Optical Properties of Symmetry-breaking Tetrahedral Nanoparticle

Spectrally rich but geometrically simple plasmonic metallic nanoparticles are highly favored in nanophotonics. Yet, they remain elusive owing to the symmetry-induced mode degeneracy and interband transitions-induced plasmonic damping. Hence, most nanoparticles display a single major extinction peak originating from the lowest-order dipole resonance. In this study, we uncover that even a simple tetrahedral nanoparticle supports rich spectral features due to symmetry breaking. This discovery runs counter to the reported gold tetrahedral nanoparticles where only a single extinction peak was observed. We find, in the case of a tetrahedral nanoparticle, the plasmonic quadrupole vertex mode becomes a bright mode and hybridizes with the dipole vertex mode, which splits the extinction peak and contributes to the spectral diversity and tunability. The peak splitting is also found to be sensitively dependent on the roundness of vertices and edges. Furthermore, the tetrahedral depolarization factors are determined using the previously generalized absorption coefficient. We envision that this work will not only help fill the gap in understanding optical properties enriched by symmetry breaking but also guide superior probe design by combining spectral tunability with geometric simplicity of the nanoparticle.