We have fabricated nanometer-scale gold dipole antennas designed to be resonant at optical frequencies. On resonance, strong field enhancement in the antenna feed gap leads to white-light supercontinuum generation. The antenna length at resonance is considerably shorter than one-half the wavelength of the incident light. This is in contradiction to classical antenna theory but in qualitative accordance with computer simulations that take into account the finite metallic conductivity at optical frequencies. Because optical antennas link propagating radiation and confined/enhanced optical fields, they should find applications in optical characterization, manipulation of nanostructures, and optical information processing.

In this review we describe fundamentals of scanning near-field optical microscopy with aperture probes. After the discussion of instrumentation and probe fabrication, aspects of light propagation in metal-coated, tapered optical fibers are considered. This includes transmission properties and field distributions in the vicinity of subwavelength apertures. Furthermore, the near-field optical image formation mechanism is analyzed with special emphasis on potential sources of artifacts. To underline the prospects of the technique, selected applications including amplitude and phase contrast imaging, fluorescence imaging, and Raman spectroscopy, as well as near-field optical desorption, are presented. These examples demonstrate that scanning near-field optical microscopy is no longer an exotic method but has matured into a valuable tool.

We investigate numerically the spectrum of plasmon resonances for metallic nanowires with a nonregular cross section, in the $20–50\hspace{0.5em}\mathrm{nm}$ range. We first consider the resonance spectra corresponding to nanowires whose cross sections form different simplexes. The number of resonances strongly increases when the section symmetry decreases: A cylindrical wire exhibits one resonance, whereas we observe more than five distinct resonances for a triangular particle. The spectral range covered by these different resonances becomes very large, giving to the particle-specific distinct colors. At the resonance, dramatic field enhancement is observed at the vicinity of nonregular particles, where the field amplitude can reach several hundred times that of the illumination field. This near-field enhancement corresponds to surface-enhanced Raman scattering (SERS) enhancement locally in excess of ${10}^{12}.$ The distance dependence of this enhancement is investigated and we show that it depends on the plasmon resonance excited in the particle, i.e., on the illumination wavelength. The average Raman enhancement for molecules distributed on the entire particle surface is also computed and discussed in the context of experiments in which large numbers of molecules are used.

We study experimentally and numerically the electromagnetic resonances in split ring resonators (SRRs), around 1 GHz. For an individual SRR, we show that both electric and magnetic fields can induce resonances, the magnetic one being the strongest. The utilization of such resonant structures as efficient microwave filter is also demonstrated. The coupling between two or more SRRs can be quite complex and strongly depends on their geometrical arrangement. For small separation distances, very strong coupling, leading to sharp resonances with high quality factors are observed. In that case a magnetic field circulation which connects neighboring elements is established. The practical implications of these results for the fabrication of a left-handed metamaterial are discussed.

The optical properties of plasmonic dipole and bowtie nanoantennas are investigated in detail using the Green’s tensor technique. The influence of the geometrical parameters (antenna length, gap dimension and bow angle) on the antenna field enhancement and spectral response is discussed. Dipole and bowtie antennas confine the field in a volume well below the diffraction limit, defined by the gap dimensions. The dipole antenna produces a stronger field enhancement than the bowtie antenna for all investigated antenna geometries. This enhancement can reach three orders of magnitude for the smallest examined gap. Whereas the dipole antenna is monomode in the considered spectral range, the bowtie antenna exhibits multiple resonances. Furthermore, the sensitivity of the antennas to index changes of the environment and of the substrate is investigated in detail for biosensing applications; the bowtie antennas show slightly higher sensitivity than the dipole antenna.