Perforated semishells: far-field directional control and optical frequency magnetic response

Broadband directional scattering through a phase difference acquired in composite nanoparticles

We study the broadband scattering of light by composite nanoparticles through the Born approximation, FEM simulations, and measurements. The particles consist of two materials and show broadband directional scattering. From the analytical approach and the subsequent FEM simulations, it was found that the directional scattering is due to the phase difference between the fields scattered by of each of the two materials of the nanoparticle. To confirm this experimentally, composite nanoparticles were produced using ion-beam etching. Measurements of SiO2 / Au composite nanoparticles confirmed the directional scattering which was predicted by theory and simulations.

Core-Shell Nano-Antenna Configurations for Array Formation with More Stability Having Conventional and Non-Conventional Directivity and Propagation Behavior

The enhancement of optical characteristics at optical frequencies deviates with the choice of the arrangement of core-shell nanoparticles and their environment. Likewise, the arrangements of core-shell nanoparticles in the air over a substrate or in liquid solution makes them unstable in the atmosphere. This article suggests designing a configuration of an active spherical coated nanoparticle antenna and its extended array in the presence of a passive dielectric, which is proposed to be extendable to construct larger arrays. The issue of instability in the core-shell nanoantenna array models is solved here by inserting the passive dielectric. In addition to this, the inclusion of a dielectric in the array model reports a different directivity behaviour than the conventional array models. We found at first that the combination model of the active coated nanoparticle and passive sphere at the resonant frequency can excite a stronger field with a rotated polarization direction and a propagation direction different from the incident plane-wave. Furthermore, the extended 2D array also rotates the polarization direction and propagation direction for the vertical incident plane-wave. The radiation beam operates strong multipoles in the 2D array plane at resonant frequency (behaving non-conventionally). Nevertheless, it forms a clear main beam in the incident direction when it deviates from the resonance frequency (behaving conventionally). The proposed array model may have possible applications in nano-amplifiers, nano-sensors and other integrated optics.

Control of the emission from electric and magnetic dipoles by gold nanocup antennas

The control of the emission from electric and magnetic dipoles is highly desired for the development of optic chips. Although the emission of electric dipole has been successfully controlled by plasmonic nanoantenna, the control of magnetic dipole emission is relatively difficult. Here, we systematically study the effect of electric and magnetic modes of Au nanocups on the emission of electric and magnetic dipoles. The emission of electric dipole can be enhanced by both the electric and magnetic mode of the Au nanocup, while the emission of the magnetic dipole is only increased by the magnetic mode. The enhancement exhibits wavelength dependence. The wavelength of the largest enhancement is determined by the resonance wavelength of electric and magnetic modes. The enhancement values for electric and magnetic dipoles are determined by the near-field electric and magnetic field enhancements, respectively. More importantly, the emission pattern of magnetic dipole is greatly modified by the magnetic mode of Au nanocup. The directional emission of magnetic dipole is first time realized by use of the magnetic mode of the Au nanocup. Our findings deepen the understanding of the plasmon-controlled emission of electric and magnetic dipoles and will be very helpful to the development of the nanophotonic chips.

Magnetic Plasmon-Enhanced Second-Harmonic Generation on Colloidal Gold Nanocups

The magnetic plasmons of three-dimensional nanostructures have unique optical responses and special significance for optical nanoresonators and nanoantennas. In this study, we have successfully synthesized colloidal Au and AuAg nanocups with a well-controlled asymmetric geometry, tunable opening sizes, and normalized depths ( h/ b, where h is depth and b is the height of the templating PbS nanooctahedrons), variable magnetic plasmon resonance, and largely enhanced second-harmonic generation (SHG). The most-efficient SHG of the bare Au nanocups is experimentally observed when the normalized depth h/ b is adjusted to ∼0.78-0.79. We find that the average magnetic field enhancement is maximized at h/ b = ∼0.65 and reveal that the maximal SHG can be attributed to the joint action of the optimized magnetic plasmon resonance and the "lightning-rod effect" of the Au nanocups. Furthermore, we demonstrate for the first time that the AuAg heteronanocups prepared by overgrowth of Ag on the Au nanocups can synergize the magnetic and electric plasmon resonances for nonlinear enhancement. By the tailoring of the dual resonances at the fundamental excitation and second-harmonic wavelengths, the far-field SHG intensity of the AuAg nanocups is enhanced 21.8-fold compared to that of the bare Au nanocups. These findings provide a strategy for the design of nonlinear optical nanoantennas based on magnetic plasmon resonances and can lead to diverse applications ranging from nanophotonics to biological spectroscopy.

Leakage-free polypyrrole–Au nanostructures for combined Raman detection and photothermal cancer therapy

A novel PPy–Au nanostructure with the bifunctionality of Raman detection and photothermal therapy of cancer is reported.

Colloidal Gold Nanocups with Orientation-Dependent Plasmonic Properties

Colloidal gold nanocups are synthesized through single-vertex-initiated gold deposition on PbS nanooctahedrons and subsequent selective dissolution of the PbS component. They possess strong magnetic plasmon resonance and exhibit remarkable orientation-dependent plasmonic properties when deposited on flat substrates. They can also effectively couple s-polarized light into the interfacial region between the nanocup and substrate.

Cationic gemini surfactant-assisted synthesis of hollow Au nanostructures by stepwise reductions

A novel synthetic approach was developed for creating versatile hollow Au nanostructures by stepwise reductions of Au(III) upon the use of cationic gemini surfactant hexamethylene-1,6-bis(dodecyl dimethylammonium bromide) (C12C6C12Br2) as a template agent. It was observed that the Au(I) ions obtained from the reduction of Au(III) by ascorbic acid can assist the gemini surfactant to form vesicles, capsule-like, and tube-like aggregates that subsequently act as soft templates for hollow Au nanostructures upon further reduction of Au(I) to Au(0) by NaBH4. It was demonstrated that the combination of C12C6C12Br2 and Au(I) plays a key role in regulating the structure of the hollow precursors not only because C12C6C12Br2 has a stronger aggregation ability in comparison with its single chain counterpart but also because the electrostatic repulsion between head groups of C12C6C12Br2 is greatly weakened after Au(III) is converted to Au(I), which is in favor of the construction of vesicles, capsule-like, and tube-like aggregates. Compared with solid Au nanospheres, the resultant hollow nanostructures exhibit enhanced electrocatalytic activities in methanol oxidation, following the order of elongated nanocapsule > nanocapsule > nanosphere. Benefiting from balanced interactions between the gemini surfactant and Au(I), this soft-template method may present a facile and versatile approach for the controlled synthesis of Au nanostructures potentially useful for fuel cells and other Au nanodevices.

Localized surface plasmon resonances in spatially dispersive nano-objects: phenomenological treatise

Nonlocal optical response of materials, important at the nanometric scale, influences numerous optical phenomena, such as electromagnetic field confinement and spectral characteristics of plasmonic resonances. Here, we present a general phenomenological approach to account for nonlocal material polarizabilities in nanoscale metal particles. The problem of nonlocal plasmonic resonances is formulated by an integro-differential equation in a space domain and solved by adopting its weak form, implemented in the finite element method, thus, dispensing with the requirements on additional boundary conditions. As an example, nonlocal smearing effects in plasmonic nanorods of various cross sections and nanotubes have been considered. Clear signature of nonlocality manifests itself in the interference fringes in the potential profile and a significant frequency shift of the localized surface plasmon resonances. These effects are especially important for nanoparticles with geometrical features comparable to the de Broglie wavelengths of electrons participating in the light-matter interactions. The proposed method provides a universal tool for phenomenological account of nonlocalities of any kind with the only requirement of linearity in system's response.

Tunable plasmon resonances and two-dimensional anisotropy of angular optical response of overlapped nanoshells

Symmetry breaking of metallic nanoparticles results in many unique optical properties. We use the discrete dipole approximation method to study the optical properties of overlapped nanoshells which further break the rotational symmetry compared with the semishells. The optical properties of the nanoparticles can be tuned from the visible to near infrared regime by varying the geometry parameters and the hybrid components of nanoparticles. The calculated extinction spectra show the two-dimensional anisotropy of the angular optical response of the nanoparticles. The plasmon hybridization model provides a way to interpret the resonance modes of the nanoparticles. The tunable plasmon resonances, the enhanced local fields and the anisotropic optical properties suggest that the overlapped nanoshells have potential applications in surface-enhanced spectroscopy and "smart" coating in windows or display devices.

Unidirectional spaser in symmetry-broken plasmonic core-shell nanocavity

The spaser, a quantum amplifier of surface plasmons by stimulated emission of radiation, is recognized as a coherent light source capable of confining optical fields at subwavelength scale. The control over the directionality of spasing has not been addressed so far, especially for a single-particle spasing nanocavity where optical feedback is solely provided by a plasmon resonance. In this work we numerically examine an asymmetric spaser – a resonant system comprising a dielectric core capped by a metal semishell. The proposed spaser emits unidirectionally along the axis of the semishell; this directionality depends neither on the incident polarization nor on the incident angle of the pump. The spasing efficiency of the semishell-capped resonator is one order of magnitude higher than that in the closed core-shell counterpart. Our calculations indicate that symmetry breaking can serve as a route to create unidirectional, highly intense, single-particle, coherent light sources at subwavelength scale.

Transformation-optics description of plasmonic nanostructures containing blunt edges/corners: from symmetric to asymmetric edge rounding

The sharpness of corners/edges can have a large effect on the optical responses of metallic nanostructures. Here we deploy the theory of transformation optics to analytically investigate a variety of blunt plasmonic structures, including overlapping nanowire dimers and crescent-shaped nanocylinders. These systems are shown to support several discrete optical modes, whose energy and line width can be controlled by tuning the nanoparticle geometry. In particular, the necessary conditions are highlighted respectively for the broadband light absorption effect and the invisibility dips that appear in the radiative spectrum. More detailed discussions are provided especially with respect to the structures with asymmetric edge rounding. These structures can support additional subradiant modes, whose interference with the neighboring dipolar modes results in a rapid change of the scattering cross-section, similar to the phenomenon observed in plasmonic Fano resonances. Finite element numerical calculations are also performed to validate the analytical predictions. The physical insights into blunt nanostructures presented in this work may be of great interest for the design of broadband light-harvesting devices, invisible and noninvasive biosensors, and slowing-light devices.

Silver-assisted colloidal synthesis of stable, plasmon resonant gold patches on silica nanospheres

Patchy particles possessing heterogeneous surface composition show great promise as self-organizing building blocks for new classes of hierarchical functional structures. A major hurdle is the scalable synthesis of stable patches on nanosized core particles with arbitrarily defined patch number and coverage. So far, few methods have been reported which could be expected to meet these challenges. Recently, we described the heterogeneous nucleation and growth of silver patches on silica nanospheres via a template free colloidal route. The patches produced, although tunable in size and number and showing interesting plasmon resonant properties, were rather unstable and degraded rapidly during attempts to process them further. In the present work, therefore, we set out to explore if related approaches can be employed to produce patchy particles involving gold, which is known to be more stable. The differences between typical patch precursors Ag(+) and [AuCl(x)(OH)(4-x)](-) and their respective interactions with amorphous silica make this a significant challenge. We show that preformed small silver patches in addition to the presence of a reducing agent are necessary for the formation of gold patches conformal to the silica nanosphere surface. Systematic study of the process parameters and their influence on the patchy particle morphology as well as in-depth analytical transmission electron microscopy investigation of the patch composition reveal that patches spread over the silica surface via a cycle of galvanic dissolution and redeposition of silver. The resulting gold patchy particles remain stable during subsequent storage or washing and display tunable plasmon resonances within the visible and near-IR spectrum.

Two-dimensional angularly selective optical properties of gold nanoshell with holes

We studied the optical extinction properties of Au nanoshell with two holes by the discrete-dipole approximation method. We found that the extinction spectra of the nanoparticles are sensitive to the angle between the polarization vector of the incident light and either symmetrical axis of the hole on nanoshell and also the sizes of two holes. The nanostructure we proposed provides the additional dimensional angularly selectivity of the optical properties and the plasmon resonances redshift comparing with the nanocup. In addition, the conception of the "two-dimensional" symmetry breaking of the nanoparticle is suggested which can induce the two-dimensional spatial asymmetry of optical properties of nanoparticles.

Optically induced interaction of magnetic moments in hybrid metamaterials

We propose a novel type of hybrid metal-dielectric structures composed of silicon nanoparticles and split-ring resonators for advanced control of optically induced magnetic response. We reveal that a hybrid "metamolecule" may exhibit a strong distance-dependent magnetic interaction that may flip the magnetization orientation and support "antiferromagnetic" ordering in a hybrid metamaterial created by a periodic lattice of such metamolecules. The propagation of magnetization waves in the hybrid structures opens new ways for manipulating artificial "antiferromagnetic" ordering at high frequencies.

Semishells: versatile plasmonic nanoparticles

Localized surface plasmon excitations in metal nanostructures have a strong impact on light scattering, absorption, and local field intensities at the nanoscale. Tweaking the nanoparticle shape, size, and material enables researchers to engineer the resonance wavelength position, the nanoparticles' local field enhancement, and their scattering properties. In particular, by breaking the symmetry of originally symmetric nanostructures, additional degrees of freedom can be explored. One particular example of a highly investigated nanostructure is the so-called semishell (or nanocup or nanocrescent moon). In this issue of ACS Nano, King et al. report on the angular and spectral scattering properties of plasmonic semishells and the effect of a high-index substrate on these properties.

Angle- and spectral-dependent light scattering from plasmonic nanocups

As optical frequency nanoantennas, reduced-symmetry plasmonic nanoparticles have light-scattering properties that depend strongly on geometry, orientation, and variations in dielectric environment. Here we investigate how these factors influence the spectral and angular dependence of light scattered by Au nanocups. A simple dielectric substrate causes the axial, electric dipole mode of the nanocup to deviate substantially from its characteristic cos(2) θ free space scattering profile, while the transverse, magnetic dipole mode remains remarkably insensitive to the presence of the substrate. Nanoscale irregularities of the nanocup rim and the local substrate permittivity have a surprisingly large effect on the spectral- and angle-dependent light-scattering properties of these structures.

Orientation-preserving transfer and directional light scattering from individual light-bending nanoparticles

A nanocup, or semishell, is an asymmetric plasmonic "Janus" nanoparticle with electric and magnetic plasmon modes; the latter scatters light in a direction controlled by nanoparticle orientation, making it the nanoscale analog of a parabolic antenna. Here we report a method for transferring nanocups from their growth substrate to oxide-terminated substrates that precisely preserves their three-dimensional orientation, enabling their use as nanophotonic components. This enables us to selectively excite and probe the electric and magnetic plasmon modes of individual nanocups, showing how the scattered light depends on the direction of incoming light and the orientation of this nanoparticle antenna.

Near-field optical properties of silver nanocylinders arranged in a Pascal triangle

The Pascal triangle is a geometric representation of binomial coefficients in triangular form. We utilize this formalism to deterministically arrange silver nanocylinders of different sizes (30, 60, and 90 nm) on a triangle and numerically study their near-field optical properties. We show that near-field intensities at specific points on this triangle depend on the wavelength and angle of incidence. From the wavelength-dependent studies at various junctions of nanocylinders, we obtain maximum near-field intensity at 350 and 380 nm. By varying the angle of incidence of the TM-polarized plane wave, we find systematic variation in the near-field intensity at different junctions of the geometry. Our study will lead to insights in designing controllable electromagnetic hot spots for chip-based plasmonic devices.

Magnetic field concentrator for probing optical magnetic metamaterials

Development of all dielectric and plasmonic metamaterials with a tunable optical frequency magnetic response creates a need for new inspection techniques. We propose a method of measuring magnetic responses of such metamaterials within a wide range of optical frequencies with a single probe. A tapered fiber probe with a radially corrugated metal coating concentrates azimuthally polarized light in the near-field into a subwavelength spot the longitudinal magnetic field component which is much stronger than the perpendicular electric one. The active probe may be used in a future scanning near-field magnetic microscope for studies of magnetic responses of subwavelength elementary cells of metamaterials.