Octupole plasmon resonance improves light enhancement by a metal nanodimer.

Metal nanoparticles are extensively used in science and technology to resonantly confine and enhance optical fields. Highest enhancement factors are achieved in nanosized gaps of metal dimers. It is commonly assumed that higher-order plasmon resonances, such as electric quadrupole and octupole, are in nanoparticles much weaker than a dipole resonance. Indeed, in the classical multipole expansion that deals with the scattered fields, these "dark" multipoles can be invisible. In this work, we show that an octupole resonance in a metal nanodimer can lead to a substantially larger field enhancement than a dipole resonance. The effect is explained by the fact that the near-field enhancement provided by the excited electric currents can be strong when the excitation is dark. This finding extends the design principles of a plasmonic nanostructure toward higher-order multipoles that, being naturally narrowband, can be useful for a variety of applications, especially in plasmonic sensing and detection.