Effect of laser beam propagation through the plasmonic nanoparticles suspension

Abstract

Gold nanoparticles are excellent light absorbers at the surface plasmon resonance wavelength. This absorption process generates highly energetic electrons near the surface that enhances the local electric field on the gold nanoparticles surface, and results in modification of the refractive index. The present work focuses on interaction of the laser, both near and far from the resonance wavelength, with colloidal solution of plasmonic nanoparticles. For the laser beam interaction, we first prepared the gold nanoparticles by sol gel method and characterized the absorption spectrum in terms of particle size and morphology. The absorption peak in visible range confirmed to the formation of Au nanoparticles. Therefore, a laser at 532 nm was selected to study light matter interaction with Au nanoparticles in solution. The interaction produced diffraction rings, which changed shape over time due to convection of colloidal nanoparticles. In another experiment, a 405 nm laser with the same power produced only a laser spot on the screen. Thus, the 532 nm is closer to the surface plasmon excitation and is responsible for producing a pattern of diffraction rings in the far-field region. The produced diffraction rings provide a very simple and sensitive technique to find out induced nonlinear refractive index for the plasmonic nanoparticles. Finding plasmonic resonances holds importance for variety of applications in plasmonic sensing and these rings could be used to characterize the resonances.