnetic plasmonic structures. A new mechanism by utilizing higher order magnetic plasmon resonance is proposed to enhance the transmission. Numerical simulation results show that 80 % electromagnetic energy can be transmitted through the middle 50-nm-thick continuous gold film in near-infrared regime. The excitation of the second-order magnetic plasmons and the propagating surface plasmons, as well as the interaction between them accounts for such a high transmission. The interaction of magnetic plasmons and surface plasmons leads to new hybrid modes, and the coupled oscillator model is introduced to analyze this hybridization. This work extends the application range of higher order magnetic plasmons and may have potential in transparent electrode and electromagnetic energy transfer applications.     

Tunable Localized Surface Plasmon Resonances in a New Graphene-Like Si2BN’s Nanostructures

The optical response of a new graphene-like material Si₂BN’s nanostructures and some kinds of hybrid structures formed by Si₂BN and metal nanoparticles was studied by using time-dependent density functional theory (TDDFT). We found that the periodic structures of Si₂BN have wider absorption ranges than graphene. When the impulse excitation polarizes in different directions (armchair-edge direction and zigzag-edge direction), the absorption spectra of Si₂BN nanostructures would be different (optical anisotropy). And in the hybrid structures, the increase of metal nanoparticles’ number brings the absorption intensity strengthening and red shift, which means a stronger ability of localized surface plasmon tuning. Also, the different metal nanoparticles were used to form the hybrid structures; they show an obviously different property as well. In addition, in the kinds of situations mentioned above, the plasmons were produced in visible region. This investigation provides an improved understanding of the plasmon enhancement effect in graphene-like photoelectric devices.

     

Enhancing the Dual-Band Guiding Capabilities of Coaxial Spoof Plasmons via use of Transmission Line Concepts

We derive closed analytical forms for the response of coaxial spoof plasmons, aided by transmission line concepts under the effective complex surface impedance framework. This constitutes a powerful platform to improve as well as to elucidate designs with enhanced performances. In particular, we propose a dual-band spoof plasmon waveguiding geometry with the higher order slow-wave mode operating well below the regime governed by dispersion of periodic guides (Bragg reflections at Brillouin zone boundaries), that is, diffraction. The analysis is supported by eigen mode numerical calculations. As an example in a waveguide device context, we demonstrate the dual-band planar routing ability of elliptical–coaxial cable-based spoof plasmons along a straight chain as well as a Y-splitter.     

Unidirectional Optical Transmission of a Dual Metallic Grating with Grooves

In this paper, a dual grating structure for unidirectional transmission is presented. The forward and backward transmission performances have been investigated by finite element method. To enhance the forward transmission and to suppress the backward transmission simultaneously, we suggested to cut grooves on the surfaces of one of the gratings, and the effects of the grooves on the optical transmission have been studied. The numerical simulation results reveal that the transmission contrast ratio and the optical unidirectional transmission of the structure can be improved markedly by properly arranging the size and the position of the grooves. The forward transmission can be more than 90%, while the backward transmission transmittance is less than 5%.

     

Sensitive label-free biosensors by using gap plasmons in gold nanoslits

The detection sensitivities of gap plasmons in gold nanoslit arrays are studied and compared with surface plasmons on outside surface. The nanoslit arrays were fabricated in a 130 nm-thick gold film with various slit widths. For transverse-magnetic (TM) incident wave, the 600 nm-period nanoslit array shows two distinguishable transmission peaks corresponding to the resonances of gap plasmons and surface plasmons, respectively. The surface sensitivities for both modes were compared by coating thin SiO(2) film and different biomolecules on the nanoslit arrays. Our experimental results verify gap plasmons are more sensitive than conventional surface plasmons. Its detection sensitivity increases with the decrease of slit width. The gap plasmon is one order of magnitude sensitive than the surface plasmon for slit widths smaller than 30 nm. We attribute this high sensitivity to the large overlap between biomolecules and nanometer-sized gap plasmons.     

Optically responsive nanoparticle layers for the label-free analysis of biospecific interactions in array formats

A novel nanocomposite surface is prepared by coating surface-adsorbed dielectric colloidal particles with a contiguous layer of gold nanoparticles. The resulting surface shows pronounced optical extinction in reflection with the extinction peaks located in the UV–Vis and NIR region of the electromagnetic spectrum. The peak positions of these maxima change very sensitively with the adsorption of organic molecules onto the surface. For the adsorption of a monolayer of octadecanethiol, we observe a peak shift of 55 nm on average, which is about five times that of established label-free sensing methods based on propagating and localized surface plasmons. In a first proof-of-principle experiment, the interaction of peptides with specific antibodies has been detected without labeling by means of a fiber-optical set-up with microscopic lateral resolution. To avoid crosstalk in high-density arrays, the optically responsive surface areas can be locally separated on a micro- or even nanometer scale. Accordingly, the newly developed optically responsive surfaces are well suited for integration into high-density peptide or DNA arrays as demanded in genomics, proteomics, and biomedical research in general.     

Alkali Pretreatment of Wheat Straw (Triticum aestivum) at Boiling Temperature for Producing a Bioethanol Precursor

Polyamines have been shown to be necessary for the activity of the extracellular ice–nucleating matter (EIM) from the ice–nucleating bacterium, Erwinia uredovora KUIN-3. When this bacterium was cultured in the presence of methylglyoxal bis(guanylhydrazone), MGBG (2 mM), the ice–nucleating activity of the EIM significantly decreased. Further, the thermal (25–40°C) and pH (alkaline region) stabilities of the activity were stimulated by the addition of spermidine. This phenomenon only occurred in the class A and B structures, and it showed that the hydrophobicities of the class A and B structures in the EIM increased with the addition of spermidine as judged by the freezing difference spectra. We then found by using fluorescent reagents that the physiological roles of spermidine in the EIM controlled the charge, free-amino groups, and hydrophobicities on the surface of the EIM. In conclusion, one could predict that spermidine took part in the charge of the surface, the control of hydrophobicity, and the stability of protein conformation in the class A and B structures in the EIM, and is a critical component in the class A and B nucleating structures.     

Perfect Terahertz Absorption with Graphene Surface Plasmons in the Modified Otto Configuration

Perfect terahertz (THz) absorption in the modified Otto configuration with the insertion of monolayer graphene sheet has been numerically demonstrated. This perfect absorption originates from the enhancement of the electrical field owing to the excitation of the transverse magnetic (TM) polarized surface plasmons at the interface of two dielectrics with monolayer graphene. It is found that the absorption peak occurs at the specific incident angles, which can be employed for realizing the angular absorbers. We further demonstrate that the angle of the peak absorption and the corresponding wavelength can be manipulated by changing the Fermi energy of monolayer graphene sheet via electrostatic biasing. Moreover, the behaviors of the perfect absorption are strongly dependent on the dielectric constants and thicknesses of the surrounding dielectrics.     

Plasmon-Enhanced Fluorescence Biosensors: a Review

Surfaces of metallic films and metallic nanoparticles can strongly confine electromagnetic field through its coupling to propagating or localized surface plasmons. This interaction is associated with large enhancement of the field intensity and local optical density of states which provides means to increase excitation rate, raise quantum yield, and control far field angular distribution of fluorescence light emitted by organic dyes and quantum dots. Such emitters are commonly used as labels in assays for detection of chemical and biological species. Their interaction with surface plasmons allows amplifying fluorescence signal (brightness) that accompanies molecular binding events by several orders of magnitude. In conjunction with interfacial architectures for the specific capture of target analyte on a metallic surface, plasmon-enhanced fluorescence (PEF) that is also referred to as metal-enhanced fluorescence (MEF) represents an attractive method for shortening detection times and increasing sensitivity of various fluorescence-based analytical technologies. This review provides an introduction to fundamentals of PEF, illustrates current developments in design of metallic nanostructures for efficient fluorescence signal amplification that utilizes propagating and localized surface plasmons, and summarizes current implementations to biosensors for detection of trace amounts of biomarkers, toxins, and pathogens that are relevant to medical diagnostics and food control.

     

Optical Near-Field Enhancement with Graphene Bowtie Antennas

In this paper, the optical near-field enhancement of graphene bowtie antennas is numerically investigated at terahertz frequencies using boundary element method. The enhanced field intensity at the gap region is a result of the mutual coupling between two triangular elements upon the excitation of graphene plasmons. Firstly, wide plasmon frequency tunability is demonstrated by changing the chemical potential of graphene without the need to alter the antenna geometry. Secondly, by varying the tip angle and radius of curvature of the graphene antennas, the field intensity enhancement at the gap center of the two-element antennas is systematically studied. It is found that graphene bowtie antennas with two round-cornered equilateral triangles have superior performance to other two-element antennas, such as ribbon pair, sharp-cornered bowtie, and disk pair antennas. Last but not least, by applying a moderate chemical potential of 0.4 eV to graphene bowtie antennas, we found that the field intensity enhancement at gap center is about 220 times as much as using gold of comparable sizes. In short, graphene bowtie antennas of rounded corners give rise to considerable near-field enhancement and are promising for a wide range of applications such as molecular sensing at terahertz frequencies.

     

Analysis of Stacked Structures Composed of Arrays of Thick Slits: an Accurate Analytical Circuit Model

In this paper, a stacked structure composed of periodic arrays of one-dimensional thick slits embedded in a conventional dielectric medium is investigated in the subwavelength regime. Arrays of thick slits are known to support extraordinary transmission resonances. When periodically embedded in multilayered structures, they demonstrate band gap properties, which can produce flat passband regions in some structures, applicable to filter designs. In addition, by adjusting the parameters of the structures, they can be designed to create epsilon-near-zero and negative permittivity metamaterials. The analysis is carried out based on a simple and accurate analytical solution. The employed circuit model includes a transmission line corresponding to the slits, terminated by two surface admittances at the interfaces. The surface admittances assume the role of the diffractive modes and dominate the limitations of the usual analytical surface admittances obtained through heuristic approaches. A Π network of lumped elements equivalent to this circuit model is introduced in the present paper. This network helps to find the source of extraordinary resonances. Finally, the electromagnetic wave transmission through the stacked structure is studied and the effects of the thickness of the slits and dielectric slabs on the transmission spectra are analyzed. The results are compared to those obtained by full wave simulations, showing good agreement.

     

Coarse-grained molecular simulation of self-assembly nanostructures of CTAB on nanoscale graphene

Coarse-grained molecular dynamics simulation has been performed to study the aggregated morphology of the cationic surfactant, cetyltrimethylammonium bromide (CTAB), adsorbed on nanoscale graphene surfaces. The CTAB surfactants can self-assemble on graphene to form various supramolecular morphologies and structures. The effect of packing density, thickness of graphene sheet and width of graphene nanoribbon on the CTAB–graphene self-assembly has been investigated. The buoyant densities of various graphene–CTAB assemblies were calculated, which increase with surfactant coverage and number of graphene layers. This result demonstrates that density gradient can be used to isolate graphenes with various layers. This simulation provides larger-scale microscopic insight into the supramolecular self-assembly nanostructures for the CTAB surfactants aggregated on graphene, which could be valuable to guide fabrication of graphene-based hybrid nanocomposites.     

A Tunable Perfect THz Metamaterial Absorber with Three Absorption Peaks Based on Nonstructured Graphene

In this paper, a graphene-based tunable multi-band terahertz absorber is proposed and numerically investigated. The proposed absorber can achieve perfect absorption within both sharp and ultra-broadband absorption spectra. This wide range of absorption is gathered through a unique combination of periodically cross- and square-shaped dielectrics sandwiched between two graphene sheets; the latter enables it to offer more absorption in comparison with the traditional single-layer graphene structures. The aforementioned top layer is mounted on a gold plate separated by a Topas layer with zero volume loss. Furthermore, in our proposed approach, we investigated the possibility of changing the shapes and sizes of the dielectric layers instead of the geometry of the graphene layers to alleviate the edge effects and manufacturing complications. In numerical simulations, parameters, such as graphene Fermi energy and the dimensions of the proposed dielectric layout, have been optimally tuned to reach perfect absorption. We have verified that the performance of our dielectric layout called fishnet, with two widely investigated dielectric layouts in the literature (namely, cross-shaped and frame-and-square). Our results demonstrate two absorption bands with near-unity absorbance at frequencies of 1.6–2.3 and 4.2–4.9 THz, with absorption efficiency of 98% in 1.96 and 4.62 THz, respectively. Moreover, a broadband absorption in the 7.77–9.78 THz is observed with an absorption efficiency of 99.6% that was attained in 8.44–9.11 THz. Finally, the versatility provided by the tunability of three operation bands of the absorber makes it a great candidate for integration into terahertz optoelectronic devices.

     

Förster Resonance Energy Transfer Between Molecules in the Vicinity of Graphene-Coated Nanoparticles

The recent demonstration of the plasmonic-enhanced Förster resonance energy transfer (FRET) between two molecules in the vicinity of planar graphene monolayers is further investigated using graphene-coated nanoparticles (GNP). Due to the flexibility of these nanostructures in terms of their geometric (size) and dielectric (e.g., core material) properties, greater tunability of the FRET enhancement can be achieved employing the localized surface plasmons. It is found that while the typical characteristic graphene plasmonic enhancements are manifested from using these GNPs, even higher enhancements can be possible via doping and manipulating the core materials. In addition, the broadband characteristics are further expanded by the closely spaced multipolar plasmon resonances of the GNPs.     

Plasmonic Properties of Gold Nanostructures on Gold Film

This paper reports on a systematic study of the plasmonic properties of periodic arrays of gold cylindrical nanoparticles in contact with a gold thin film. Depending on the gold film thickness, it observes several plasmon bands. Using a simple analytical model, it is able to assign all these modes and determine that they are due to the coupling of the grating diffraction orders with the propagating surface plasmons travelling along the film. With finite difference time domain (FDTD) simulations, it demonstrates that large field enhancement occurs at the surface of the nanocylinders due to the resonant excitation of these modes. By tilting the sample, it also observes the evolution of the spectral position of these modes and their tuning through nearly the whole visible range is possible. Such plasmonic substrates combining both advantages of the propagative and localised surface plasmons could have large applications in enhanced spectroscopies.