Gold Nanorod Dimers Research Articles

Atomic-Scale Confinement of Resonant Optical Fields

In the presence of matter, there is no fundamental limit preventing confinement of visible light even down to atomic scales. Achieving such confinement and the corresponding resonant intensity enhancement inevitably requires simultaneous control over atomic-scale details of material structures and over the optical modes that such structures support. By means of self-assembly we have obtained side-by-side aligned gold nanorod dimers with robust atomically defined gaps reaching below 0.5 nm. The existence of atomically confined light fields in these gaps is demonstrated by observing extreme Coulomb splitting of corresponding symmetric and antisymmetric dimer eigenmodes of more than 800 meV in white-light scattering experiments. Our results open new perspectives for atomically resolved spectroscopic imaging, deeply nonlinear optics, ultrasensing, cavity optomechanics, as well as for the realization of novel quantum-optical devices.

Dispersible Surface‐Enhanced Raman Scattering Nanosheets

Ultrathin and flexible silica nanosheets, synthesized with gold nanorod dimers embedded uniformly throughout, can be dispersed in solution and deposited onto arbitrary surfaces. These novel materials conform and maintain the as-synthesized density of dimers, allowing them to be used reliably in labeling and detection applications.

Dispersible Gold Nanorod Dimers with Sub-5 nm Gaps as Local Amplifiers for Surface-Enhanced Raman Scattering

We report the synthesis of solution-dispersible, 35 nm diameter gold nanorod dimers with gaps as small as ∼2 nm for surface-enhanced Raman scattering (SERS). Using on-wire lithography (OWL), we prepared tailorable dimers in high yield and high monodispersity (∼96% dimers) that produce both large and reproducible SERS signals with enhancement factors of (6.8 ± 0.7) × 10(8) for single dimers in air and 1.2 × 10(6) for ensemble-averaged solution measurements. Furthermore, we show that these structures, which are the smallest ever made by OWL, can be used to detect molecules on flat surfaces and in aqueous solutions. When combined, these attributes with respect to sensitivity, reproducibility, and tailorability lead to a novel and powerful local amplification system for SERS applications.

Magnetoelectrically coupled polariton excitation in a plasmonic crystal composed of nanorod dimers

In this work, the long wavelength optical properties of a plasmonic crystal, composed of gold nanorod dimers arranged parallel, have been studied. Due to the strong coupling between incident light and the oscillation of free electrons inside nanorod dimers, the magnetically induced and/or magnetoelectrically coupled plasmonic polaritons can be excited. A theoretical demonstration has been proposed and coupled equations that show similar profiles to the Huang–Kun equations for ionic crystals have been deduced, indicating the constitutive abnormalities and polaritonic bandgap effect. The analogy between the magnetoelectrically coupled metamaterials and ionic crystals may shed light on physical explanations, as well as constitutive parameter retrieval, for the magnetoelectric metamaterials.

Plasmonic behaviors of gold dimers perturbed by a single nanoparticle in the gap

Modulation of resonance modes of plasmonic nanostructures at a single nanostructure level is of great importance for the development of integrated photonic chips. Recent progress of the fabrication techniques including double electron beam lithography, direct laser printing, and electron beam induced deposition has allowed the local fabrication of sub-50 nm size nanoparticles (NPs) of various materials, which provides a potential experimental realization of local impact on plasmonic behaviors of single nanostructures. In this article, we theoretically investigate that the presence of a single NP in the midgap may give rise to the excitation of new resonance modes and/or large spectral shifts of resonance modes for different plasmonic dimers (gold bowtie, nanodisk and nanorod dimers), therefore resulting in the modification of the far-field and near-field optical properties. The modification can be controlled by changing the deposition position, particle size, and material composition of the single NP. Moreover, our calculations imply that the plasmonic dimers perturbed locally by a single NP of the metal-insulator phase transition material (vanadium dioxide) in the gap can potentially act as very effective optical switch components in future photonic chips.

Infrared spectroscopic and electron microscopic characterization of gold nanogap structurefabricated by focused ion beam

Using infrared spectroscopy of plasmonic resonances and mapping of elemental compositionand structure, we investigated the correlation between optical and structuralproperties of nanometre-scale gaps in gold nanorod dimers fabricated by electronbeam lithography (EBL) and focused ion beam (FIB) milling. In spite of theirvery similar scanning electron microscopy (SEM) images, a fully cut nanogapand a shallower cut with slight imperfection near the gap region were clearlydistinguished by their strongly different infrared plasmonic resonance behaviour.The differences in the infrared spectra are related to different structural andchemical results from elaborated cross-sectional transmission electron micrographsand energy dispersive x-ray spectrometry (EDX) mapping of the gap region.

Tunable SERS in Gold Nanorod Dimers through Strain Control on an Elastomeric Substrate

In this Letter we provide experimental verification of the interparticle distance dependence of the SERS enhancement factor in 1 μm gold gapped nanorods. Au dimers are fabricated from electrochemically grown heterogeneous Au-Ag-Au nanorods and deposited on a stretchable elastomer film which allows active and reversible tuning of the interparticle gap on the sub-5-nm level. Significantly, this technique allows the distance dependence to be tracked using a single dimer, thereby avoiding enhancement factor reproducibility issues arising from morphological differences in disparate nanoparticle pairs.

Effects of Symmetry Breaking and Conductive Contact on the Plasmon Coupling in Gold Nanorod Dimers

We have explored the consequences of symmetry breaking on the coupled surface plasmon resonances in individual dimers of gold nanorods using single-particle dark-field scattering spectroscopy and numerical simulations. Pairs of chemically grown nanorods can exhibit wide variation in sizes, gap distances, and relative orientation angles. The combination of single-particle spectroscopy and theoretical analysis allowed us to discern the effects of specific asymmetry-inducing parameters one at a time. The dominant influence of symmetry breaking occurred for longitudinal resonances in strongly coupled nanorods in linear end-to-end configurations. In particular, we found that the normally dark antibonding dimer mode becomes visible when the sizes of the two nanorods are different. In addition, we observed a conductively coupled plasmon mode that was red-shifted by at least 250 nm from the bonding plasmon mode for the corresponding nontouching geometry. Gaining detailed insight into how symmetry breaking influences coupled surface plasmon resonances of individual nanorod dimers is an important step toward the general understanding of the optical properties of assemblies of chemically synthesized nanorods with unavoidable irregularities in size and orientation.

Angle- and Energy-Resolved Plasmon Coupling in Gold Nanorod Dimers

The plasmon coupling in the dimers of Au nanorods linked together at their ends with dithiol molecules has been studied. The plasmon coupling in the dimers composed of similarly sized nanorods gives antibonding and bonding plasmon modes. The plasmon wavelengths of the two modes have been found to remain approximately unchanged, with the scattering intensity ratio between the antibonding and bonding modes decaying rapidly as the angle between the nanorods is increased. This plasmon coupling behavior agrees with that obtained from both electrodynamic calculations and modeling on the basis of the dipole-dipole interaction. The electric field in the gap region is largely enhanced for the bonding mode, while that for the antibonding mode is even smaller than the far field, highlighting the importance of selecting appropriate plasmon modes for plasmon-enhanced spectroscopies. An anti-crossing-like behavior in the plasmon coupling energy diagram has further been revealed for linearly end-to-end assembled dimers composed of differently sized nanorods. This result will be useful for plasmonic applications where the plasmon wavelength is required to be controllable but without sacrificing the electric field enhancement.

Plasmon Coupling of Gold Nanorods at Short Distances and in Different Geometries

The experimentally determined scattering spectra of discrete, crystalline, gold nanorod dimers arranged side-to-side, end-to-end, at right angles in different orientations and also with longitudinal offsets are reported along with the electron micrographs of the individual dimers. The spectra exhibit both red- and blue-shifted surface plasmon resonances, consistent with the plasmon hybridization model. However, the plasmon coupling constant for gold dimers with less than a few nanometers separation between the particles does not obey the exponential dependence predicted by the Universal Plasmon Ruler equation. The experimentally determined spectra are compared with electrodynamic calculations and the interactions between the individual rod plasmons in different dimer orientations are elucidated.