Selectivity and Switchability of Hot Spot Activation in Nanolens-like Nanosphere Heterotrimers

Near-field coupling between metal nanoparticles creates regions of strongly confined electric fields at the junctions of the particles, known as hot spots. Nanoparticle assemblies with multiple hot spots offer opportunities for optical control. Linear trimers of self-similar nanoparticles with a hierarchical arrangement of monomers in terms of particle size along the chain, often explored as nanolens configurations, focus electromagnetic energy in the vicinity of the smallest particle. In contrast with the traditional nanolens configurations, the current study embarks on an exploration of the optical control of hot spots using nanolens-like trimers of nanospheres with compositional asymmetry and with a constant gap size using finite-difference time-domain simulations. We analyzed the optical control features of a plethora of dimeric and trimeric configurations by examining their electric field intensity distribution profiles at their extinction maxima. Of the six trimeric systems considered, two of them were found promising for achieving selective excitation of hot spots, while two others were found useful for accomplishing hot spot switching. For the systems demonstrating optical control features, a wavelength dependence study was conducted by exciting them at two commonly employed laser wavelengths, namely 457 and 633 nm. Our simulations exemplify that a balanced choice of composition and size in the proposed nanolens-like configurations entails them with considerable optical control of hot spots, a prediction that is worth investigating in surface-enhanced spectroscopy experiments.