Gold nanoparticle absorption under a nanoscale tip illuminated by surface-plasmon polaritons

This research numerically calculated the optical absorption of gold nanoparticles (AuNP) in the presence of metallic (Au) and dielectric (Si) AFM probes, illuminated by a surface plasmon polaritons on an infinite gold substrate. Nanoscale probes localize and enhance the field between the apex of the tip and the particle. However, the absorption of the nanoparticle is not always enhanced; in fact, under a gold tip, the absorption is suppressed for a 50 nm diameter AuNP. After fitting the numerical absorption data with the equation of a driven damped harmonic oscillator (HO), it was found that the AFM tip modifies both the driving force (F-0), consisting of the free carrier charge (q) and the driving field (E), and the overall damping of the oscillator (beta). The enhancement or suppression of absorption with different tips can be understood in terms of competition between beta and F-0. Introducing the metallic tip increases beta and decreases F-0, resulting in reduced absorption. Introducing the dielectric tip, although it increases beta, it also increases F-0, resulting in overall absorption enhancement. Therefore, one most consider both beta and F-0 to control the absorption of nanoparticles under Surface Plasmon Polaritons.