Plasmonic resonant modes in highly symmetric multi-branches sea-urchin like nanostructures

The electrostatic charge distribution model and the finite-difference time-domain method were used to provide a physically intuitive interpretation of how the shape and the number of branches affect the localized surface plasmon resonances of 3D sea-urchin like nanoparticles. We consider both planner and spherical structures with pins of cylindrical, triangular and hexagonal shape. Better alignment between surface charges of opposite sign leads to higher resonant energy states. More pin numbers provide more surface distribution area to spread dipole charges which leads to worse charges alignment and redshift of dipole mode. However, as the pin number increases, the quadrupole surface charge was pushed toward the pin tip due to Coulomb repulsion. Better charge alignment caused the quadrupole blueshift. Other effects such as convex and concave shape and dielectric materials are also discussed. Our finding may provide a designing rule to synthesize multi-branch nanoparticles for bio-chemical sensing applications.