The plasmonic modes of a nano-antenna formed by a nanoparticle/thin film hybrid system are investigated. Plasmonic
nano-antennas are well-known for their capabilities to concentrate electromagnetic wave into extreme small region and
couple the emission from active materials in proximity to the antennas into far-field region. Previously, we have shown
through direct measurement of emission profile images that the nano-antennas not only enhance Raman emission but
also systematically direct inelastic emission to the far-field through the dipole mode. We also showed that high order
modes of the hybrid structure can be detected. Here, the higher order plasmonic modes are characterized through
imaging, variable angle linearly polarized excitation, and simulation. Through spectral simulation with improved
resolution, two distinct modes are found to contribute to the broad band high order mode. One has dipole-like behavior
and the other has quadrupole-like behavior. The modes are characterized both through near-field distribution and farfield
scattering profiles. The quadrupole-like mode can be excited by both p- and s-polarized light whereas the dipolelike
mode is only excited by p-polarized light. These high order modes are not as bright as the dipole mode in the farfield
spectrum but have substantial near field enhancement which can be utilized for surface-enhancing spectroscopy and
sensing. In addition, characterization of high order modes may serve to clarify the interaction between nano-antenna and
active materials and will lead to design rules for applications of active plasmonic structures in integrated optical circuits.
Optical metamaterials characterized by several unique properties are introduced and characterized. The structures
comprise continuous metal films sandwiched between dense periodic arrays of optically thin metal strips or patches
separated by a small distance. The structures' electromagnetic properties are described by means of a modification of the
cavity model typically used to characterized microwave patch antennas. It is shown that the presented structures can
operate as negative index metamaterials that comprise deeply subwavelength periodic unit cells, are tunable for
operation in the near-infrared and visible spectra, and can be manufactured using standard methods and materials. In
addition, the presented structures can operate as an optical filter that, due to the presence of several resonances, transmits
fields for certain (controllable) wavelength bands, which are (nearly) independent of the angle of incidence and
polarization. The presented structures also can support arbitrary polarized surface waves that can have a high
wavenumber and can exhibit unusual dispersion relations.
In this paper, we introduce an analytic effective medium theory of plasmonic metamaterials founded on electrostatic
eigenfunctions of plasmon states. The emphasis is on the sub-wavelength particles and metamaterials
with unit cell much smaller than the optical wavelength. The theory covers plasmonic structures with arbitrary
degree of symmetry: from completely asymmetric (including chiral) structures to fully isotropic ones. We also
review several previously reported theoretical techniques used for calculating the effective parameters of plasmonic
metamaterials in connection with our new theory. Several examples of negative permittivity and negative
permeability plasmonic metamaterials are used to illustrate the theory.
A new surface-enhanced coherent anti-Stokes Raman scattering (CARS) diagnostic of chiral molecules using one-dimensional arrays of metallic nanocylinders is reported. It is found that such structures can be made biresonant, with one resonance arising from guided resonance (GR) of the periodic structure and the another--from the surface plasmon resonance (SPR). Enormous enhancements of the CARS signal can be expected when the pump laser beam is tuned to GR and the emitted anti-Stokes signal is tuned to SPR. Peak field enhancement corresponding to GR is systematically studied for metallic and dielectric nanorods as a function of the incidence angle, material losses, and nanorod diameter. Using coupled dipole approximation (CDA), we provide analytic estimates of the maximum local field enhancements and resonance widths, and find optimum parameters for the field amplification in our essentially two-dimensional geometry. It is shown that the maximum field enhancement at GR is always limited by resistive losses, which can never be completely cancelled by the far-field dipolar interaction. Full electromagnetic simulations supplement CDA-based calculations and qualitatively confirm their findings. Two types of CARS applications are envisioned: one is based on simultaneous enhancement of both two pump waves and of the emission of the anti-Stokes signal, and another one is designed specifically for detection of chiral molecules in crossed fields with very low background noise.