Electromagnetic coupling in near-field scattering by small homogeneous and heterogeneous nanoaggregates.
Progress in near-field optical spectroscopy research on metal nanoparticles demands a better understanding of the role played by particle-particle interactions and a deeper insight of the influence of the incident field wavelength. This is particularly true for scanning near-field optical microscopy (SNOM), where the mechanism by which some components of the evanescent illuminating field are transformed into propagating field components that carry information about the sample is at the core of the image formation and where the role played by the interactions between sample and tip remains a still open problem. In this perspective, we investigate numerically the optical behavior of small aggregates of spherical nanoparticles, taking into account the electromagnetic coupling between all particles and the apertureless tip. The tip is modeled as a sphere made of different materials characterized by appropriate dielectric functions. We find that the tip material affects both qualitatively and quantitatively the SNOM images; more important, from the analysis of the calculated scattering cross section, the resonance plasmon location of the whole (aggregate + tip) system undergoes detectable changes, if the tip is constituted of the same material of the sample, as the tip is situated in different positions. This modification of the plasmon frequencies induces a nontrivial variation of the near-field intensity as a function of the tip position and the resulting SNOM image can be distorted with respect to the actual shape of the sample. No simple arguments can be used to relate the value of the local field on the tip surface to the scattering cross section value; depending on the tip material, the comparison between these two measurements can help to clarify the role of basic interactions in the scattering mechanism.
Bruzzone
S
S
Università degli Studi di Pisa, Dipartimento di Chimica e Chimica Industriale, Via Risorgimento 35, 56100 Pisa Italy. sama@dcci.unipi.it
Malvaldi
M
M
Arrighini
G P
GP
Guidotti
C
C