  Optical polaritons appear when a material excitation strongly couples to the
optical mode. Such strong coupling between molecular transitions and optical
cavities results in far-reaching opportunities in modifying fundamental
properties of chemical matter. More recently an exciting prospect of
cavity-free polaritons has emerged by matter sustaining the optical mode with
its geometry. Here we show how strong coupling of the interband transition and
surface plasmons can be engineered in nickel at the nanoscale to realize
cavity-free optical polaritons inside metals. Using electron energy-loss
spectroscopy, we demonstrate that in thin films and nanoantennas the
propagation and radiation losses result in a broadening of the plasmon
linewidth and a transition from strong to weak coupling. Further, higher-order
plasmon resonances couple to the interband transition, and the multipolar
coupled states acquire the field profile of the plasmon. Our results provide a
fundamental understanding of plasmon-interband coupling in metals and establish
the base for the design of unforeseen photocatalytic and magneto-optical
nanosystems.
