Two Particle Enhanced Nano Raman Microscopy and Spectroscopy

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 λ = 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.