Nanoscale Engineering of Optical Strong Coupling inside Metals

摘要

Optical polaritons appear when a material excitation strongly couples toan optical mode. Such strong coupling between molecular transitions andoptical cavities results in far-reaching opportunities in modifying fundamentalproperties of chemical matter. More recently an exciting prospect of self-coupledpolaritons has emerged by matter sustaining the optical mode with itsgeometry. Here, it is shown how strong coupling of the interband transitionand surface plasmons can be engineered in nickel at the nanoscale to realizeself-coupled plasmon–interband polaritons inside metals. Using electronenergy-loss spectroscopy, it is demonstrated that in nickel thin films andnanoantennas the propagation and radiation losses result in a broadening ofthe plasmon linewidth and a transition from strong to weak coupling. Further,higher-order plasmon resonances couple to the interband transition, and themultipolar-coupled states acquire the field profile of the plasmon. The resultsprovide a fundamental understanding of plasmon–interband coupling inmetals and establish the base for the design of photocatalytic and magnetoopticalnanosystems.