Fundamentals of Plasmonics.- Electromagnetics of Metals.- Surface Plasmon Polaritons at Metal / Insulator Interfaces.- Excitation of Surface Plasmon Polaritons at Planar Interfaces.- Imaging Surface Plasmon Polariton Propagation.- Localized Surface Plasmons.- Electromagnetic Surface Modes at Low Frequencies.- Applications.- Plasmon Waveguides.- Transmission of Radiation Through Apertures and Films.- Enhancement of Emissive Processes and Nonlinearities.- Spectroscopy and Sensing.- Metamaterials and Imaging with Surface Plasmon Polaritons.- Concluding Remarks.

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Plasmonics: Fundamentals and Applications

Recent developments have greatly improved the sensitivity of optical sensors based on metal nanoparticle arrays and single nanoparticles. We introduce the localized surface plasmon resonance (LSPR) sensor and describe how its exquisite sensitivity to size, shape and environment can be harnessed to detect molecular binding events and changes in molecular conformation. We then describe recent progress in three areas representing the most significant challenges: pushing sensitivity towards the single-molecule detection limit, combining LSPR with complementary molecular identification techniques such as surface-enhanced Raman spectroscopy, and practical development of sensors and instrumentation for routine use and high-throughput detection. This review highlights several exceptionally promising research directions and discusses how diverse applications of plasmonic nanoparticles can be integrated in the near future.

Biosensing with plasmonic nanosensors

Localized surface plasmon resonance (LSPR) spectroscopy of metallic nanoparticles is a powerful technique for chemical and biological sensing experiments. Moreover, the LSPR is responsible for the electromagnetic-field enhancement that leads to surface-enhanced Raman scattering (SERS) and other surface-enhanced spectroscopic processes. This review describes recent fundamental spectroscopic studies that reveal key relationships governing the LSPR spectral location and its sensitivity to the local environment, including nanoparticle shape and size. We also describe studies on the distance dependence of the enhanced electromagnetic field and the relationship between the plasmon resonance and the Raman excitation energy. Lastly, we introduce a new form of LSPR spectroscopy, involving the coupling between nanoparticle plasmon resonances and adsorbate molecular resonances. The results from these fundamental studies guide the design of new sensing experiments, illustrated through applications in which researchers use both LSPR wavelength-shift sensing and SERS to detect molecules of chemical and biological relevance.

https://www.photonics.ethz.ch/fileadmin/user_upload/Courses/NanoOptics/willets.pdf

Localized Surface Plasmon Resonance Spectroscopy and Sensing

The unprecedented ability of nanometallic (that is, plasmonic) structures to concentrate light into deep-subwavelength volumes has propelled their use in a vast array of nanophotonics technologies and research endeavours. Plasmonic light concentrators can elegantly interface diffraction-limited dielectric optical components with nanophotonic structures. Passive and active plasmonic devices provide new pathways to generate, guide, modulate and detect light with structures that are similar in size to state-of-the-art electronic devices. With the ability to produce highly confined optical fields, the conventional rules for light-matter interactions need to be re-examined, and researchers are venturing into new regimes of optical physics. In this review we will discuss the basic concepts behind plasmonics-enabled light concentration and manipulation, make an attempt to capture the wide range of activities and excitement in this area, and speculate on possible future directions.

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Plasmonics for extreme light concentration and manipulation.

This feature article highlights work from the authors' laboratories on the synthesis, assembly, reactivity, and optical applications of metallic nanoparticles of nonspherical shape, especially nanorods. The synthesis is a seed-mediated growth procedure, in which metal salts are reduced initially with a strong reducing agent, in water, to produce ∼4 nm seed particles. Subsequent reduction of more metal salt with a weak reducing agent, in the presence of structure-directing additives, leads to the controlled formation of nanorods of specified aspect ratio and can also yield other shapes of nanoparticles (stars, tetrapods, blocks, cubes, etc.). Variations in reaction conditions and crystallographic analysis of gold nanorods have led to insight into the growth mechanism of these materials. Assembly of nanorods can be driven by simple evaporation from solution or by rational design with molecular-scale connectors. Short nanorods appear to be more chemically reactive than long nanorods. Finally, optical applica...

Anisotropic metal nanoparticles: Synthesis, assembly, and optical applications

Metallic bowtie nanoantennas should provide optical fields that are confined to spatial scales far below the diffraction limit. To improve the mismatch between optical wavelengths and nanoscale objects, we have lithographically fabricated Au bowties with lengths $\ensuremath{\sim}75\text{ }\text{ }\mathrm{n}\mathrm{m}$ and gaps of tens of nm. Using two-photon-excited photoluminescence of Au, the local intensity enhancement factor relative to that for the incident diffraction-limited beam has been experimentally determined for the first time. Enhancements $g{10}^{3}$ occur for 20 nm gap bowties, in good agreement with theoretical simulations.

/pdf/improving-the-mismatch-between-light-and-nanoscale-objects-egywhoitsz.pdf

Improving the Mismatch between Light and Nanoscale Objects with Gold Bowtie Nanoantennas

Optical spectroscopy at the ultimate limit of a single molecule has grown over the past dozen years into a powerful technique for exploring the individual nanoscale behavior of molecules in complex local environments. Observing a single molecule removes the usual ensemble average, allowing the exploration of hidden heterogeneity in complex condensed phases as well as direct observation of dynamical state changes arising from photophysics and photochemistry, without synchronization. This article reviews the experimental techniques of single-molecule fluorescence spectroscopy and microscopy with emphasis on studies at room temperature where the same single molecule is studied for an extended period. Key to successful single-molecule detection is the need to optimize signal-to-noise ratio, and the physical parameters affecting both signal and noise are described in detail. Four successful microscopic methods including the wide-field techniques of epifluorescence and total internal reflection, as well as confocal and near-field optical scanning microscopies are described. In order to extract the maximum amount of information from an experiment, a wide array of properties of the emission can be recorded, such as polarization, spectrum, degree of energy transfer, and spatial position. Whatever variable is measured, the time dependence of the parameter can yield information about excited state lifetimes, photochemistry, local environmental fluctuations, enzymatic activity, quantum optics, and many other dynamical effects. Due to the breadth of applications now appearing, single-molecule spectroscopy and microscopy may be viewed as useful new tools for the study of dynamics in complex systems, especially where ensemble averaging or lack of synchronization may obscure the details of the process under study.

Methods of single-molecule fluorescence spectroscopy and microscopy

Single molecule confocal microscopy is used to study fluorescence intermittency of individual ZnS overcoated CdSe quantum dots (QDs) excited at 488 nm The confocal apparatus permits the distribution of “on” and “off” times (ie, periods of sustained fluorescence emission and darkness) to be measured over an unprecedentedly large dynamic range (109) of probability densities, with nonexponential behavior in τoff over a 105 range in time scales In dramatic contrast, these same τoff distributions in all QDs are described with remarkable simplicity over this 109-fold dynamic range by a simple inverse power law, ie, P(τoff)∝1/τoff1+α Such inverse power law behavior is a clear signature of distributed kinetics, such as predicted for (i) an exponential distribution of trap depths or (ii) a distribution of tunneling distances between QD core/interface states This has important statistical implications for all previous studies of fluorescence intermittency in semiconductor QDs and may have broader implications for other systems such as single polymer molecules

Nonexponential “blinking” kinetics of single CdSe quantum dots: A universal power law behavior

Metallic “bowtie” nanoantennas consisting of two opposing tip-to-tip Au triangles have been fabricated with triangle lengths of 75 nm and gaps ranging from 16 to 488 nm. For light polarized along the line between the two triangles, the plasmon scattering resonance first blue-shifts with increasing gap, and then red-shifts as the particles become more and more uncoupled, while perpendicularly polarized excitation shows little dependence upon gap size. This behavior may be approximately understood in a coupled-dipole approximation as changes in the phase between static dipole−dipole interactions and dipole radiative interaction effects.

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Gap-Dependent Optical Coupling of Single “Bowtie” Nanoantennas Resonant in the Visible

Single molecule confocal microscopy is used to investigate the detailed kinetics of fluorescence intermittency in colloidal II–VI (CdSe) semiconductor quantum dots. Two distinct modes of behavior are observed corresponding to (i) sustained “on” episodes (τon) of rapid laser absorption/fluorescence cycling, followed by (ii) sustained “off” episodes (τoff) where essentially no light is emitted despite continuous laser excitation. Both on-time and off-time probability densities follow an inverse power law, P(τon/off)∝1/τon/offm, over more than seven decades in probability density and five decades in time. Such inverse power law behavior is an unambiguous signature of highly distributed kinetics with rates varying over 105-fold, in contrast with models for switching between “on” and “off” configurations of the system via single rate constant processes. The unprecedented dynamic range of the current data permits several kinetic models of fluorescence intermittency to be evaluated at the single molecule level a...

D.P. Fromm

Papers

Improving the Mismatch between Light and Nanoscale Objects with Gold Bowtie Nanoantennas

Methods of single-molecule fluorescence spectroscopy and microscopy

Nonexponential “blinking” kinetics of single CdSe quantum dots: A universal power law behavior

Gap-Dependent Optical Coupling of Single “Bowtie” Nanoantennas Resonant in the Visible

``On''/``off'' fluorescence intermittency of single semiconductor quantum dots