Electrochemical crystallization of plasmonic nanostructures

We show how gold recrystallizes when under the influence of electrochemical potentials. This "cold annealing" occurs without charge transfer reactions and preserves nanoscale structural features. By performing the process on plasmonic nanostructures, grain growth is monitored noninvasively by optical spectroscopy. In this way, the influence from crystal structure on plasmon resonances can be investigated independently. Observed spectral changes are in excellent agreement with analytical models and changes in electron relaxation time and plasma frequency are calculated.     

Building plasmonic nanostructures with DNA

Plasmonic structures can be constructed from precise numbers of well-defined metal nanoparticles that are held together with molecular linkers, templates or spacers. Such structures could be used to concentrate, guide and switch light on the nanoscale in sensors and various other devices. DNA was first used to rationally design plasmonic structures in 1996, and more sophisticated motifs have since emerged as effective and versatile species for guiding the assembly of plasmonic nanoparticles into structures with useful properties. Here we review the design principles for plasmonic nanostructures, and discuss how DNA has been applied to build finite-number assemblies (plasmonic molecules), regularly spaced nanoparticle chains (plasmonic polymers) and extended two- and three-dimensional ordered arrays (plasmonic crystals).     

Nanoscale control of Ag nanostructures for plasmonic fluorescence enhancement of near-infrared dyes

Potential utilization of proteins for early detection and diagnosis of various diseases has drawn considerable interest in the development of protein-based detection techniques. Metal induced fluorescence enhancement offers the possibility of increasing the sensitivity of protein detection in clinical applications. We report the use of tunable plasmonic silver nanostructures for the fluorescence enhancement of a near-infrared (NIR) dye (Alexa Fluor 790). Extensive fluorescence enhancement of ～2 orders of magnitude is obtained by the nanoscale control of the Ag nanostructure dimensions and interparticle distance. These Ag nanostructures also enhanced fluorescence from a dye with very high quantum yield (7.8 fold for Alexa Fluor 488, quantum efficiency (Qy) = 0.92). A combination of greatly enhanced excitation and an increased radiative decay rate, leading to an associated enhancement of the quantum efficiency leads to the large enhancement. These results show the potential of Ag nanostructures as metal induced fluorescence enhancement (MIFE) substrates for dyes in the NIR “biological window” as well as the visible region. Ag nanostructured arrays fabricated by colloidal lithography thus show great potential for NIR dye-based biosensing applications.      

Near-field behavior of zone-plate-like plasmonic nanostructures

The near-field behavior of a new plasmonic structure, the plasmonic micro-zone-plate (PMZP), is presented. The PMZP can realize superfocusing at a working distance on the micrometer scale and a resolving power beyond the diffraction limit. Compared with conventional Fresnel zone plates (CFZPs), its unique characteristics of a significantly elongated depth of focus (DOF) and focal length will make autofocusing easier for the relevant optical systems. These characteristics imply that it is possible to realize a free feedback control system for autofocusing systems in which probe scanning is performed with a constant working distance from the probe to the sample surface, provided that the flatness variation of the sample substrate is within the DOF. Moreover, unlike the CFZPs, there is no series of focal points appearing for beam propagation in the near-field region with a propagation distance ranging from lambda to 8 lambda or even longer. In addition, transmission properties in the near-field region are investigated by means of a computational simulation based on a finite-difference time-domain numerical algorithm. Peak transmission wavelength shifts were observed while the metal film thickness was changed. Focusing characteristics were analyzed for different numerical apertures of the PMZPs.     

Chiral metamaterial composed of three-dimensional plasmonic nanostructures

We introduce a top-down fabricated metamaterial composed of three-dimensional, chiral, plasmonic nanostructures for visible and near-infrared wavelengths. Based on a combined spectroscopic and interferometric characterization, the entire complex transmission response in terms of a Jones matrix is disclosed. Particularly, the polarization output state of light after propagation through the nanostructures can be decoded from the measurements for any excitation configuration. We experimentally found a rotation of the polarization azimuth of linearly polarized light exceeding 50° at wavelengths around 1.08 μm. This corresponds to a specific rotation which is significantly larger than that of any linear, passive, and reciprocal medium reported to date.     

Monolithic integration of continuously tunable plasmonic nanostructures

We demonstrate precise three-dimensional integration of smooth bumps, grooves, and apertures in optically thick metal films using template stripping. Patterned silicon wafers are used as high-quality, reusable templates. The heights or depths of the metallic features are controlled to within 2 nm, giving continuously tunable optical properties with sharp and intense plasmonic resonances. Furthermore, we demonstrate a pick-and-place template stripping method in situ, enabling versatile three-dimensional micromanipulation, imaging, and characterization of nanoscale devices.     

The formation of periodic diffractive plasmonic nanostructures with implanted copper nanoparticles by local ion etching of silica glass

Silica glass was subjected to a low-energy implantation with 40-keV Cu⁺ ions at a dose of 7.5 × 10¹⁶ ions/cm² and an ion-beam current density of 5 μA/cm² through a surface metal-wire mask with square holes of ～40 μm. The formation of copper nanoparticles in the glass was determined from the occurrence of characteristic plasmon optical absorption and through the detection of particles using an atomic force micro- scope. The formation of periodic surface microstructures via the local etching of silica glass during implantation was observed using a scanning electron microscope. The operating efficiency of the diffractive optical plasmonic element based on silica glass microstructures with metallic copper nanoparticles was shown during its sounding by the emission of a helium-neon laser.     

纳米金属粉体制备及其表面等离子共振应用

介绍了金、银等金属纳米结构的形貌可控的合成工艺进展，详述了纳米金属粉体的表面等离子共振特性、原理，展望了它们一些独特的应用，包括电磁场的区域增强、光学成像、光传榆、比色传感器和纳米范围的光波导等。     

Structural colors: from plasmonic to carbon nanostructures

In addition to colorant-based pigmentation, structure is a major contributor to a material's color. In nature, structural color is often caused by the interaction of light with dielectric structures whose dimensions are on the order of visible-light wavelengths. Different optical interactions including multilayer interference, light scattering, the photonic crystal effect, and combinations thereof give rise to selective transmission or reflection of particular light wavelengths, which leads to the generation of structural color. Recent developments in nanofabrication of plasmonic and carbon nanostructures have opened another efficient way to control light properties at the subwavelength scale, including visible-light wavelength selection, which can produce structural color. In this Concept, the most relevant and representative achievements demonstrated over the last several years are presented and analyzed. These plasmonic and carbon nanostructures are believed to offer great potential for high-resolution color displays and spectral filtering applications.     

Excitation and reemission of molecules near realistic plasmonic nanostructures

The enhancement of excitation and reemission of molecules in close proximity to plasmonic nanostructures is studied with special focus on the comparison between idealized and realistically shaped nanostructures. Numerical experiments show that for certain applications choosing a realistic geometry closely resembling the actual nanostructure is imperative, an idealized simulation geometry yielding significantly different results. Finally, a link between excitation and reemission processes is formed via the theory of optical reciprocity, allowing a transparent view of the electromagnetic processes involved in plasmon-enhanced fluorescence and Raman-scattering.     

Optical near-field mapping of plasmonic nanoprisms

The optical local-field enhancement on nanometer length scales provides the basis for plasmonic metal nanostructures to serve as molecular sensors and as nanophotonic devices. However, particle morphology and the associated surface plasmon resonance alone do not uniquely reflect the important details of the local field distribution. Here, we use interferometric homodyne tip-scattering near-field microscopy for plasmonic near-field imaging of crystalline triangular silver nanoprisms. Strong spatial field variation on lengths scales as short as 20 nm are observed sensitively depending on structural details and environment. The poles of the dipole and quadrupole plasmon modes, as identified by phase-sensitive probing and calculations performed in the discrete dipole approximation (DDA), reflect the particle symmetry. Together with the observation that the largest enhancement is not necessarily found to be associated with the tips of the nanoprisms, our results provide critical information for the selection of particle geometries as building blocks for plasmonic device applications.     

Optical forces in hybrid plasmonic waveguides

We demonstrate that in a hybrid plasmonic system the optical force exerted on a dielectric waveguide by a metallic substrate is enhanced by more than 1 order of magnitude compared to the force between a photonic waveguide and a dielectric substrate. A nanoscale gap between the dielectric waveguide and the metallic substrate leads to deep subwavelength optical energy confinement with ultralow mode propagation loss and hence results in the enhanced optical forces at low input optical power, as numerically demonstrated by both Maxwell's stress tensor formalism and the coupled mode theory analysis. Moreover, the hybridization between the surface plasmon modes and waveguide modes allows efficient optical trapping of single dielectric nanoparticle with size of only several nanometers in the gap region, manifesting various optomechanical applications such as nanoscale optical tweezers.     

Optical properties of ZnO nanostructures

We present a review of current research on the optical properties of ZnO nanostructures. We provide a brief introduction to different fabrication methods for various ZnO nanostructures and some general guidelines on how fabrication parameters (temperature, vapor-phase versus solution-phase deposition, etc.) affect their properties. A detailed discussion of photoluminescence, both in the UV region and in the visible spectral range, is provided. In addition, different gain (excitonic versus electron hole plasma) and feedback (random lasing versus individual nanostructures functioning as Fabry-Perot resonators) mechanisms for achieving stimulated emission are described. The factors affecting the achievement of stimulated emission are discussed, and the results of time-resolved studies of stimulated emission are summarized. Then, results of nonlinear optical studies, such as second-harmonic generation, are presented. Optical properties of doped ZnO nanostructures are also discussed, along with a concluding outlook for research into the optical properties of ZnO.     

Optical studies of polyaniline nanostructures

An investigative infrared spectroscopic study is undertaken of nano polyaniline (PANI) samples differing in size and morphology in order to explore the sensitivity of spectral behavior on these factors. IR spectroscopy is used for studying the changes in the interaction between polymeric molecules as parts of nanodimensional structures. A time dependent interfacial oxidative polymerization of aniline monomer is conducted in order to obtain the desired nano PANI samples in doped form. The morphological changes in samples so obtained are characterized using scanning electron microscopy (SEM). The SEM images of samples obtained using the time dependent interfacial polymerization show preferential formation of 1D nano/micro structures with dimensions varying with reaction time intervals, which is also confirmed by transmission electron microscopy (TEM). X-ray diffraction study is undertaken to assess the crystalline nature of the samples. The IR spectra and the X-ray diffraction pattern of samples reflect these morphological variations. A comparative study of bulk polyaniline sample obtained using standard procedures is also undertaken. Significant variations in the conductivity of different polyaniline samples are also observed. The 10 min sample shows significantly enhanced conductivity as compared to the 20 min and 24 h. PANI samples having well defined nanofibers.     

Optical spin Hall effects in plasmonic chains

Observation of optical spin Hall effects (OSHEs) manifested by a spin-dependent momentum redirection is presented. The effect occurring solely as a result of the curvature of the coupled localized plasmonic chain is regarded as the locally isotropic OSHE, while the locally anisotropic OSHE arises from the interaction between the optical spin and the local anisotropy of the plasmonic mode rotating along the chain. A wavefront phase dislocation was observed in a circular curvature, in which the dislocation strength was enhanced by the locally anisotropic effect.     

Epitaxial self-alignment: A new route to hybrid active plasmonic nanostructures

Concepts of lateral ordering of epitaxial semiconductor quantum dots (QDs) are for the first time transferred to hybrid nanostructures for active plasmonics. We review our recent research on the self-alignment of epitaxial nanocrystals of In and Ag on ordered one-dimensional In(Ga)As QD arrays and isolated QDs by molecular beam epitaxy. By changing the growth conditions the size and density of the metal nanocrystals are easily controlled and the surface plasmon resonance wavelength is tuned over a wide range in order to match the emission wavelength of the QDs. Photoluminescence measurements reveal large enhancement of the emitted light intensity due to plasmon enhanced emission and absorption down to the single QD level.     

Optical properties of ZnO-based core-shell nanostructures

ZnO nanorods were prepared by electrodeposition, and the effect of different shells (MgO and TiO_x) on their optical properties has been studied. Shells have also been prepared using a simple solution based method. The two types of shells exhibited different effects on the optical properties of ZnO nanorods, mainly due to different annealing temperature required for synthesis process. The improvement in the emission intensity from the samples under optical excitation did not result in changes in the emission spectra or increase in emission intensity under electrical excitation for the same bias voltage. However, improved luminance for the same bias current was observed for most core-shell structures.     

Optical impedance matching using coupled plasmonic nanoparticle arrays

Silver nanoparticle arrays placed on top of a high-refractive index substrate enhance the coupling of light into the substrate over a broad spectral range. We perform a systematic numerical and experimental study of the light incoupling by arrays of Ag nanoparticle arrays in order to achieve the best impedance matching between light propagating in air and in the substrate. We identify the parameters that determine the incoupling efficiency, including the effect of Fano resonances in the scattering, interparticle coupling, as well as resonance shifts due to variations in the near-field coupling to the substrate and spacer layer. The optimal configuration studied is a square array of 200 nm wide, 125 nm high spheroidal Ag particles, at a pitch of 450 nm on a 50 nm thick Si(3)N(4) spacer layer on a Si substrate. When integrated over the AM1.5 solar spectral range from 300 to 1100 nm, this particle array shows 50% enhanced incoupling compared to a bare Si wafer, 8% higher than a standard interference antireflection coating. Experimental data show that the enhancement occurs mostly in the spectral range near the Si band gap. This study opens new perspectives for antireflection coating applications in optical devices and for light management in Si solar cells.