The spectral properties of two spherical metallic nanoparticles of 80 nm in diameter are examined with regard to the interparticle distance and relative polarization of the excitation light. One Au nanoparticle is attached to a scanning fiber probe and the second to a scanning substrate. This configuration allows three-dimensional and arbitrary manipulation of both distance and relative orientation with respect to the incident light polarization. As supported by numerical simulations, a periodic modulation of the coupled plasmon resonance is observed for separations smaller than 1.5 microm. This interparticle coupling affects the scattering cross section in terms of spectral position and spectral width as well as the integral intensity of the Mie-scattered light.

The distance- and polarization-dependent near-field enhancement of two coupling metal nanoparticles (MNPs) is analyzed by means of the novel scanning particle enhanced Raman spectroscopy (SPRM) technique. In contrast to single MNP Raman experiments, the near-field coupling between two dissimilar MNPs as followed here leads to a Raman hot spot yielding an extra enhancement factor of 17.6 and 20, as proven here both in experiment and in theory. Three-dimensional electric field calculations for our two-particle arrangements were performed using the semianalytical multiple-multipole method. An excellent agreement is found to our experiments, in which we inspect the interaction between a "scanning" 30 nm gold MNP (Au30) and a "fixed" 80 nm Au MNP (Au80). The Au80 MNP is attached to the apex of an optical fiber manipulator and exposed to the Gaussian focus of a high NA = 1.45 objective at lambda = 532 nm. A monolayer of 1-octanethiol molecules covering the Au80 MNP serves as the electric field prober when scanning the Au30 MNP through the optical focus. This constellation allows recording the Raman signatures from a very low number of well-confined molecules. Moreover, also the spectral and spatial dependence could be explored with a superb sensitivity and very low integration time.

Using scattering-type near-field infrared microscopy in combination with a free-electron laser, intersublevel transitions in buried single InAs quantum dots are investigated. The experiments are performed at room temperature on doped self-assembled quantum dots capped with a 70 nm GaAs layer. Clear near-field contrast of single dots is observed when the photon energy of the incident beam matches intersublevel transition energies, namely the p-d and s-d transition of conduction band electrons confined in the dots. The observed room-temperature line width of 5-8 meV of these resonances in the mid-infrared range is significantly below the inhomogeneously broadened spectral lines of quantum dot ensembles. The experiment highlights the strength of near-field microspectroscopy by demonstrating signals from bound-to-bound transitions of single electrons in a probe volume of the order of (100 nm)(3).

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