We introduce a numerical method that enables efficient modeling of light scattering by large, disordered ensembles of non-spherical particles incorporated in stratified media, including when the particles are in close vicinity to each other, to planar interfaces, and/or to localized light sources. The method consists of finding a small set of fictitious polarizable elements-or numerical dipoles-that quantitatively reproduces the field scattered by an individual particle for any excitation and at an arbitrary distance from the particle surface. The set of numerical dipoles is described by a global polarizability matrix that is determined numerically by solving an inverse problem relying on fullwave simulations. The latter are classical and may be performed with any Maxwell's equations solver. Spatial non-locality is an important feature of the numerical dipoles set, providing additional degrees of freedom compared to classical coupled dipoles to reconstruct complex scattered fields. Once the polarizability matrix describing scattering by an individual particle is determined, the multiple scattering problem by ensembles of such particles in stratified media can be solved using a Green tensor formalism and only a few numerical dipoles, thereby with a low physical memory usage, even for dense systems in close vicinity to interfaces. The performance of the method is studied with the example of large high-aspect-ratio high-index dielectric cylinders. The method is easy to implement and may offer new possibilities for the study of complex nanostructured surfaces, which are becoming widespread in emerging photonic technologies.

Minimizing the luminescence lifetime while maintaining a high emission quantum yield is paramount in optimizing the excitation cross-section, radiative decay rate, and brightness of quantum solid-state light sources, particularly at room temperature, where non-radiative processes can dominate. In that sense, plasmon-based optical nanoantennas can feature strongly enhanced and confined optical fields to enhance excitation probabilities and fluorescence decay rates. Their morphology and their coupling to luminescent emitters can be engineered to minimize non-radiative losses and optimize their overall brightness. We demonstrate here that short DNA strands are an excellent template to introduce individual fluorescent molecules in dimers of gold nanoparticles in order to achieve single photon emission with decay rates enhanced by more than two orders of magnitude (M. P. Busson et al, Nat. Commun. 3, 962 (2012)). The coupling between single dye molecules and plasmonic gap antennas can be further optimized by selecting nanostructures where the transition dipole of the emitter is aligned with the gold particle dimer axis (M. P. Busson & S. Bidault, Nano Lett. 14, 284 (2014)). Furthermore, by using dimers of 60 and 80 nm diameter gold particles, we demonstrate the assembly of nanostructures exhibiting single-photon emission with lifetimes that can fall below 10 ps and typical quantum yields in a 45−70% range (S. Bidault et al, ACS Nano 10, 4806 (2016)). These data are in excellent agreement with theoretical calculations and demonstrate that millions of bright fluorescent nanostructures, with radiative lifetimes below 100 ps, can be produced in parallel.

It is now well established that radiative decay of quantum emitters can be strongly modified by their environment. In this paper we present an exact, within the weak-coupling approximation, multipole expression to compute the Lamb (frequency) shift induced by an arbitrary set of resonant scatterers on a nearby quantum emitter, using multi-scattering theory. We also adopt a Quasi-Normal Mode description to account for the line shape of the Lamb shift spectrum in the near-field of a plasmonic nanosphere. It is then shown that the Lamb shift resonance can be blue-shifted as the size of the nanoparticle increases, suggesting that nanoparticles may be used to tune this resonant interaction. Finally, a realistic calculation of the Lamb shift is made for a dimer configuration.

Polarizability expressions are commonly used in optics and photonics to model the light scattering by small particles. Models based on Taylor series of the scattering coefficients of the particles fail to predict the morphologic resonances hosted by dielectric particles. Here, we propose to use the factorization of the special functions appearing in the expression of the Mie scattering coefficients to derive point-like models. These models can be applied to reproduce both Mie resonances of dielectric particles and plasmonic resonances of metallic particles. They provide simple but robust tools to predict accurately the electric and magnetic Mie resonances in dielectric particles.

Point-like models are simplified expressions of the polarizability that are commonly employed to study the light scattering by small particles. These models, such as the quasistatic approximation, have played a key role in plasmonics in the understanding of the resonant light interaction with metallic nanoparticles. However, these models fail to predict morphological resonances hosted by dielectric particles. This study aims to derive point like models that can describe dipolar resonances of particles made of either positive or negative permittivity.

This work studies the limits of far- and near-field electromagnetic responses of subwavelength scatterers, like the unitary limit of lossless scatterers and the ideal absorption limit of lossy particles. These limit behaviors are described in terms of analytic formulas that approximate finite-size effects while rigorously including radiative corrections. This analysis predicts the electric and/or magnetic limit responses of both metallic and dielectric nanoparticles while quantitatively describing near-field enhancements.

Minimizing the luminescence lifetime while maintaining a high emission quantum yield is paramount in optimizing the excitation cross-section, radiative decay rate, and brightness of quantum solid-state light sources, particularly at room temperature, where nonradiative processes can dominate. We demonstrate here that DNA-templated 60 and 80 nm diameter gold nanoparticle dimers, featuring one fluorescent molecule, provide single-photon emission with lifetimes that can fall below 10 ps and typical quantum yields in a 45-70% range. Since these colloidal nanostructures are obtained as a purified aqueous suspension, fluorescence spectroscopy can be performed on both fixed and freely diffusing nanostructures to quantitatively estimate the distributions of decay rate and fluorescence intensity enhancements. These data are in excellent agreement with theoretical calculations and demonstrate that millions of bright fluorescent nanostructures, with radiative lifetimes below 100 ps, can be produced in parallel.

DNA self-assembly is a flexible and robust technique to produce hybrid nano-structures. Here, we use a short DNA double-strandto position organic dye molecules in the gap of gold nanoparticle dimers that act as antennas for light (Figure 1-a). These nanoantennascan enhance the spontaneous emission rates of dye molecules by more than two orders of magnitude [1-2]. However, the efficiency of the emitter-antenna interaction strongly depends on the size of the plasmonic particles. We have demonstrated that DNA-templated 60 and 80 nm diameter gold nanoparticle dimers,featuring one fluorescent molecule, provide single-photon emission with lifetimes that can fall below 10 ps and typicalquantum yields in a 45−70% range (Figure 1-b) [2].

Plasmonic optical antennas enhance and control the emission of quantum sources in the far-field. Interestingly, the antenna concept can also be applied to enhance the electric field produced by a quantum emitter in the near-field and increase the rate of Forster resonance energy transfer (FRET) between two nearby donor and acceptor dipole emitters. However, plasmonic antennas also influence numerous other photophysical processes such as the donor excitation intensity and decay dynamics and the acceptor emission yield, which compete with the observation of FRET. Understanding the balance between FRET and these processes and monitoring FRET under intense resonant optical confinement in plasmonic nanoantennas have remained challenging open questions. Here, we use DNA-driven self-assembly to accurately produce 40 and 60 nm gold nanoparticle dimer antennas containing a single FRET pair located in the center of a 14 nm gap. The spontaneous donor decay rate constants are increased by 2 orders of magnitude, creating high local densities of optical states (LDOS) to explore the link between LDOS and FRET. The antennas induce a 5-fold increase of Forster energy transfer rate constants associated with reduced transfer efficiencies, in good agreement with numerical simulations. The strong antenna emitter interaction leads to the surprising association of an enhanced acceptor emission with a weak transfer efficiency. Our measurements exemplify the competition between radiative and nonradiative processes in complex nanophotonic systems and highlight geometrical parameters and design rules to optimize nanoantennas for nonradiative energy harvesting.

We demonstrate that the electromagnetic fields scattered by particles made of different materials can be equalized. Emphasize is placed first in metallic nanoparticles that host localized surface plasmons and it is shown that their electromagnetic fields can be identically reproduced with dielectric particles. We derive the explicit formulas relating the different constitutive parameters that yield identical electromagnetic responses. This method provides the dielectric permittivities of spherical particles that reproduce the strong near electric field intensities observed around metallic particles featuring localized surface plasmon resonances in optics or near infrared frequencies. We also demonstrate the ability of homogenous dielectric particles to host the magnetic resonances predicted for exotic materials with negative permeability.

Plasmonic antennas offer extremely promising strategies to enhance single molecule fluorescence sensing and breach the limitations set by diffraction. However, the technical difficulty and limited availability of top-down nanofabrication techniques enabling nanometer gap sizes are limiting the impact of plasmonic antennas for biochemical and biophysical applications. Here we demonstrate the effectiveness of self-assembled nanoparticle gap antennas to enhance single molecule fluorescence detection at high concentrations. For a dimer of 80 nm gold nanoparticles with 6 nm gap, we isolate detection volumes down to 70 zL (equivalent to lambda(3)/3600) and achieve 600-fold fluorescence enhancement, microsecond transit time, and operation of fluorescence correlation spectroscopy at concentrations exceeding 10 mu M. We quantify the near-field detection volume and the fluorescence enhancement for different self-assembled nanoantenna designs using fluorescence correlation spectroscopy. The combination of the fabrication simplicity with the large fluorescence enhancement makes the self-assembled colloidal nanoparticle gap antennas optimal to extend a wide variety of single-molecule applications toward the biologically relevant micromolar concentration regime.

We demonstrate that the electromagnetic fields scattered by metallic particles hosting localized surface plasmons can be accurately reproduced by dielectric particles. We derive analytic formulas relating the permittivities of the dielectric and metallic particles that yield identical dipolar electromagnetic responses. This equivalence between dielectric and metallic particles permits the use of well-known pointlike dipolar models to predict the dipolar resonances of dielectric particles.

We present a detailed formalism allowing analytical calculations of the radiative properties of nanoantennas. This formalism does not rely on dipole approximations and utilizes multipolar multiple-scattering theory. The improvement in both accuracy and calculation speeds offered by this formulation provides significant advantages that are used in this work to study Yagi-Uda-type nanoantennas. We provide a study that questions the necessity of the reflector particle in nanoantennas.

We report the design of highly efficient optical antennas employing a judicious synthesis of metallic and dielectric materials. In the proposed scheme, a pair of metallic coupled nanoparticles permits large enhancements in both excitation strength and radiative decay rates, while a high refractive index dielectric microsphere is employed to efficiently collect light without spoiling the emitter quantum efficiency. Our simulations indicate potential fluorescence rate enhancements of 3 orders of magnitude over the entire optical frequency range.