Quadrupole plasmon resonance mode in nanocrescent/nanodisk structure:Local field enhancement and tunability in the visible light region

We propose a metallic nanostructure consisting of a nanodisk in a nanocrescent. At the quadrupole plasmon resonance wavelengths of the nanocrescent/nanodisk structures, the local electric field amplitudes at the crescent tips are 15 times higher than those of the single nanocrescents. In addition, the quadrupole resonance wavelengths are tunable in the visible region while the peak widths keep less than 5 nm. We study the mechanisms of the local field enhancement (LFE), and find that the coupling between the quadrupole resonance modes of the nanogap and the nanocrescent result into the high LFE factor.