The small size of plasmonic nanostructures compared to the wavelength of light is one of their most distinct and defining characteristics. It results in the strong compression of an incident wave to intense hot spots which have been used most remarkably for molecular sensing and nanoscale lasers. But another important direction for research is to use this ability to design miniaturized interconnects and modulators between fast, loss-less photonic components. In this situation one is looking for the smallest optical nanostructure possible while trying to mitigate losses. Here we show that despite their high absorption, conductors are still the best materials to reach the sub-wavelength regime for optical antennae when compared to polar crystals and high-index dielectrics, two classes of material which have shown a lot of potential recently for nanophotonic applications. It is demonstrated through both Mie theory and numerical calculations that the smallest possible, efficient, radiating antenna has a length L > λ res /20 in all cases (this length is typically L = λ res /2 in microwave engineering), including the redshifting mechanism induced by a background or substrate refractive index, the effect of material loss and that of shape. In addition, we show that although the assembly of individual particles can further increase the miniaturization factor, it strongly increases the size-mismatch in detriment of the overall efficiency, thus making this method unfit for radiating antennae. By identifying the relevant dimensionless properties for conductors, polar materials and high index dielectrics, we present an unified understanding of the behaviour of sub-wavelength nanostructures which are at the heart of current nanophotonic research and cast the upper achievable limits for optical antennae crucial to the development of real-life implementation.Magnetic and electric coherence in forward-and back-scattered electromagnetic waves by a single dielectric subwavelength sphere," Nature Communications 3 (2012). 25

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.

We push the fabrication limit of gold nanostructures to the exciting sub-nanometer regime, in which light-matter interactions have been anticipated to be strongly affected by the quantum nature of electrons in metals. Doing so allows us to (1) evaluate the validity of classical electrodynamics to describe plasmonic effects at this length scale and (2) witness the gradual (instead of sudden) evolution of plasmon modes when two gold nanoprisms are brought into contact. Using electron energy-loss spectroscopy and transmission electron microscope imaging, we investigated nanoprisms separated by gaps of only 0.5 nm and connected by conductive bridges as narrow as 3 nm. Good agreement of our experimental results with electromagnetic calculations and LC circuit models evidence the gradual evolution of the plasmonic resonances toward the quantum coupling regime. We demonstrate that down to the nanometer length scales investigated classical electrodynamics still holds, and a full quantum description of electrodynamics phenomena in such systems might be required only when smaller gaps of a few angstroms are considered. Our results show also the gradual onset of the charge-transfer plasmon mode and the evolution of the dipolar bright mode into a 3λ/2 mode as one literally bridges the gap between two gold nanoprisms.

On the basis of conformal transformation, a general strategy is proposed to design plasmonic nanostructures capable of an efficient harvesting of light over a broadband spectrum. The surface plasmon modes propagate toward the singularity of these structures where the group velocity vanishes and energy accumulates. A considerable field enhancement and confinement is thus expected. Radiation losses are also investigated when the structure dimension becomes comparable to the wavelength.

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