Near Field Scanning Optical Imaging of Gold Nanoparticles in the Sub-Wavelength Limit

TL;DR

This paper demonstrates near-field scanning optical microscopy (NSOM) imaging of gold nanoparticles smaller than the wavelength of visible light, revealing sub-wavelength details through evanescent wave interactions and surface plasmon resonance effects.

Contribution

It presents a novel application of NSOM to image sub-wavelength gold nanoparticles, highlighting the role of evanescent waves and surface plasmon resonance in achieving super-resolution.

Findings

NSOM can resolve gold nanoparticles smaller than the wavelength of light.

Evanescent waves facilitate imaging beyond the diffraction limit.

Surface plasmon resonance influences image contrast.

Abstract

The near-field scanning optical microscopic (NSOM) imaging of Au nanoparticles with size in the sub-wavelength limit (<wavelength/2N.A.) is reported. The NSOM imaging technique can resolve the objects which is beyond the scope of optical microscope using visible light (wavelength=500 nm) with objectives having a numerical aperture (N.A.) close to unity. The role of evanescent waves which is an exponentially decaying field with higher momenta i.e., lower wavelengths compared to that of normal light, in the metal dielectric interface is realized for imaging of noble metal nanostructures with sub-wavelength dimension in the near field. However, the confined light with components of evanescent waves, emanating from the NSOM probe, interacts with the oscillating dipoles present in the sub-diffraction limited nanostructures and produce propagating waves, which can be recorded by the far field detector. The light-matter interactions of Au nanoparticles of diameters in the range of 10-150 nm probed by the NSOM technique with a visible excitation of 532 nm are reported. The strong surface plasmon resonance (SPR) related absorption of Au nanoparticles is envisaged for explaining the contrast variations in the recorded NSOM images.