Absorption Modulation of Plasmon Resonant Nanoparticles in the Presence of an AFM Tip

We numerically calculated the optical absorption cross section (C_abs) of a silver nanoparticle (AgNP) and a gold nanoparticle (AuNP) in the presence of metallic (gold) and dielectric (silicon) atomic force microscope (AFM) probes, illuminated by transverse magnetic polarized, total internally reflected waves. Both 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. Fitting the numerical absorption data to a driven damped harmonic oscillator model revealed that the AFM tip modifies both the driving force (F₀), consisting of the free carrier charge and the driving field, and the overall damping of the oscillator (β). These effects can be complementary or competing, and they combine to either enhance or suppress absorption. Therefore, under an Si tip, C_abs of a AuNP is enhanced while C_abs of a AgNP is suppressed. In contrast, an Au tip suppresses the absorption cross section for both Au and Ag NPs.