Manipulation and Raman Spectroscopy with Optically Trapped Metal Nanoparticles Obtained by Pulsed Laser Ablation in Liquids

We investigate experimentally and theoretically optical trapping of metal nanoparticles and aggregates. In particular, we show how light forces can be used to trap individual gold nanoaggregates of controlled size and structure obtained by laser ablation synthesis in solution. Due to their surface charge, no agglomeration of isolated nanoparticles was observed during trapping experiments and reliable optical force measurements of isolated and aggregated nanoparticles was possible through an analysis of the Brownian motion in the trap. We show how the field-enhancement properties of these nanostructures enables surface-enhanced Raman spectroscopy of molecules adsorbed on aggregates optically trapped in a Raman tweezers setup. We finally discuss calculations of extinction and optical forces based on a full electromagnetic scattering theory for aggregated gold nanostructures where the occurrence of plasmon resonances at longer wavelength play a crucial role in the enhancement of the trapping forces.