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Metals support surface plasmons at optical wavelengths and have the ability to localize light to subwavelength regions. The field enhancements that occur in these regions set the ultimate limitations on a wide range of nonlinear and quantum optical phenomena. We found that the dominant limiting factor is not the resistive loss of the metal, but rather the intrinsic nonlocality of its dielectric response. A semiclassical model of the electronic response of a metal places strict bounds on the ultimate field enhancement. To demonstrate the accuracy of this model, we studied 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.

A novel active negative index metamaterial that derives its gain from an electron beam is intro- duced. The metamaterial consists of a stack of equidistant parallel metal plates perforated by a periodic array of holes shaped as complementary split-ring resonators. It is shown that this structure supports a negative-index transverse magnetic electromagnetic mode that can resonantly interact with a relativistic electron beam. Such metamaterial can be used as a coherent radiation source or a particle accelerator.

We experimentally and theoretically study electromagnetic properties of optically thin silicon carbide (SiC) membranes perforated by an array of sub-wavelength holes. Giant absorption and transmission is found using Fourier Transformed Infrared (FTIR) microscopy and explained by introducing a frequency-dependent effective permittivity \\(\\epsilon_{\\rm eff}(\\omega)\\) of the perforated film. The value of \\(\\epsilon_{\\rm eff}(\\omega)\\) is determined by the excitation of two distinct types of hole resonances: a delocalized slow surface polariton (SSP) whose frequency is largely determined by the array period, and a localized surface polariton (LSP) which corresponds to the resonances of an isolated hole. Only SSPs are shown to modify \\(\\epsilon_{\\rm eff}(\\omega)\\) strongly enough to cause giant transmission and absorption.