Near-Field Coupling Method for Subwavelength Surface Plasmon Polariton Waveguides

A near-field coupling method for studying propagation properties of surface plasmon polariton (SPP) in subwavelength dielectric-loaded SPP waveguides (DLSPPWs) is presented. In this method, a tapered fiber probe is employed to generate a nanometer optical spot. When this spot is near the entrance of the DLSPPW with its polarization parallel to the waveguide, a strong guiding wave is observed by a leakage radiation microscope. For DLSPPWs with a dielectric height of about 600 nm, we observed SPP waves with zigzag propagation patterns at 650 nm wavelength. Such zigzag propagation results in a great reduction of propagation loss. In addition, the zigzag wave has a strong optical confinement. The bending loss for an L-bend DLSPPW is only about 0.4 dB.     

Near-Field Imaging of Surface Plasmon Polaritons Excited by Chains of Gold Nanodiscs

We present a study of the near-field pattern created by chains of gold nanodiscs situated on a gold thin film and illuminated at oblique incidence. Each disc generates surface plasmon polaritons that propagate on the gold surface. The created waves interfere between them and with the illuminating beam. We observed that when the discs are separated by a distance smaller than the half wavelength, the chain behaves like a continuous ridge. When the discs separation increases, a complex periodic pattern appears and extends up to several wavelengths from the chain. For some specific separation distances, a directional emission of surface plasmon is also observed. The experimental results are in good agreement with numerical simulations performed by considering each disk as an independent dipole-like surface plasmon source.     

Molecular Optical Imaging with Radioactive Probes

Background

Optical imaging (OI) techniques such as bioluminescence and fluorescence imaging have been widely used to track diseases in a non-invasive manner within living subjects. These techniques generally require bioluminescent and fluorescent probes. Here we demonstrate the feasibility of using radioactive probes for in vivo molecular OI.

Methodology/Principal Findings

By taking the advantages of low energy window of light (1.2–3.1 eV, 400–1000 nm) resulting from radiation, radionuclides that emit charged particles such as β⁺ and β⁻ can be successfully imaged with an OI instrument. In vivo optical images can be obtained for several radioactive probes including 2-deoxy-2-[¹⁸F]fluoro-D-glucose ([¹⁸F]FDG), Na¹⁸F, Na¹³¹I, ⁹⁰YCl₃ and a ⁹⁰Y labeled peptide that specifically target tumors.

Conclusions/Significance

These studies demonstrate generalizability of radioactive OI technique. It provides a new molecular imaging strategy and will likely have significant impact on both small animal and clinical imaging.     

On Plasmon Polariton Propagation Along Metallic Nano-Chain

The collective wave type plasmon polariton self–modes in the metallic (Au, Ag) nano-chain were determined and analyzed with respect to the nano-sphere size and chain separation parameters. At some regions for parameters, the undamped modes were identified when the interaction had been assumed as the near-field-zone dipole coupling. These modes were found on the rim of stability of the linear theory, which indicates artifact of the model of near-field coupling. Inclusion of the medium- and far-field zone contributions to dipole interaction removes, however, instability and allows for fully analytical demonstration of quenching of irradiation losses of plasmon polaritons in the chain to the level of only ohmic attenuation. The plasmon polariton dispersion and the group velocity of plasmon polariton wave packets were examined with respect to nano-sphere and chain parameters and mode polarization. Previous numerical results related to long-range plasmon polariton propagation in the chain are transparently interpreted within the analytical approach.     

Three-Dimensional Manipulations of Surface Plasmon Polariton Wave Propagation

The plasmonic integrated circuit, a potential application of surface plasmon polaritons (SPPs), can manipulate an SPP wave propagating on a metal surface in a way similar to electronic circuits. Here, we propose the concept of three-dimensional (3D) SPP wave manipulation: control of an SPP wave propagating in both the horizontal direction and the vertical direction. A hole set in the film can guide an SPP wave in the vertical direction. In the horizontal direction, two holographic groove patterns are used to focus an incident SPP wave on one surface of the film to the hole and control the divergent SPP waves transmitted from the hole on the other metal surface, respectively. The holographic groove patterns are designed via the methodology of surface electromagnetic wave holography. 3D finite-difference time-domain method simulations show a good performance of the 3D manipulation via these designed holographic groove patterns.

     

Coupled Mode Theory for Surface Plasmon Polariton Waveguides

In allusion to special modes supported by surface plasmon polariton (SPP) waveguides, explicit expression for mode coupling coefficient which plays a central role in coupled mode theory is firstly redefined by adding the longitudinal electric field component. The mode coupling coefficients calculated by the proposed formula improve greatly compared with the coupled mode theory suited to conventional optical waveguides, and reasonable explanations from the point of view of physics and mathematics have been given. Afterwards, the coupling lengths, the transmission lengths, the normalized power exchanges, and the cross talk performances of adjacent parallel SPP waveguides with varying waveguide separation distances D and waveguide lengths L are investigated at telecom wavelength. The results are encouraging as they indicate that the coupled mode theory is developed in a self-consistent manner by retaining the longitudinal electric field component in the derivation and neglecting it only when the waveguides structure satisfies the weakly guiding situations. As a result, the new mode coupling coefficient formula for SPP waveguides considered in this paper is an important complement in the theory of SPP waveguides.     

Tunable Multi-Port Surface Plasmon Polariton Excitation with Nanostructures

Surface plasmon polaritons (SPPs) have appealing features such as tighter spatial confinement and higher local field intensity. Manipulation of surface plasmon polaritons on metal/dielectric interface is an important aspect in the achievement of integrated plasmonic circuit beyond the diffraction limit. Here, we introduce a design of pin cushion structure and a holographic groove pattern structure for tunable multi-port SPPs excitation and focusing. Free space light is coupled into SPPs through momentum matching conditions. Both nanostructures are capable of tunably controlling of SPPs depending on the incident polarizations, while the holographic method provides more flexibility of wavelength-dependent excitations. Furthermore, a quantitative method is applied to calculate the efficiencies of excitation for both nanostructures under different conditions, including radially polarized incident beams. These results can work as a guidance and be helpful to further choice of the suitable design strategies for variable plasmonic applications such as beam splitter, on-chip spectroscopy, and plasmonic detectors.     

Performance of Scanning Near-Field Optical Microscope Probes with Single Groove and Various Metal Coatings

We investigate the performance of a simple corrugated aperture scanning near-field optical microscope (SNOM) probe with various cladding metals. The probes have only one corrugation, however, they offer increased transmission over both uncorrugated probes and those with many grooves. Enhancement of light throughput results from excitation of surface plasmons at the corrugation at the core–cladding interface. We show how the choice of metal influences radiation properties of grooved probes.     

A Dielectric-Thickness-Adjusting Method for Manipulating Graphene Surface Plasmon Polariton

We propose and numerically investigate a dielectric-thickness-adjusting method to manipulate the graphene surface plasmon polariton (SPP). The dispersion relationships of graphene SPP at different dielectric thickness are derived by solving the analytic equations. In addition, the SPP effective index at cutoff dielectric thickness is obtained according to different dielectric permittivity and working frequencies. As a typical application, a plasmonic Bragg reflector is designed by alternately depositing dielectric gratings along the transverse direction of the SPP propagation. The performance of the Bragg reflector is analyzed at different grating thickness, and the effective index at cutoff thickness is verified by numerical simulation. The proposed method will have important potential prospects in designing graphene-based wave trapping and slow wave devices in future.     

Graphene-based Superlens for Subwavelength Optical Imaging by Graphene Plasmon Resonances

Plasmonics - Recently, graphene plasmons with an excellent tenability by doping or gating have been drawing increasing interest. In this work, we designed graphene-based superlens to achieve...     

Imaging of Oil/Monoglyceride Networks by Polarizing Near-Field Scanning Optical Microscopy

Polarizing near-field scanning optical microscopy (NSOM) was applied for visualization of lipid coagel structures. The technique ensures obtaining polarization contrast images at micro- and nanoscale resolution. Comparison to the polarizing light microscopy images revealed that the same fractal structural organization persists also at submicron scale, at the level of primary ordered structures creation. Many long birefringent needle-shaped primary crystallites were imaged in the corn oil:monoglyceride samples, and lower amount of smaller oval-shaped primary crystallites—in the olive oil:monoglyceride samples. Unlike atomic force microscopy, polarizing NSOM brought direct evidence on the physical state of specific features. Compared to the polarizing light microscopy, polarizing NSOM provided additional information on the structural organization of oil–monoglyceride coagels at the micro- and submicron scale.     

Novel Ridge-Type Gold Film Waveguide for Surface Plasmon Polariton Laser

Plasmonics - Surface plasmon polariton lasers are the basis for photonic circuits, but their losses, thresholds, and some other problems remain thorny issues. In this study, we put forward a novel...     

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.     

Direct Observation of Surface Plasmon Polariton Propagation and Interference by Time-Resolved Imaging in Normal-Incidence Two Photon Photoemission Microscopy

Plasmonics - Time-resolved imaging of the propagation and interference of isolated ultrashort surface plasmon polariton wave packets is demonstrated using two photon photoemission microscopy. The...     

Anticrossing Behavior of Surface Plasmon Polariton Dispersions in Metal-Insulator-Metal Structures

The anticrossing behavior of dispersion curves of the surface plasmon polaritons supported by metal-insulator-metal structures are studied experimentally and theoretically. Samples consisting of a poly(methyl methacrylate) layer sandwiched by Ag films are prepared and their angle- and wavelength-scan attenuated total reflection spectra are measured. From an analysis of the angle-scan spectrum, the coupled-mode nature of the surface plasmon polariton modes is suggested. The dispersion relations obtained from the wavelength-scan spectra exhibit clearly the anticrossing behavior that arises from the coupling of the modes. The experimental dispersion relations are in good agreement with theoretical ones.     

Highly Sensitive Refractive Index Sensing with Surface Plasmon Polariton Waveguides

Two prototypical transducer structures are proposed, including a single-waveguide (SW) and Mach–Zehnder interferometer (MZI), implemented with surface plasmon polariton waveguides. Formulas of the output power with structural parameters are deduced respectively. The sensitivities are found to be proportional to S ₁ for SW and S ₂ for MZI, which are dependent on waveguide parameters. Maximizing S ₁ or S ₂ maximizes the corresponding sensitivity, leading to optimized waveguide designs and preferred operating wavelengths. Sensitivity parameters S ₁ and S ₂ are calculated for fundamental modes of V grooves, triangular wedges, and dielectric-loaded surface plasmon polariton waveguides (DLSPPWs), as a function of measured material refractive index n _c (n _c ?=?1.3～1.6, representative refractive index of biochemical matter), at wavelength λ?=?1.55 μm. Finally, the sensitivity S ₂ is analyzed as a function of work wavelength for DLSPPWs with different ridge thickness and specific fluidic SPP waveguide for biochemical sensing is presented. The results offer foundations for application of surface plasmon polariton waveguides in biochemical sensing.     

Coupling Behaviors of Surface Plasmon Polariton and Localized Surface Plasmon with an InGaN/GaN Quantum Well

Plasmonics - Surface plasmon (SP) coupling behaviors of an InGaN/GaN quantum well (QW) with surface plasmon polariton (SPP) induced on a smooth Ag-film/GaN interface and localized surface plasmon...     

Bionanofabrication by Near-Field Optical Methods

Near-field optical methods offer unique potential in nanofabrication, because they provide the capacity to initiate highly selective chemical transformations with nanometer scale precision. The basic principles behind scanning near-field photolithography (SNP), in which a scanning near-field optical microscope coupled to a UV laser is used to initiate surface chemical reactions, are described. The fundamental principles underlying the patterning of self-assembled monolayers by SNP are described, and the resolution limits and the basic principles that enable routine achievement of sub-50 nm resolution are discussed. Illustrations are provided of the application of SNP to the patterning of protein molecules on gold surfaces. The patterning of molecular adsorbates on oxide surfaces, including the fabrication of highly miniaturized arrays of DNA on silicon dioxide, is also described. It is argued that SNP holds great promise for the organization of biomolecules on nanometer length scales.     

Generating Surface Plasmon Polariton Airy Beam with Dielectric Relief Holographical Structures

Plasmonics - Surface plasmon polariton (SPP) Airy beams are very attractive in theory and applications. We propose a new method to design dielectric relief holographical structures for controlling...     

Fully Metal-Coated Scanning Near-Field Optical Microscopy Probes with Spiral Corrugations for Superfocusing under Arbitrarily Oriented Linearly Polarised Excitation

We study the effect of a spiral corrugation on the outer surface of a fully metal-coated scanning near-field optical microscopy (SNOM) probe using the finite element method. The introduction of a novel form of asymmetry, devoid of any preferential spatial direction and covering the whole angular range of the originally axisymmetric tip, allows attaining strong field localization for a linearly polarised mode with arbitrary orientation. Compared to previously proposed asymmetric structures which require linearly polarised excitation properly oriented with respect to the asymmetry, such a configuration enables significant simplification in mode injection. In fact, not only is the need for the delicate procedure to generate radially polarised beams overcome, but also the relative alignment between the linearly polarised beam and the tip modification is no longer critical.