Metals support surface plasmons at optical wavelengths and have the ability to localize light to sub-wavelength regions. The field enhancements that occur in these regions set the ultimate limitations on a wide range of nonlinear and quantum optical phenomena. Here we show that the dominant limiting factor is not the resistive loss of the metal, but the intrinsic nonlocality of its dielectric response. A semi-classical model of the electronic response of a metal places strict bounds on the ultimate field enhancement. We demonstrate the accuracy of this model by studying the optical scattering from gold nanoparticles spaced a few angstroms from a gold film. The bounds derived from the models and experiments impose limitations on all nanophotonic systems.

To move nanophotonic devices such as lasers and single-photon sources into the practical realm, a challenging list of requirements must be met, including directional emission 1-5 , room-temperature and broadband operation 6-9 , high radiative quantum efficiency 1,4 and a large spontaneous emission rate 7 . To achieve these features simultaneously, a platform is needed for which the various decay channels of embedded emitters can be fully understood and controlled. Here, we show that all these device requirements can be satisfied by a film-coupled metal nanocube system with emitters embedded in the dielectric gap region. Fluorescence lifetime measurements on ensembles of emitters reveal spontaneous emission rate enhancements exceeding 1,000 while maintaining high quantum efficiency (>0.5) and directional emission (84% collection efficiency). Using angle-resolved fluorescence measurements, we independently determine the orientations of emission dipoles in the nanoscale gap. Incorporating this information with the threedimensional spatial distribution of dipoles into full-wave simulations predicts time-resolved emission in excellent agreement with experiments.Typical luminescent emitters have relatively long emission lifetimes (∼10 ns) and non-directional emission. Unfortunately, these intrinsic optical properties are poorly matched to the requirements of nanophotonic devices. For example, in single-photon sources, fast radiative rates are required for operation at high frequencies, and directionality is needed to achieve a high collection efficiency 10 . In addition, with plasmonic lasers, enhanced spontaneous emission into the cavity mode can reduce the lasing threshold 9 . As a result, much work has focused on modifying the photonic environment of emitters to enhance 11 the spontaneous emission rate, known as the Purcell effect 12 . Early approaches concentrated on integrating emitters into dielectric optical microcavities and showed modest emission rate enhancements [13][14][15] . However, dielectric cavities require high quality factors for large rate enhancements, which makes these cavities mismatched with the spectrally wide emission from inhomogeneously broadened or room-temperature emitters. Plasmonic nanostructures are a natural solution to the spectral mismatch problem because of their relatively broad optical resonances and high field enhancements [16][17][18] . Despite these advantages and the capability for emission rate enhancement 19 , many plasmonic structures suffer from unacceptably high non-radiative decay due to intrinsic losses in the metal, or have low directionality of emission 7 . In plasmonic structures, the Purcell factor (defined as the fractional increase in total emission rate) has contributions from an increased radiative rate and from an increased non-radiative rate due to metal losses. It is therefore critical to specify the fraction of energy emitted as radiation, known as the radiative quantum efficiency (QE). From knowledge of the Purcell factor and the QE, the enhancement in the ...

Efficient and tunable absorption is essential for a variety of applications, such as the design of controlled emissivity surfaces for thermophotovoltaic devices1; tailoring of the infrared spectrum for controlled thermal dissipation2; and detector elements for imaging3. Metamaterials based on metallic elements are particularly efficient as absorbing media, because both the electrical and the magnetic properties of a metamaterial can be tuned by structured design4. To date, metamaterial absorbers in the infrared or visible range have been fabricated using lithographically patterned metallic structures2,5–9, making them inherently difficult to produce over large areas and hence reducing their applicability. We demonstrate here an extraordinarily simple method to create a metamaterial absorber by randomly adsorbing chemically synthesized silver nanocubes onto a nanoscale thick polymer spacer layer on a gold film –making no effort to control the spatial arrangement of the cubes on the film– and show that the film-coupled nanocubes provide a reflectance spectrum that can be tailored by varying the geometry. Each nanocube is the optical analog of the well-known grounded patch antenna, with a nearly identical local field structure that is modified by the plasmonic response of the metal dielectric function, and with an anomalously large absorption efficiency that can be partly attributed to an interferometric effect10. The absorptivity of large surface areas can be controlled using this method, at scales out of reach of lithographic approaches like e-beam lithography otherwise required to manipulate matter at the nanometer scale.

A metallic nanoparticle positioned over a metal film offers great advantages as a highly controllable system relevant for probing field-enhancement and other plasmonic effects. Because the size and shape of the gap between the nanoparticle and film can be controlled to subnanometer precision using relatively simple, bottom-up fabrication approaches, the film-coupled nanoparticle geometry has recently been applied to enhancing optical fields, accessing the quantum regime of plasmonics, and the design of surfaces with controlled reflectance. In the present work, we examine the plasmon modes associated with a silver nanocube positioned above a silver or gold film, separated by an organic, dielectric spacer layer. The film-coupled nanocube is of particular interest due to the formation of waveguide cavity-like modes between the nanocube and film. These modes impart distinctive scattering characteristics to the system that can be used in the creation of controlled reflectance surfaces and other applications. We perform both experimental spectroscopy and numerical simulations of individual nanocubes positioned over a metal film, finding excellent agreement between experiment and simulation. The waveguide mode description serves as a starting point to explain the optical properties observed.

The radiative processes associated with fluorophores and other radiating systems can be profoundly modified by their interaction with nanoplasmonic structures. Extreme electromagnetic environments can be created in plasmonic nanostructures or nanocavities, such as within the nanoscale gap region between two plasmonic nanoparticles, where the illuminating optical fields and the density of radiating modes are dramatically enhanced relative to vacuum. Unraveling the various mechanisms present in such coupled systems, and their impact on spontaneous emission and other radiative phenomena, however, requires a suitably reliable and precise means of tuning the plasmon resonance of the nanostructure while simultaneously preserving the electromagnetic characteristics of the enhancement region. Here, we achieve this control using a plasmonic platform consisting of colloidally synthesized nanocubes electromagnetically coupled to a metallic film. Each nanocube resembles a nanoscale patch antenna (or nanopatch) whose plasmon resonance can be changed independent of its local field enhancement. By varying the size of the nanopatch, we tune the plasmonic resonance by ∼ 200 nm, encompassing the excitation, absorption, and emission spectra corresponding to Cy5 fluorophores embedded within the gap region between nanopatch and film. By sweeping the plasmon resonance but keeping the field enhancements roughly fixed, we demonstrate fluorescence enhancements exceeding a factor of 30,000 with detector-limited enhancements of the spontaneous emission rate by a factor of 74. The experiments are supported by finite-element simulations that reveal design rules for optimized fluorescence enhancement or large Purcell factors.

In this concept, we present the basic assumptions and techniques underlying the hydrodynamic model of electron response in metals and demonstrate that the model can be easily incorporated into computational models. We discuss the role of the additional boundary conditions that arise due to nonlocal terms in the modified equation of motion and the ultimate impact on nanoplasmonic systems. The hydrodynamic model captures much of the microscopic dynamics relating to the fundamental quantum mechanical nature of the electrons and reveals intrinsic limitations to the confinement and enhancement of light around nanoscale features. The presence of such limits is investigated numerically for different configurations of plasmonic nanostructures.

Multiscale plasmonic systems (e.g. extended metallic nanostructures with sub-nanometer interdistances) play a key role in the development of next-generation nano-photonic devices. An accurate modeling of the optical interactions in these systems requires an accurate description of both quantum effects and far-field properties. Classical electromagnetism can only describe the latter, while Time-Dependent Density Functional Theory (TD-DFT) can provide a full first-principles quantum treatment. However, TD-DFT becomes computationally prohibitive for sizes that exceed few nanometers, which are instead very important for most applications. In this article, we introduce a method based on the quantum hydrodynamic theory (QHT) that includes nonlocal contributions of the kinetic energy and the correct asymptotic description of the electron density. We show that our QHT method can predict both plasmon energy and spill-out effects in metal nanoparticles in excellent agreement with TD-DFT predictions, thus allowing reliable and efficient calculations of both quantum and far-field properties in multiscale plasmonic systems.

We present a study of the second-order nonlinear optical properties of metal-based metamaterials. A hydrodynamic model for electronic response is used, in which nonlinear surface contributions are expressed in terms of the bulk polarization. The model is in good agreement with published experimental results, and clarifies the mechanisms contributing to the nonlinear response. In particular, we show that the reported enhancement of second-harmonic in split-ring resonator based media is driven by the electric rather than the magnetic properties of the structure.PACS numbers: 42.65. Ky, 81.05.Xj, 78.67.Pt, 73.20.Mf Metamaterials (MMs) are artificially structured media whose collective electromagnetic properties derive from the geometry of sub-wavelength inclusions. To date, the most common MM designs have made use of inclusions formed by conducting materials that function as subwavelength electrical circuits. These conductor based MMs have proven adept at mimicking a wide variety of linear electromagnetic responses, providing a new venue to explore otherwise inaccessible concepts 1 . In the context of nonlinear response, however, artificial materials may offer even greater opportunities, due to the inherently inhomogeneous local field distribution that exists within and around MM inclusions. By carefully structuring the inclusion geometry, extremely large field enhancement regions can be produced that can dominate and enhance the effective nonlinear response of the composite.The enhancement of nonlinear processes by MMs has been demonstrated at radio and microwave frequencies, using packaged components, such as varactor diodes, to introduce nonlinearity into the gaps of metal MM inclusions 2 . However, to achieve nonlinear optical materials at higher wavelengths, a simple scaling of these prototype structures to higher frequencies (e.g., beyond a few terahertz) will not suffice. First, the response of most metals changes from conductor-like to dielectric-like at frequencies above a few terahertz, with absorption increasing significantly as the fields are able to penetrate further into the metal. Second, packaged semiconductor components are not readily available at frequencies above 100 GHz.While metals and conductors may possess undesirable properties at optical wavelengths, such as increased absorption, they also possess unique and potentially advantageous properties. In addition to large field enhancements, metal nanostructures also support intrinsic nonlinearities that relate to the dynamics of free and bound charge carriers. As a result, metals possess some of the largest nonlinear susceptibilities known. Examples include the large χ (3) values of gold or silver, for example, suggesting that metals can serve both to form the linear MM response by tailoring the structure, while serving as the source of nonlinearity for nonlinear optical MMs.The second-order nonlinearity in metals arises from both volume and surface contributions. Nonlinear surface contributions are strictly related to the response of the elec...