The response of gold nanoparticle dimers is studied theoretically near and beyond the limit where the particles are touching. As the particles approach each other, a dominant dipole feature is observed that is pushed into the infrared due to interparticle coupling and that is associated with a large pileup of induced charge in the interparticle gap. The redshift becomes singular as the particle separation decreases. The response weakens for very small separation when the coupling across the interparticle gap becomes so strong that dipolar oscillations across the pair are inhibited. Lowerwavelength, higher-order modes show a similar separation dependence in nearly touching dimers. After touching, singular behavior is observed through the emergence of a new infrared absorption peak, also accompanied by huge charge pileup at the interparticle junction, if initial interparticle-contact is made at a single point. This new mode is distinctly different from the lowest mode of the separated dimer. When the junction is made by contact between flat surfaces, charge at the junction is neutralized and mode evolution is continuous through contact. The calculated singular response explains recent experiments on metallic nanoparticle dimers and is relevant in the design of nanoparticle-based sensors and plasmon circuits.

The optical response of ring-shaped gold nanoparticles prepared by colloidal lithography is investigated. Compared to solid gold particles of similar size, nanorings exhibit a redshifted localized surface plasmon that can be tuned over an extended wavelength range by varying the ratio of the ring thickness to its radius. The measured wavelength variation is well reproduced by numerical calculations and interpreted as originating from coupling of dipolar modes at the inner and outer surfaces of the nanorings. The electric field associated with these plasmons exhibits uniform enhancement and polarization in the ring cavity, suggesting applications in near-infrared surface-enhanced spectroscopy and sensing.

The rapid emergence of nanoplasmonics as a novel technology has been driven by recent progress in the fabrication, characterization, and understanding of metal-nanoparticle systems. In this review, we highlight some of the key advances in each of these areas. We emphasize the basic physical understanding and experimental techniques that will enable a new generation of applications in nano-optics. Abstract The rapid emergence of nanoplasmonics as a novel technology has been driven by recent progress in the fabrication, characterization, and understanding of metal-nanoparticle systems. In this review, we highlight some of the key advances in each of these areas. We emphasize the basic physical understanding and experimental techniques that will enable a new generation of applications in nano-optics.Critical areas driving the emergence of metal-nanoparticle plasmonics: fabrication, characterization, and theory.

Modern nanotechnology opens the possibility of combining nanocrystals of various materials with very different characteristics in one superstructure. Here we study theoretically the optical properties of hybrid molecules composed of semiconductor and metal nanoparticles. Excitons and plasmons in such a hybrid molecule become strongly coupled and demonstrate novel properties. At low incident light intensity, the exciton peak in the absorption spectrum is broadened and shifted due to incoherent and coherent interactions between metal and semiconductor nanoparticles. At high light intensity, the absorption spectrum demonstrates a surprising, strongly asymmetric shape. This shape originates from the coherent internanoparticle Coulomb interaction and can be viewed as a nonlinear Fano effect which is quite different from the usual linear Fano resonance.

The optical properties of coupled metallic nanorods are studied to investigate the use of coupled plasmonic structures in field-enhanced spectroscopies. Light scattering by coupled nanorods is calculated with the boundary element method, including retardation. The modes of coupled nanorod systems are calculated by the boundary charge method and discussed in terms of their symmetry. Similar scattering behavior for isolated nanorods and pairs of nanorods can mask the very different local responses that produce near-field enhancement. The response of isolated rods redshifts with increasing rod length because intrarod restoring forces are reduced. The near-and far-field responses increase monotonically with increasing rod length ͑increasing polarization along the rod͒. For coupled nanorods, coupling localizes charge at the gap between the rod ends and splits degenerate modes. The localized charge depolarizes the intrarod response and provides an additional redshift. Moreover, the near-field enhancement in the gap between the nanorods is dramatically increased by coupling-induced charge localization at the gap. For short nanorods, the near-field response in coupled systems is determined by the geometry of the rod ends that define the gap. For longer nanorods, the response in coupled systems is determined by the rod length. Changing the dimensions and geometry of the nanorods to modify the interrod coupling has a major effect on the local-field enhancement. The effects of the environment and the actual metallic material do not have as big an influence on the field enhancement.

We describe the physical properties of excitons in hybrid complexes composed of semiconductor and metal nanoparticles. The interaction between individual nanoparticles is revealed as an enhancement or suppression of emission. Enhanced emission comes from electric field amplified by the plasmon resonance, whereas emission suppression is a result of energy transfer from semiconductor to metal nanoparticles. The emission intensity and energy transfer rate depend strongly on the geometrical parameters of the superstructure and the physical and material properties of the nanoparticles. In particular, the emission enhancement effect appears for nanoparticles with relatively small quantum yield, and silver nanoparticles have stronger enhancement compared to gold ones. Using realistic models, we review and analyze available experimental data on energy transfer between nanoparticles. In hybrid superstructures conjugated with polymer linkers, optical emission is sensitive to environmental parameters such as, for example, temperature. This sensitivity comes from expansion or contraction of a linker. With increasing temperature, emission of polymer-conjugated complexes can decrease or increase depending on the organization of a superstructure. The structures described here have potential as sensors and actuators.

We study the light scattering and surface plasmon resonances of Au nanorods that are commonly used as optical nanoantennas in analogy to dipole radio antennas for chemical and biodetection field-enhanced spectroscopies and scanned-probe microscopies. With the use of the boundary element method, we calculate the nanorod near-field and far-field response to show how the nanorod shape and dimensions determine its optical response. A full mapping of the size (length and radius) dependence for Au nanorods is obtained. The dipolar plasmon resonance wavelength lambda shows a nearly linear dependence on total rod length L out to the largest lengths that we study. However, L is always substantially less than lambda/2, indicating the difference between optical nanoantennas and long-wavelength traditional lambda/2 antennas. Although it is often assumed that the plasmon wavelength scales with the nanorod aspect ratio, we find that this scaling does not apply except in the extreme limit of very small, spherical nanoparticles. The plasmon response depends critically on both the rod length and radius. Large (500 nm) differences in resonance wavelength are found for structures with different sizes but with the same aspect ratio. In addition, the plasmon resonance deduced from the near-field enhancement can be significantly red-shifted due to retardation from the resonance in far-field scattering. Large differences in near-field and far-field response, together with the breakdown of the simple scaling law must be accounted for in the choice and design of metallic lambda/2 nanoantennas. We provide a general, practical map of the resonances for use in locating the desired response for gold nanoantennas.

In this communication, we study the optical response of a semiconductor quantum dot (SQD) coupled with a metal nanoparticle (MNP). In particular, we explore the relationship between the size of the constituents and the response of the system. We identify, here, three distinct regimes of behavior in the strong field limit that each exhibit novel properties. In the first regime, we find that the energy absorption spectrum displays an asymmetrical Fano shape (as previously predicted). It occurs when there is interference between the applied field and the induced field produced by the SQD at the MNP. When the coupling is increased by increasing the size of the SQD, we find a double peaked Fano structure in the response. This second peak occurs when the induced field becomes stronger than the external field. As the coupling is further increased by increasing the sizes of both the SQD and the MNP, we find a regime of bistability. This originates when the self-interaction of the SQD becomes significant. We explore these three regimes in detail and set bounds on each.