Ultrafast Multipolar Plasmon for Unidirectional Optical Switching in a Hemisphere‐Nanoshell Array

This study reports ultrafast multipolar plasmons in a large‐area periodic array of hemisphere gold nanoshells. A dipolar plasmon is observed as a resonance mode at longer wavelengths, where the free electrons are driven to oscillate through the nanoshells and the corresponding resonance spectrum is determined by the size of the hemisphere nanoshells. However, a hexapolar plasmon is observed to resonate at shorter wavelengths, where the free electrons are driven to oscillate in the inner shell and in the surrounding gold film and this inner‐shell “bright” dipole induces charge redistribution in the surrounding gold film, leading to the formation of a quadrupolar “dark” plasmon. Different physical mechanisms of these two resonance modes lead to different interactions between plasmonic electrons and photons, resulting in different lifetimes of the plasmonic dynamics. Both resonance modes can be exploited to achieve ultrafast optical switching devices. Large‐area structures with simple fabrication techniques enable potentially practical applications.