Plasmonic Modes of Ag Nanoshell Excited by Bi-Dipole

We studied plasmonic dipole, quadrupole, and sextupole modes of Ag nanoshell (NS) excited by a pair of aligned radial electric dipoles (bi-dipole) in symmetric and antisymmetric configurations by using dyadic Green’s functions. The mutual excitation rate and the radiative and nonradiative powers of bi-dipole in the presence of Ag NS were analyzed. Our results show that these modes are in accordance with the surface plasmon resonances of Ag NS irradiated by a polarized plane wave. In addition, the mutual excitation rate retains local maxima at these modes. Moreover, the quadrupole and octupole modes are only excited in the cases of the symmetric radial bi-dipole, while the dipole and sextupole modes are only excited in the cases of the antisymmetric ones. The dipole mode is broadband, while the other higher-order modes are narrowband. Moreover, all of these plasmonic modes are red-shifted as the ratio of the core radius to the shell thickness increases.     

Effect of Edge Rounding on the Extinction Properties of Hollow Metal Nanoparticles

The optical responses of metal nanoparticles induced by subtle variations in geometry, especially by the rounding of the edges and corners, have generated great interest at present due to the requirement of fabricating refined structures of metal nanoparticles and theoretical simulations of the real particles. We study the effect of both inner and outer edge rounding on the optical properties of gold nanobox and gold nanobox dimer with small interparticle distances by using the discrete dipole approximation method. The shift of extinction peaks, the electric field distribution, and the variation of refractive index sensitivities by changing the curvature of the inner and outer edges of gold nanobox are investigated. We demonstrate that the optical properties of nanobox are more sensitive to the outer edge rounding than the inner edge rounding. By edge rounding of two very close gold nanoboxes, the blue shift of the dipolar and the quadrupolar plasmonic resonances of nanobox dimer are shown. Comparing with the inner edge rounding of nanobox dimer, we find that rounding of the outer edges causes the larger shift of the quadrupolar mode and approximate shift of the dipole mode.     

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.     

Total internal reflection fluorescence. Measurement of spatial and orientational distributions of fluorophores near planar dielectric interfaces

The fluorescence collected from a fluorophore which is near a planar interface and is excited by a laser beam that is totally internally reflected at the interface depends on the direction of the absorption and emission transition dipole moments of the fluorophore with respect to the interface, on the distance from the fluorophore to the interface, on the angle of incidence and polarization direction of the exciting beam, and on properties of the collection optics. Expressions are derived for the excitation and subsequent emission and collection of fluorescence from a population of fluorophores near a planar interface. Presented is a general model-independent method of obtaining characteristic parameters of the spatial and orientational distribution of the population of fluorophores, from a measure of the fluorescence collected as a function of the polarization and the incidence angle of the totally internally reflected laser beam. The method is illustrated with several simulation calculations.     

Plasma asymmetric dipole antenna excited by a surface wave

A study is made of a quarter-wave asymmetric dipole antenna in which the conducting rod is replaced by a plasma column with an electron density much higher than the critical density. The parameters of such an antenna are determined by the exited surface wave, which affects the electromagnetic field structure in the near-field zone. It is shown analytically, numerically, and experimentally that the resonant length of the plasma dipole antenna is close to one-quarter of the length of the surface wav and that the conversion efficiency of plasma antenna power into radiation can be no worse than that of a metal dipole antenna. It is also shown experimentally that the plasma in a dipole antenna can be self-consistently excited by an RF oscillator and that the excited RF oscillations can be efficiently radiated into the surrounding space.     

Spatial Distribution of Optical Near-Fields in Plasmonic Gold Sphere Segment Voids

We present a comprehensive experimental and computational study on the electromagnetic field distribution in sphere segment void arrays. Surface plasmon polaritons can be excited in these void arrays, resulting in greatly enhanced electromagnetic fields. With the scanning near-field optical microscope (SNOM) we are able to measure the electromagnetic field distribution at the sample surface. For this purpose, an array of relatively large voids with a sphere diameter of 900 nm was fabricated, allowing for an easy access of the scanning glass-fibre tip and yielding very detailed scans. Complementary, finite-difference time-domain (FDTD) calculations on a complete void array have been performed and compared with the SNOM intensity maps and experimental reflectivity data. We show in a direct way both the existence of extended and localised modes in the Au void array for three different void depths. We also show and discuss the changes that the modes undergo for the different void depths and excitation wavelengths. Moreover, since the simulations were performed for two different void geometries, one containing perfectly spherical void surfaces and another more realistic one, which considers the presence of interstitial wall holes and other imperfections, as observed in scanning electron micrographs, we were able to determine by comparison with the experiment under which conditions an array of idealised sphere segment voids is a meaningful model. This demonstrates that both SNOM and FDTD simulations are powerful tools for understanding the plasmonic response of metallic nanostructures, thus enabling, for instance, a design for applications in ultra-sensitive optical detection.     

Plasmon-Enhanced Photoluminescence of SiC Quantum Dots for Cell Imaging Applications

Strong photoluminescence enhancement of chemically inert and biocompatible SiC quantum dots (QDs) ensured by their near-field coupling with multipolar localized plasmons is experimentally demonstrated. The main physical mechanisms responsible for this phenomenon are described with the use of three-dimensional FDTD simulations. Nano-Ag/SiN_X/glass plasmonic substrates were shown to be efficiently used for significant luminescence enhancement of fibroblast cells labeled with the SiC QDs. The proposed approach allows a plasmon-induced enhancement of fluorescent cell imaging.     

Plasmon Resonances in V-Shaped Gold Nanostructures

Using numerical simulations, we examine the change in plasmon resonance behavior in gold nanorod structures that have a V shape. The reduction in symmetry compared to linear rods causes two different longitudinal-type resonances to appear in a single structure, and the relative intensity and hybridization of these can be controlled by varying the angle of the arms of the ??V.?? The resonances may also be selectively excited by controlling the polarization of the incident light, thereby providing a convenient way to control a nanoscale optical electric field using far-field parameters. For example, the wavelength at which a strong resonance occurs in the V-shaped structures studied can be switched between 630 and 900?nm by a 90° rotation of the polarization of the incident light. Due to the symmetry of the targets, there will be three types of special near-field location; a location at which the electric field intensity is enhanced by either resonance, a location at which the electric field intensity is enhanced by the 630?nm resonance but not by the 890?nm resonance, and a location at which the electric field intensity is enhanced by the 890?nm resonance but not by the 630?nm one.     

Near-Field Optical Analysis of Plasmonic Nano-Probes for Top-Illumination Tip-Enhanced Raman Scattering

Top-illumination tip-enhanced Raman scattering (TI-TERS) has recently emerged as a promising near-field vibrational spectroscopy method that can be adapted on an upright optical microscope. With a relatively simplified optics, TI-TERS can probe both opaque and transparent samples making them a promising tool in nanoscale chemical analysis. One of the critical components of TI-TERS is the plasmonic nano-tip used to enhance the Raman spectroscopic signature. Herein, we numerically studied the near-field optical properties of conventional gold tip (20 nm radius of curvature) and two varieties of optical antenna-based tips in the context of TI-TERS. Optical antenna-based tips included a 40-nm gold nanoparticle attached to a dielectric tip and a 50-nm equilateral gold nanotriangle attached to a dielectric tip. We evaluated the Raman enhancement spectra as a function of experimental variables such as underlying substrate and angle of the tip with respect to substrate normal. Our analysis revealed that conventional gold tip facilitates superior enhancement and optical antenna-based tips facilitate superior spectral bandwidth and lateral resolution in TI-TERS configuration. Tips with higher enhancement can be harnessed for ultra-sensitive measurements, and tips with broader spectral bandwidth can be utilized to enhance both Stokes and anti-Stokes component of the Raman spectra.     

High-resolution immunocytochemical labeling of replicas with ultrasmall gold.

The use of 10-15-nm gold probes in freeze-fracture immunocytochemistry sometimes results in poor immunogold labeling. Replica sites are labeled with only one or two gold particles, making it unlikely that the labeling depicts the true distribution of antigen. In this study, the feasibility of using ultrasmall ( approximately 1.4-nm) gold probes for immunocytochemical labeling of replicas was examined. When HLA Class I in neutrophil membrane replicas was labeled with various sized immunogold particles as the secondary detection system, the apparent distribution density was inversely related to the size of the particles (1.4-nm > 5-nm >10-nm >15-nm). Indeed, the density of the apparent distribution of HLA Class I labeled with 1.4-nm gold particles was about sevenfold greater than when labeling was carried out with the 10-nm gold particles. Similar results were obtained with CD16, another neutrophil membrane protein. Silver enhancement was required to visualize the 1.4-nm gold particles, but this procedure did not adversely affect replica membranes. These results suggest that, when followed by silver enhancement, 1.4-nm gold particles are effective probes for achieving high-resolution immunocytochemical labeling of replicas.(J Histochem Cytochem 47:569-573, 1999)     

Fluorescence recovery spectroscopy as a probe of slow rotational motions.

Pump-and-probe techniques can be used to follow the slow rotational motions of fluorescent labels bound to macromolecules in solution. A strong pulse of polarized light initially anisotropically depletes the ground-state population. A continuous low-intensity beam of variable polarization then probes the anisotropic ground-state distribution. Using an additional emission polarizer, the generated fluorescence can be recorded as it rises towards its prepump value. A general theory of fluorescence recovery spectroscopy (FRS) is presented that allows for irreversible depletion processes like photobleaching as well as slowly reversible processes like triplet formation. In either case, rotational motions modulate recovery through cosine-squared laws for dipolar absorption and emission processes. Certain pump, probe, and emission polarization directions eliminate the directional dependence of either dipole and simplify the resulting expressions. Two anisotropy functions can then be constructed to independently monitor the rotations of either dipole. These functions are identical in form to the anisotropy used in fluorescence depolarization measurements and all rotational models developed there apply here with minor modifications. Several setups are discussed that achieve the necessary polarization alignments. These include right-angle detection equipment that is commonly available in laboratories using fluorescence methods.     

Influence of the electron density on the characteristics of terahertz waves generated under laser–cluster interaction

A theory of generation of terahertz radiation under laser–cluster interaction, developed earlier for an overdense cluster plasma [A. A. Frolov, Plasma Phys. Rep. 42. 637 (2016)], is generalized for the case of arbitrary electron density. The spectral composition of radiation is shown to substantially depend on the density of free electrons in the cluster. For an underdense cluster plasma, there is a sharp peak in the terahertz spectrum at the frequency of the quadrupole mode of a plasma sphere. As the electron density increases to supercritical values, this spectral line vanishes and a broad maximum at the frequency comparable with the reciprocal of the laser pulse duration appears in the spectrum. The dependence of the total energy of terahertz radiation on the density of free electrons is analyzed. The radiation yield is shown to increase significantly under resonance conditions, when the laser frequency is close to the eigenfrequency of the dipole or quadrupole mode of a plasma sphere.     

Complex Polarization Response in Plasmonic Nanospirals

Archimedean nanospirals exhibit many far-field resonances that result from the lack of symmetry and strong intra-spiral plasmonic interactions. Here, we present a computational study, with corroborating experimental results, on the plasmonic response of the 4π Archimedean spiral as a function of incident polarization, for spirals in which the largest linear dimension is less than 550 nm. We discuss the modulation of the near-field structure for linearly and circularly polarized light in typical nanospiral configurations. Computational studies of the near-field distributions excited by circularly polarized light illustrate the effects of chirality on plasmonic mechanisms, while rotation of linearly polarized light provides a detailed view of the effects of broken symmetry on nanospiral fields in any given direction in the plane of the spiral. The rotational geometry exhibits a preference for circular polarization that increases near-field enhancement compared to excitation with linearly polarized light and exchanges near-field configurations and resonant modes. By analyzing the effects of polarization and wavelength on the near-field configurations, we also show how the nanospiral could be deployed in applications such as tunable near-field enhancement of nonlinear optical signals from chiral molecules.     

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.     

Nano-Raman Spectroscopy: Surface Plasmon Emission, Field Gradients, and Fundamentally Near Field Propagation Effects

Nano-Raman spectra differ from far-field Raman spectra. The differences result from a strong electric field gradient near the metal tip, propagation, and polarization, but the dependence upon probe-sample distance can only be explained by the inclusion of surface plasmons and the near-field, non-propagating terms of the dipole emission. A simple model based upon these components accurately describes distance-dependent data measured with a near-field scanning optical microscope. Our essentially near-field model will apply generally to Raman spectroscopy near a nanoscale conductor.     

Simulation and Analytical Study of Optical Complex Field in Nano-corral Slits Plasmonic Lens

Although spiral plasmonic lens has been proposed as circular polarization analyzer, there is no such plasmonic nanostructure available for linear polarization. In the current work, we have designed nano-corral slits (NCS) plasmonic lens, which focuses the x- and y-polarized light into spatially distinguished plasmonic fields. We have calculated analytically and numerically the electric field intensity and phase of the emission from nano-corral slits plasmonic lens with different pitch lengths under various polarizations of the illumination. It has been shown that one can control the wave front of the output beam of these plasmonic lenses by manipulating the illumination of both circular and linear polarization. Our theoretical study in correlation with FDTD simulation has shown that NCS plasmonic lens with pitch length equal to λ_spp produces scalar vortex beam having optical complex fields with helical wave front and optical singularity at the center under circular polarization of light. When NCS lens (pitch = λ_spp) is illuminated with linearly polarized light, it exhibits binary distribution of phase with same electric field intensity around the center. However, with pitch length of 0.5λ_spp, NCS shows linear dichroism under linearly polarized illumination unlike spiral plasmonic lens (SPL) eliminating the use of circularly polarized light. Optical complex fields produced by these NCS plasmonic lenses may find applications for faster quantum computing, data storage, and telecommunications.

     

Multipole representations of current generators in a volume conductor

As far as the potential distribution outside the current generators is concerned, any current source distribution may be replaced by a suitable collection of multipoles. If these current generators lie close to the geometrical center of the volume conductor, a central dipole is a good approximation for potentials at surface points which are at considerable distances from the center. For better accuracy and for points close to the center, additional singularities such as a central quadrupole, a central octopole, etc., should be included. Potential expressions due to such multipoles in a spherical conductor can be obtained in closed forms by means of the “interior sphere theorem”. This paper presents a method for determining successively better multipole representations of the current generators in a homogeneous conducting sphere by measuring surface potentials at a successively increasing number of points. It is shown that Einthoven's triangle and Wilson's tetrahedron in the theory of electrocardiography are first and second approximations of this method. This concept also applies to conductors of other shapes. This investigation was supported by The National Heart Institute under a research grant H-2263(c).     

Application of surface plasmon coupled emission to study of muscle

Muscle contraction results from interactions between actin and myosin cross-bridges. Dynamics of this interaction may be quite different in contracting muscle than in vitro because of the molecular crowding. In addition, each cross-bridge of contracting muscle is in a different stage of its mechanochemical cycle, and so temporal measurements are time averages. To avoid complications related to crowding and averaging, it is necessary to follow time behavior of a single cross-bridge in muscle. To be able to do so, it is necessary to collect data from an extremely small volume (an attoliter, 10(-18) liter). We report here on a novel microscopic application of surface plasmon-coupled emission (SPCE), which provides such a volume in a live sample. Muscle is fluorescently labeled and placed on a coverslip coated with a thin layer of noble metal. The laser beam is incident at a surface plasmon resonance (SPR) angle, at which it penetrates the metal layer and illuminates muscle by evanescent wave. The volume from which fluorescence emanates is a product of two near-field factors: the depth of evanescent wave excitation and a distance-dependent coupling of excited fluorophores to the surface plasmons. The fluorescence is quenched at the metal interface (up to approximately 10 nm), which further limits the thickness of the fluorescent volume to approximately 50 nm. The fluorescence is detected through a confocal aperture, which limits the lateral dimensions of the detection volume to approximately 200 nm. The resulting volume is approximately 2 x 10(-18) liter. The method is particularly sensitive to rotational motions because of the strong dependence of the plasmon coupling on the orientation of excited transition dipole. We show that by using a high-numerical-aperture objective (1.65) and high-refractive-index coverslips coated with gold, it is possible to follow rotational motion of 12 actin molecules in muscle with millisecond time resolution.     

Nonlinear ineraction of an annular electron beam with azimuthal surface waves

A nonlinear theory is developed that describes the interaction between an annular electron beam and an electromagnetic surface wave propagating strictly transverse to a constant external axial magnetic field in a cylindrical metal waveguide partially filled with a cold plasma. It is shown theoretically that surface waves with positive azimuthal mode numbers can be efficiently excited by an electron beam moving in the gap between the plasma column and the metal waveguide wall. Numerical simulations prove that, by applying a constant external electric field oriented along the waveguide radius, it is possible to increase the amplitude at which the surface waves saturate during the beam instability. The full set of equations consisting of the waveenvelope equation, the equation for the wave phase, and the equations of motion for the beam electrons is solved numerically in order to construct the phase diagrams of the beam electrons in momentum space and to determine their positions in coordinate space (in the radial variable-azimuthal angle plane).     

Determination of the orientation distribution of adsorbed fluorophores using TIRF. I. Theory.

The spectroscopic technique total internal reflection fluorescence can be used for determination of the orientation of adsorbed fluorescent molecules. The underlying theory is presented in general terms and elaborated in detail for the case that the fluorescent group is a porphyrin ring. It is shown that order parameters of the orientation distribution can be obtained if both the fluorescence intensity and its polarization are measured as functions of the polarization of the incident laser beam. From these order parameters an approximation of the orientation distribution can be derived by the maximum-entropy method.