Observation of nanometric metallic particles with an apertureless scanning near-field optical microscope

The near field of an isolated gold metallic nanoparticle laid on a quartz substrate has been investigated using an apertureless scanning near-field optical microscope (SNOM). We performed a near-field absorption spectra of a gold nanoparticle, emphasizing an electromagnetic resonance. In this study, we were interested in the process of image formations in SNOM. In this context, we prove that the optical image contain near-field information by varying different parameters such as wavelength, amplitude oscillation of the tip, polarization of the light, size of the particles. The atomic force microscopy images recorded simultaneously with the optical images were an useful tool to characterize the particle shape and therefore to show the sphere deformation when deposited on the substrate. We used several hypothesis to explain the nanoparticle resonance position with the help of established calculations and complementary experiments. The optical response of a metallic nanoparticle is also suited in order to understand the physical phenomena involved in the surface enhanced raman scattering and more generally to improve the resolution in local spectroscopy.