Selectivity and Switchability\nof Hot Spot Activation\nin Nanolens-like Nanosphere Heterotrimers

Near-field coupling between metal nanoparticles creates\nregions\nof strongly confined electric fields at the junctions of the particles,\nknown as hot spots. Nanoparticle assemblies with multiple hot spots\noffer opportunities for optical control. Linear trimers of self-similar\nnanoparticles with a hierarchical arrangement of monomers in terms\nof particle size along the chain, often explored as nanolens configurations,\nfocus electromagnetic energy in the vicinity of the smallest particle.\nIn contrast with the traditional nanolens configurations, the current\nstudy embarks on an exploration of the optical control of hot spots\nusing nanolens-like trimers of nanospheres with compositional asymmetry\nand with a constant gap size using finite-difference time-domain simulations.\nWe analyzed the optical control features of a plethora of dimeric\nand trimeric configurations by examining their electric field intensity\ndistribution profiles at their extinction maxima. Of the six trimeric\nsystems considered, two of them were found promising for achieving\nselective excitation of hot spots, while two others were found useful\nfor accomplishing hot spot switching. For the systems demonstrating\noptical control features, a wavelength dependence study was conducted\nby exciting them at two commonly employed laser wavelengths, namely\n457 and 633 nm. Our simulations exemplify that a balanced choice of\ncomposition and size in the proposed nanolens-like configurations\nentails them with considerable optical control of hot spots, a prediction\nthat is worth investigating in surface-enhanced spectroscopy experiments.