Tunable coupled plasmon modes via nanoshell particle chains

Coupled plasmon modes have been studied theoretically in periodic chains of nanoshell particles embedded in a graded dielectric host. These chains not only sustain a variety of localized modes as unshelled nanoparticle chains, but also offer precise control of the localization-delocalization transition among these modes by varying the permittivity contrast and/or core-shell radius ratio. By optimizing these parameters, the upper band can be tuned into higher frequencies while the lower band can be tuned into the optical communication frequencies for practical application. We also discuss the Ohmic loss effects in the metallic component of the nanoshells.     

Band structure of two-dimensional square lattice photonic crystals of circular dispersive metamaterial rods

By virtue of the efficiency of the Dirichlet-to-Neumann map method, the details of the band structure of a two-dimensional square lattice photonic crystal composed of dispersive metamaterial circular rods in air background has been studied. We show that there are two flat bands at the band structure of the system for both H-polarization and E-polarization. These flat bands are created around the magnetic resonance frequency, surface plasmon frequency and magnetic surface plasmon frequency. We realized that the modes with frequencies lying above the resonance frequency behave like resonant cavity modes created in a single metallic cylindrical waveguide. While, due to the relatively large and imaginary refractive index of the metamaterial rods at the frequencies lying below the resonance frequency, the modes are localized modes with negligible penetration into the rods. Moreover, the modes are localized at the interface of the cylindrical metamaterial rods and the air background for the frequencies around the surface plasmon frequency and the magnetic surface plasmon frequency.     

A time-dependent density functional theory investigation of plasmon resonances of linear Au atomic chains

We report theoretical studies on the plasmon resonances in linear Au atomic chains by using ab initio time-dependent density functional theory. The dipole responses are investigated each as a function of chain length. They converge into a single resonance in the longitudinal mode but split into two transverse modes. As the chain length increases, the longitudinal plasmon mode is redshifted in energy while the transverse modes shift in the opposite direction (blueshifts). In addition, the energy gap between the two transverse modes reduces with chain length increasing. We find that there are unique characteristics, different from those of other metallic chains. These characteristics are crucial to atomic-scale engineering of single-molecule sensing, optical spectroscopy, and so on.     

Plasmon-resonance-induced enhancement of the reflection band in a one-dimensional metal nanocomposite photonic crystal

We report the realization of a one-dimensional photonic crystal consisting of alternating layers of metal nanocomposite and polymer layers. The structure shows a large change in the width of the reflection band due to the interplay between the plasmon resonance of the metal nanoparticle and the Bloch modes of the photonic crystal. The width of the reflection band is found to increase by 200% when the photonic band edge approaches the plasmon resonance of the silver nanoparticles.     

Detailed study of the flat bands appeared in two-dimensional magnetic photonic crystals with square symmetry

By virtue of the efficiency of the Dirichlet-to-Neumann map method, the details of the band structure of a two-dimensional magnetic photonic crystal having a square array of parallel circular ferrite rods in air background influenced by an external static magnetic field applied in the rod direction has been investigated. We show that there are two sets of flat bands at the band structure of the system for TM-polarization. These flat bands are created around the magnetic surface plasmon frequency and frequency in which the magnetic permeability has singular value. For the frequency around the magnetic surface plasmon, the modes are highly localized at the interface of the cylindrical ferrite rods and air background and also by approaching the modes to the magnetic surface plasmon frequency the localization length decreases and the number of field's nodes increases considerably. Moreover, we realized that the modes with frequencies lying immediately below the singular value act similar to as resonance cavity modes created in a single metallic cylindrical waveguide.     

Localized vibrations in perfect anharmonic lattices: Trapping on phonons

A theory is developed, which allows one to calculate intrinsic local modes (ILMs) in infinite lattices and to find the effect of an ILM on phonons. One prediction is the appearance of linear local modes in the lattice nearby an ILM. To check these results, high-precision MD calculations of the vibrations of diatomic chains have been carried through. The results fully confirm the predictions.     

Plasmon resonance coupling in strongly coupled gold nanotube arrays with structural defects

We theoretically investigate the transmission spectra and the field distributions with different defects in the gold nanotube arrays by using the finite-difference time-domain method.It is found that the optical properties of the nanotube arrays are strongly influenced by different defects.When there are no defects in the central nanotube,the values of peaks located at both sides of the photonic band gap have their maxima.Based on the distributions of electric field component E x and the total energy distribution of the electric and the magnetic field,we show that mainly a dipole field distribution is exhibited for the plasmon mode at the long-wavelength edge of the band gap but higher order modes of the composite are excited at the short-wavelength edge of the band gap.The plasmon resonant modes can also be controlled by introducing defects.     

Plasmonic coherent drive of an optical trap

We demonstrate that optical trapping can be driven by delocalized surface plasmon modes resonantly excited within a standing wave trap. Dynamical modifications are shown to be determined by the near-field symmetry of the plasmonic modes with negligible thermal effect. With low trapping powers and polarization control, remarkable stiffness enhancements are recorded, the larger the smaller the particle. The results can be simply modeled accounting for a coherent interaction between the plasmon field and the Gaussian standing wave of the trap.     

Plasmon modes in double-layer gapped graphene at zero temperature

Plasmon dispersions and damping rate of plasma oscillations in a double-layer gapped graphene made of two parallel mono layer gapped graphene sheets grown on dielectric separation are calculated within random-phase-approximation at zero temperature. By using long wavelength limit expansion, analytical expressions for optical and acoustic plasmon frequencies have been formed, and the formulae demonstrate that the considerable difference in analytical form for plasmon frequencies comes from the factor depending on the band gap, compared to gapless situation. Numerical results show that only large band gap decreases remarkably plasmon frequencies of two modes in the range of large wave vector. Acoustic plasmon branch becomes shorter than that in case of zero gap while optical one seems independent with small band gap. In addition, interlayer separation and carrier density affect on collective excitations and damping rate when taking into account the band gap quite similarly to those in case of zero gap.     

Infrared surface modes in small NiO particles

Small particle specimens of NiO (powder and arc-produced smoke) were studied using infrared spectroscopy. Size distribution and particle configuration were established using an electron microscope. The importance of surface modes (polaritons) as well as their dependence on particle shape was established in the interpretation of the spectra. Chain formation in the smoke suggested cylinder calculations which showed the presence of both bulk and surface modes, in agreement with measurements. In both samples particle shape effects broaden the spectral features in the vicinity of the surface mode. Because of large errors in calculating surface modes from optical constants derived from a diatomic model, it was also shown that accurately measured optical constants are required in the study of the surface mode problem.     

柱状磁光颗粒的局域表面等离激元共振及尺寸效应

柱状磁光颗粒的局域表面等离激元共振为二维磁光光子晶体的手征性边缘模的生成提供了重要的机制. 但目前对此类颗粒的局域表面等离激元共振效应的研究局限于长波长近似下的结果, 且缺乏对发生共振时的远场与近场特征的深入了解. 本文从散射理论出发, 计算并分析了柱状磁光颗粒发生局域表面等离激元共振的条件与特殊的场特征, 并讨论了颗粒尺寸对共振峰的影响. 计算结果解释了实验中观察到的二维磁光光子晶体的共振带隙与在长波长近似下得到的局域表面等离激元共振频率的明显偏移, 并展示了颗粒在较大尺寸下形成的高阶共振峰, 这可能有助于利用共振效应在磁光光子晶体中实现多模的手征边缘态.     

Commensurability oscillations in fast particle energy loss to a density modulated 2DEG in a magnetic field

We examine fast particle energy loss as a probe of the properties of a density modulated 2DEG in a normal magnetic field. In the high velocity limit, the local plasmon spectrum makes two separate contributions to energy loss for both the cases in which the particle velocity is parallel and perpendicular to the planar 2DEG. The inter-Landau band plasmon makes a distinct contribution to energy loss through excitation of the hybrid 2D magnetoplasmon. The other local plasmon, associated with the spread of a Landau level into a miniband by application of the periodic potential associated with the density modulation contributes to energy loss with features corresponding to Shubnikov-de Haas oscillations jointly with an amplitude envelope exhibiting oscillations associated with the commensurability of the modulation period and the cyclotron radius at the Fermi energy.     

Collective mode dispersions of organic chain compounds

We investigate the collective mode dispersions for the tight-binding dielectric matrix with two one-dimensional electron bands per donor and acceptor chains, and the three-dimensional long-range Coulomb electron-electron interaction within the random phase approximation. The hybridized collective modes are the result of the strong coupling between the intraband plasmon and the interband dipolar modes due to strong dipole Coulomb interactions. Our calculations show the existence of the low-energy renormalized plasmon mode above the electron-hole quasi-continuum in the long wavelength limit. The obtained modes are brought into correspondence with the optical data of quasi-one-dimensional organic conductor tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ). Namely, the renormalized plasmon and the dipolar mode are assigned to the observed excitations at respective energy scales of roughly 10 meV and 0.75 eV, explaining why lower excitation is eliminated while higher excitation persists below the temperature of the Peierls phase transition.     

Theoretical near-field optical properties of branched plasmonic nanoparticle networks

Extended branched networks of single-nanoparticle chains have recently been self-assembled from colloidal suspensions. In particular, gold nanoparticle linear chains and complex chain networks have revealed unique signatures of plasmon modes in their extinction spectra. In this Letter, we investigate theoretically their near-field optical properties and show that a real space mapping of these modes can be achieved with a photon scanning tunneling microscope setup. A distinct subwavelength patterning of the optical near-field gives rise to well-resolved photon scanning tunneling microscope images that can be used to identify the network segments able to efficiently carry optical energy.     

Charge‐Induced Shifts in Chiral Surface Plasmon Modes in Gold Nanorod Assemblies

A detailed investigation of the effect of electron injection on the chiral plasmon modes of helical nanorod assemblies is presented. An increased surface plasmon frequency of the electron gas due to the addition of electrons leads to a blue‐shift in the corresponding chiral surface plasmon modes. The mechanism behind the shift in plasmonic chirality due to nanorod charging is investigated using theoretical simulations. Charging of the nanorods alters the surface electron density, thereby modifying the plasma frequency and causing a change in the dielectric function. The nature of the plasmon shift and the intensity of chiral surface plasmons are found to be largely dependent on the extent of electron addition. At extended periods of time, the blue shifted band slowly shifts back toward the red, due to transfer of electrons back to the medium, leading to discharging of the nanorods.     

Silver fractal networks for surface-enhanced Raman scattering substrates

Based on diffusion-limited aggregation process, a convenient nanotechnique is demonstrated to obtain large silver fractal networks for a surface-enhanced Raman scattering (SERS)-active substrate. The silver fractal networks are of high SERS enhancement factor, large dynamical range. The observed SERS efficiency can be explained in terms of strongly localized plasmon modes relative to the single particle plasmon resonance.     

Diagnostics of aqueous colloids of noble metals by extinction modeling based on mie theory

Two-factor dependences of the maximum and half-width of a surface plasmon resonance band on both the average diameter of nanoparticles and the scatter in their particle-size distributions were defined for colloidal silver and gold aqueous solutions based on modeling the extinction effectiveness factor by Mie theory. The obtained three-dimensional surfaces determined the shape of calibration curves used to define the average particle diameters and the scatter in their particle-size distributions from measurements of the maximum and half-width of the surface plasmon resonance band in spectra of the silver and gold colloidal solutions. The calibration curves were correlated with experimental samples of aqueous ultradispersed media containing silver and gold nanoparticles.     

Dynamics of coupled vortices in a pair of ferromagnetic disks

We here experimentally demonstrate that gyration modes of coupled vortices can be resonantly excited primarily by the ac current in a pair of ferromagnetic disks with variable separation. The sole gyration mode clearly splits into higher and lower frequency modes via dipolar interaction, where the main mode splitting is due to a chirality sensitive phase difference in gyrations of the coupled vortices, whereas the magnitude of the splitting is determined by their polarity configuration. These experimental results show that the coupled pair of vortices behaves similar to a diatomic molecule with bonding and antibonding states, implying a possibility for designing the magnonic band structure in a chain or an array of magnetic vortex oscillators.     

A computational study of the optical response of strongly coupled metal nanoparticle chains

We study the optical response of strongly coupled metal nanoparticle chains using rigorous multiple scattering calculations. The collective resonant frequency of silver nanosphere chains and the coupling between chains are considered. The coupling between silver nanoparticle chains are understood by the transmission and reflection calculations of 2D periodic arrays of nanospheres. The results are in agreement with recent experiments. The splitting of plasmon resonance modes for different polarizations of the incident light are explored. Our results show that the transverse mode resonant wavelength is very sensitive to the inter-chain distance. Results on the effect of disorder are also presented.     

Energy-loss function of TTF-TCNQ

We investigate the energy-loss function for a previously developed model of quasi-one-dimensional metals with two one-dimensional electron bands per donor and acceptor chains and the three-dimensional long-range Coulomb electron-electron interaction within the random phase approximation. It is essentially influenced by two hybridized collective modes which result from the strong coupling of the intraband plasmon and the interband dipolar modes. Our calculations show that the spectral weights of the renormalized plasmon and the dipolar mode dominate within the long wavelength limit, while for large longitudinal wave vectors the intraband electron-hole quasi-continuumgains some experimentally observable spectral weight as the second mode approaches it. The function obtained is brought into correspondence with the data of the quasi-one-dimensional organic conductor tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) obtained from electron energy-loss spectroscopy (EELS) measurements.