Metal Nanoparticle Chains Research Articles

Integration of short gold nanoparticles chain on SOI waveguide toward compact integrated bio-sensors

We demonstrate the integration of short metal nanoparticle chains (L ≈700 nm) supporting localized surface plasmons in Silicon On Insulator (SOI) waveguides at telecom wavelengths. Nanoparticles are deposited on the waveguide top and excited through the evanescent field of the TE waveguide modes. Finite difference time domain calculations and waveguide transmission measurements reveal that almost all the TE mode energy can be transferred to nanoparticle chains at resonance. It is also shown that the transmission spectrum is very sensitive to the molecular environment of nanoparticles, thus opening the way towards ultra-compact sensors in guided plasmonics on SOI. An experimental demonstration is reported with octadecanthiol molecules for a detection volume as small as 0.26 attoliter.

Optical Interaction in a Plasmonic Metallic Nanoparticle Chain Coupled to a Metallic Film

We demonstrated the coupling of localized surface plasmons and surface plasmon polaritons modes in a system composed of a metallic particle chain separated from a thin metallic film. The results showed that: (1) the thickness of the metallic particles buried in the dielectric space, (2) the positions of the particles influence the level of interaction between localized surface plasmons and surface plasmon polaritons modes. Meanwhile, the positions of the particles and the thickness of the metallic particles control the electromagnetic enhancement and influence the electric field distributions in this system. This kind of system has a very promising candidate for biosensing and surface enhanced spectroscopy applications.

Dispersion relations in metal nanoparticle chains: necessity of the multipole approach

Lorenz–Mie multiple-scattering theory is used to perform semi-analytical calculations of the lossy dispersion relations of propagating modes in infinite chains of metallic spheres. Lossy modes are described by allowing the projection of the wavevector along the chain axis to be a complex number rather than the more common complex frequency description. We show that even when the constituent particles are much smaller than the wavelength, one generally needs to go well beyond the coupled dipole approximation to achieve stable predictions.

Giant Coupling Effect between Metal Nanoparticle Chain and Optical Waveguide

We demonstrate that the optical energy carried by a TE dielectric waveguide mode can be totally transferred into a transverse plasmon mode of a coupled metal nanoparticle chain. Experiments are performed at 1.5 μm. Mode coupling occurs through the evanescent field of the dielectric waveguide mode. Giant coupling effects are evidenced from record coupling lengths as short as ~560 nm. This result opens the way to nanometer scale devices based on localized plasmons in photonic integrated circuits.

Cascaded plasmonic metamaterials for phase-controlled enhancement of nonlinear absorption and refraction

The nonlinear optical properties of plasmon resonant metamaterials consisting of chains of metal nanoparticles are evaluated. Introducing particle size differences along the chains leads to the development of cascaded plasmon resonances exhibiting increased field enhancement and field confinement. The interplay among the different resonances on the structures induces a frequency-dependent enhancement of the nonlinear refractive and absorptive response of the metamaterial, ultimately providing larger nonlinear susceptibility enhancement factors with engineered complex phase. It is shown that cascaded structures can provide a figure of merit for nonlinear absorption that is more than an order of magnitude larger than that obtained in noncascaded structures. The presented approach could lead to new planar and integrated nonlinear optical modulation and switching media with improved performance compared to their noncascaded counterparts.

Relationship between length and surface-enhanced raman spectroscopy signal strength in metal nanoparticle chains: Ideal models versus nanofabrication

Relationship between length and surface-enhanced raman spectroscopy signal strength in metal nanoparticle chains: Ideal models versus nanofabrication

Formation of gold nanoparticles in polymeric nanowires by low-temperature thermolysis of gold mesitylene

The formation of polymer nanowires containing metal nanoparticle chains by low-temperature thermolyses of metal precursors has remained challenging. We report the block copolymer-assisted generation of locally regular chains of quasi-spherical gold nanoparticles with narrow particle diameter distribution by mild thermolysis of the non-polar gold precursor gold mesitylene inside the cylindrical nanopores of self-ordered anodic aluminium oxide (AAO). The block copolymer separates the gold mesitylene as well as the developing gold nanoparticles from the AAO pore walls so that surface nucleation and pinning of gold clusters are prevented. Growing quasi-spherical gold nanoparticles locally deform the polymer chains irreversibly adsorbed on the AAO pore walls, and the polymer chains are pushed into the space between the gold nanoparticles. The gold nanoparticles have, therefore, larger diameters and smaller specific surface than hypothetical pluglike gold entities with the same volume, the formation of which is suppressed.

Relationship between Length and Surface-Enhanced Raman Spectroscopy Signal Strength in Metal Nanoparticle Chains: Ideal Models versus Nanofabrication

We have employed capillary force deposition on ion beam patterned substrates to fabricate chains of 60 nm gold nanospheres ranging in length from 1 to 9 nanoparticles. Measurements of the surface-averaged SERS enhancement factor strength for these chains were then compared to the numerical predictions. The SERS enhancement conformed to theoretical predictions in the case of only a few chains, with the vast majority of chains tested not matching such behavior. Although all of the nanoparticle chains appear identical under electron microscope observation, the extreme sensitivity of the SERS enhancement to nanoscale morphology renders current nanofabrication methods insufficient for consistent production of coupled nanoparticle chains. Notwithstanding this fact, the aggregate data also confirmed that nanoparticle dimers offer a large improvement over the monomer enhancement while conclusively showing that, within the limitations imposed by current state-of-the-art nanofabrication techniques, chains comprising more than two nanoparticles provide only a marginal signal boost over the already considerable dimer enhancement.

Subradiant Plasmon Modes in Multilayer Metal–Dielectric Nanoshells

Subradiant plasmon modes play an important role in plasmon coupling, which can couple to the superradiant modes resulting in Fano resonances. In this study, we investigated the behaviors of the subradiant plasmons in multilayer metal–dielectric nanoshells using the generalized Mie theory. We showed that it is possible to get a pronounced Fano resonance in a single concentric nanoshell by controlling the size and geometry. The near-field coupling effects in multilayer nanoshell chains were studied by changing the alignment, particle number, and interparticle spacing. The local electric field and induced surface charge distributions were plotted to gain physical insight into the coupling mechanisms. The interactions between the particles result in a well-developed Fano resonance due to the broadening of bright plasmon modes. The subradiant plasmon modes remain robust and nearly uninfluenced by the near-field coupling and the surrounding medium. At the Fano resonance frequency, the largest field enhancement occurs in the silica layer, which is quite different from solid metal nanoparticle chains.

Analytical properties of the plasmon decay profile in a periodic metal-nanoparticle chain

We show that the spatial decay of plasmons in a periodic metal-nanoparticle chain is composed of exponential and power-law decays. Our results show a high level of similarity between the absorptive and radiative decay channels. By analyzing the poles (and the corresponding residues) of the generating function for the lattice Green's function, we explain the details of the spatial decay profile. We also present an analytical formula for the decay profile.

Traveling waves in two parallel infinite linear point-scatterer arrays

Traveling waves in two coupled parallel infinite linear point-scatterer arrays are studied analytically for the first time to our knowledge. The two arrays are considered to be generally offset in the axial direction. It is found that slow quasi-even/odd supermodes are supported, as a result of the coupling-induced splitting of the modes of the single array, in direct analogy to standard optical waveguide couplers. Exactly even/odd supermodes are supported when the axial offset is zero. Mode splitting, dispersion curves, and coupling length are numerically investigated versus the inter-element spacing, the inter-array distance, and the axial offset. Potential applications of the concept are in directional optical couplers made of metallic or dielectric nanoparticle chains.

Complex dispersion relation of a double chain of lossy metal nanoparticles

We study the propagation characteristics of optical signals in waveguides composed of a double chain of metallic nanoparticles embedded in a dielectric host. We find that the complex Bloch band diagram for the guided modes, derived by the Mie scattering theory including material losses, exhibits strong differences with respect to the previously studied single chain. The results of the model are validated through the finite element solution of the Maxwell equations.

Plasmons in disordered nanoparticle chains: Localization and transport

Disorder-induced effects on plasmon coupling in chains of metallic nanoparticles are studied within a dipole model, by considering two types of disorder: fluctuations of the particles' shapes and fluctuations of their positions. Typical localization effects are found both in the eigenmodes and in the transport behavior of the system, and an estimate of the localization length is made. It is argued that chains with deliberately introduced disorder constitute promising systems for studying localization effects of electromagnetic waves at optical frequencies under well controllable and manipulable conditions.

Surface Plasmon Resonance of a Few Particles Linear Arrays

We present a study of the enhancement of the electric field due to localized surface plasmons in a short chain of metallic nanoparticles with different shapes: spheres, cylinders and spheroids. We calculate numerically the external radiation effect on these chains and analyze besides the shape, also the influence on size, interparticle distances and number of nanoparticles, corroborating that each one plays a definitive role for the enhancement of the electric field. Particularly, we focus on the main features of the electric field in the inter-particle regions, where an enormous increasing is expected due to the longitudinal localized plasmons. The electric field distribution along the chain shows a maximum in the middle of the chain. This fact could be related to a hybridization effect as the gap between particles decreases below 2 nm, we also observe a strong enhancement with the number of nanoparticles. Also regarding the shape we find agreement with reported results on spheroids, moreover we show that lateral coupled cylinders are more flexible to tune the enhancement factor than all other.

Analytical approximations of the dispersion relation of a linear chain of metal nanoparticles

We find some useful analytical approximations of the dispersion relation of a linear chain of metal nanoparticles in the subwavelength limit where the dipolar approximation can be used. We also approximate the group velocity without a direct estimation of the derivative of the dispersion relation, that carries unavoidable error amplifications. In the end we use these results in order to get some simple recipes that evaluate the sensitivity of the dispersion relation and the propagation losses with respect to the main parameters of the chain.

Twisted chains of resonant particles: optical polarization control, waveguidance, and radiation

Linear chains of metallic nanoparticles that are sequentially rotated about the chain axis display interesting polarization-sensitive optical properties. Such twisted chains posses and extend properties of chiral gratings and general periodic gratings. They are characterized by high anisotropy and polarization sensitivity, and have subwavelength transverse dimensions. These structures are shown to support transverse modes with distinct propagation wavenumbers and radiation properties, including slow (bound) and fast (radiative) modes. They also have stop bands of different types, resulting from coupling between distinct transverse modes, as well as coupling with different higher-order diffraction modes.

Dispersive properties of linear chains of lossy metal nanoparticles

We study the propagation characteristics of optical signals in waveguides composed of linear periodic arrangements of metallic nanoparticles embedded in a dielectric host. We find the complex Bloch band diagram for the guided modes including material losses by employing Mie scattering theory as well as coupled dipole approximations. The results of the model are validated through finite element solution of the Maxwell’s equations.

Vector lensing with a single chain of metal nanoparticles

A single periodic chain of noncontacting noble-metal nanoparticles is predicted to provide focusing in a cylindrical geometry of optical fields whose frequency lies within the surface-plasmon-polariton band polarized perpendicular to the chain axis for fields dominated by that component based on an analytic nonlocal vector electrodynamic theory in the Fresnel limit. Exact closed-form expressions within the model for the coherently scattered field and the quadratic phase are obtained.

Preparation and characterization of silver nanoparticles within silicate glass ceramics via modification of ion exchange process

This work pointed out the preparation of glass-ceramic based on fluoramphibole using different alkalis. Phases were crystallized using heat-treatment at 850 °C/2.5 h and were identified using XRD. Fluorophlogopite, fluorrichterite, enstatite and cristobalite were found in the heat-treated glasses. Crystallization of fluorophlogopite or fluorrichterite was detected in samples containing high K and Na, respectively, accompanied with crystallization of enstatite in the last sample. Cristobalite was crystallized only in equal alkali-containing glass beside enstatite and richterite. Nanoparticles of silver have grown within a silicate glass via modification of ion exchange process. The metal particle diameters were detected using XRD and TEM. The particles size ranges from 4 to 10 nm. The amount of silver ions exchanged was varied according to type and amount of alkali on fluoramphibole compositions. The composites show low values of dielectric permittivity (10–30) due to the formation of interconnected metal nanoparticle chains. The resistivity of the specimen dropped from 10 8 to 10 10 Ω cm 2 to 10 4 to 10 8 Ω cm 2 after ion exchange process which shows metallic and semiconducting behavior according to the reduction temperature.

Natural Biopolymers: Novel Templates for the Synthesis of Nanostructures

Biological systems such as proteins, viruses, and DNA have been most often reported to be used as templates for the synthesis of functional nanomaterials, but the properties of widely available biopolymers, such as cellulose, have been much less exploited for this purpose. Here, we report for the first time that cellulose nanocrystals (CNC) have the capacity to assist in the synthesis of metallic nanoparticle chains. A cationic surfactant, cetyltrimethylammonium bromide (CTAB), was critical to nanoparticle stabilization and CNC surface modification. Silver, gold, copper, and platinum nanoparticles were synthesized on CNCs, and the nanoparticle density and particle size were controlled by varying the concentration of CTAB, the pH of the salt solution, and the reduction time.