Investigation of plasmon resonances in local structures by the discrete sources method

The modification of the Discrete Sources Method, which allows one to perform effective numerical analysis of plasmon resonances in local structures, is proposed and realized. The numerical algorithm allows computation of the near field with a high degree of accuracy and tracking resonances that cause an increase of the near-field intensity by several orders of magnitude.     

Analysis of spatial resonances in the field of evanescent waves by the discrete source method

Scattering properties of particles placed on a metal film deposited on a glass prism are studied in detail using a computer implementation of the discrete source method. The presence of scattering cross-section resonances in the region of angles of incidence of a plane wave has been established. The possibility of controlling the radiation directionality in the resonance range is demonstrated. Conditions that permit one to increase the resonance intensity by an order of magnitude have been found.     

Analysis of scattering properties of optical antennas by discrete sources method

Based on the discrete sources method, a mathematical model of the optical antenna is developed. Scattering properties of antennas as clusters of nanodimensional particles in a film on a substrate were analyzed. The results of computer simulation of the scattered intensity are presented.     

Enlarging the negative-index bandwidth of optical metamaterials by hybridized plasmon resonances

By exploiting the concept of internal surface plasmon polariton (I-SPP) resonances, which appear at nonsingle metallic film stacks, we have designed a metamaterial showing a negative effective index within a large frequency bandwidth. The designed structure consists of an arrangement of several fishnet layers. By properly adjusting the lattice and the thickness of the dielectric slab of the fishnet, an I-SPP mode can be excited at a certain frequency, giving rise to a negative permeability. Thus, when combining several fishnet layers, each one configured to excite an I-SPP at a slightly different frequency, the coupling among the fishnet layers will cause a plasmon hybridization effect that enables us to extend the negative-index bandwidth.     

Nanoparticle plasmon resonances in the near-static limit

Localized surface plasmon resonances of metal nanoparticles of arbitrary shape are analyzed in the near-static limit with retardation included to the second order. Starting from the electrostatic approximation, the second-order correction to the resonant dielectric constant is expressed by means of a triple surface integral. For arbitrary nanoparticles with cylindrical symmetry we show how the triple surface integral can be significantly simplified, resulting in a computationally efficient scheme for evaluation of nanoparticle plasmon eigenresonances in the near-static limit. The approach allows for calculation of both dipolar and higher-order resonances.     

Shape-dependent plasmon resonances of Ag nanostructures

Different silver nanostructures have been rapidly synthesized under microwave irradiation from a solution of silver nitrate (AgNO₃) and β$\beta$-D glucose; neither additional reducing nor capping agent were required in this soft green solution approach. Not only spherical nanoparticles, but also necklace and wires have been synthesized. The plasmon resonances of the synthesized silver nanostructures were tuned by varying the irradiation time and hence by changing size and morphology of nanostructures. The obtained nanostructures were characterized by X-Ray diffraction (XRD), Uv–Vis spectroscopy (Uv–Vis), transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM). The change of peak position and the shape of the absorption spectra were clearly observed during the whole reaction process; in fact, it was evidenced that initially Ag nanoparticles were formed, which, as reaction time elapsed, self-assembled and fused with each other to yield nanowires.     

Investigation of the resonance properties of nanodimensional rods by the method of discrete sources

Based on the method of discrete sources, a mathematical model for the analysis of the spectral characteristics of the scattering of nonradiative waves by nanodimensional rods arranged on the surface of a glass prism was constructed. It was shown that the spectral characteristics significantly depend on both the elongation of the rods and their orientation.     

Interplay of plasmon resonances in binary nanostructures

By introducing the difference permittivity ratio η=(ε ₂?ε ₀)/(ε ₁?ε ₀), the Green matrix method for computing surface plasmon resonances is extended to binary nanostructures. Based on the near field coupling, the interplay of plasmon resonances in two closely packed nanostrips is investigated. At a fixed wavelength, with varying η the resonances exhibit different regions: the dielectric effect region, resonance chaos region, collective resonance region, resonance flat region, and new branches region. Simultaneously, avoiding crossing and mode transfer phenomena between the resonance branches are observed. These findings will be helpful to design hybrid plasmonic subwavelength structures.     

Near-field calculation based on the T-matrix method with discrete sources

We propose a novel approach to compute the field scattered by a particle in the near-zone, in the framework of the transition matrix formulation. This method is based on the expansion of the total near-field in terms of discrete sources vector spherical wave functions and turns out to be particularly effective to model strongly elongated or flattened particles with high permittivity. The performances are evaluated on gold spheroids with several aspect ratios. This method results very useful to understand the complex behavior of enhanced plasmon fields in resonant metallic nanostructures.     

Understanding plasmon resonances of metal-coated colloidal crystal monolayers

Metal films deposited over two-dimensional colloidal crystals (MFoCC) constitute a low-cost periodic structure with interesting photonic and plasmonic properties. It has previously been shown that this structure exhibits a behaviour similar to the well-known Extraordinary Optical Transmission (EOT) of metallic hole arrays in planar films. Here, we explore the transmission characteristics of AgFoCC by systematic comparison with that of the bare CC. Furthermore with additional reflectivity measurements we evaluate the AgFoCC overall plasmonic response, which, notably, exhibits a strong plasmon absorption band at wavelengths larger than those of the transmitted maximum. By corroborating these results with finite-difference time-domain electromagnetic simulations, we identify a hybrid metal-dielectric propagative mode in the transmission mechanism. On the contrary a strongly localized mode is responsible for the maximum light absorption by this structure. These results shed new light on the current understanding of this highly promising plasmonic structure, being useful for the design of surface-enhanced Raman scattering and enhanced fluorescence substrates.     

Optical trapping and alignment of single gold nanorods by using plasmon resonances

We demonstrate three-dimensional trapping and orientation of individual Au nanorods by using laser light slightly detuned from their longitudinal plasmon mode. Detuning to the long-wavelength side of the resonance allows stable trapping for several minutes, with an exponential dependence of trapping time on laser power (consistent with a Kramer's escape process). Detuning to the short-wavelength side causes repulsion of the rods from the laser focus. Alignment of the long axis of the rods with the trapping laser polarization is observed as a suppression of rotational diffusion about the short axis.     

Surface plasmon polariton scattering by small ellipsoid particles

Scattering of surface plasmon polaritons (SPP’s) by small ellipsoid particles placed near a dielectric–metal interface is theoretically considered. Using the Green’s function formalism and the dipole approximation, we consider the differential and total scattering cross-sections associated with the SPP-to-SPP scattering as well as with the SPP scattering into waves propagating away from the interface, analyzing the influence of system parameters. As an example, scattering cross-sections of differently shaped gold spheroid particles placed near an air–gold interface are evaluated at the light wavelength of 800 nm. It is shown that the differential and total cross-sections depend strongly upon the particle-to-surface distance, the ratio between the major and minor axes and their orientation with respect to both the interface and the direction of SPP incidence. Implications of the obtained results to the design of SPP micro-optical components are also discussed.     

Sensitivity of surface plasmon resonances in spheroidal metal nanoparticles

The sensitivity of the surface plasmon resonance wavelength position in prolate and oblate metal nanoparticles to the refractive index of an embedding solution, nanoparticle shape, and temperature is studied theoretically. The influence of dissipation factor on plasmon resonance is discussed as well. The new approach specifying the optimal nanoparticle shape to reach the maximal sensitivity is proposed. The calculations are illustrated on the example of Na nanoparticle.     

Lighting up multipolar surface plasmon polaritons by collective resonances in arrays of nanoantennas

We demonstrate the coupling of multipolar surface plasmons with photonic modes in periodic arrays of metallic nanoantennas. This coupling leads to sharp resonances known as lattice surface modes. In spite of the weak interaction of multipolar surface plasmons with light, lattice surface modes provide an efficient radiative decay channel for emitters in the proximity of the array. We observe a tenfold emission enhancement of dyes coupled to lattice resonances. Lattice surface modes light up multipolar plasmonic resonances, opening new possibilities for fluorescence spectroscopies.     

Tuning of particle plasmon resonances in binary dielectric medium

Using light scattering spectroscopy, I demonstrate an approach to tune particle plasmon resonance in a binary dielectric media where silver nano-rods are embedded partially both in Anodic Aluminum Oxide (AAO) matrix and in air. A systematic experimental study under a controlled variation of the degree of embedding of nano-rods in AAO matrix has been presented. Experimental results have been interpreted based on the Drude model. Finite Difference Time Domain (FDTD) method has been employed to calculate the nature of the silver nano-rod resonance at the experimental conditions. Both the simulation results and theory corroborate the experimental findings.     

Laser-induced forces in metallic nanosystems: the role of plasmon resonances

We present calculations of the laser-induced force between metallic nanospheres, similar and dissimilar in character, and that between a metallic nanosphere and a planar surface. When the separation between these objects is in the 0.5-2 nm range, we find very strong resonances in the laser-induced force associated with excitation of plasmon resonances. Measurement of such forces will provide direct access to the plasmon enhancements of laser fields so critical to optical spectroscopy in the nanoenvironment.     

Optical absorption resonances of trapped particles

We have previously shown the possibility of the existence of a one-photon resonance with homogeneous width in the center of an absorption line of trapped particles. The results of a detailed investigation of this resonance are given in the first part of the present paper. The second part is devoted to a two-photon resonance in a standing-wave field: the time of coherent interaction with the field is increased for trapped particles, which permits the elimination of transit resonance broadening.     

Analysis of the light scattering properties of a gold nanorod on a plane surface via discrete sources method

The Discrete Sources Method has been extended to analyze the scattering behavior of a gold nanorod deposited on a plane surface in an evanescent wave field. In this case the system particle-surface demonstrates no axial symmetry and it is not possible to apply the conventional DSM approach for axisymmetric systems used before. In this paper DSM algorithm for non-axisymmetric systems is presented. The rigorous computer model based on DSM, which allows to take into account all features of the scattering problem including particle-surface interaction has been realized. Computer simulation analysis of the scattering spectra of gold nanorod is presented.