Nanocrescent is a typical sharp-tip structure. By changing the geometrical parameters, the dipole peak can be red shifted to the near infrared region. The concentric and non-concentric ring/disk cavities were examples utilizing particle-particle interaction. The sharp resonance peak at the near-infared region was due to the antisymmetric coupling of the disk and the ring dipolar plasmons.For the above two kinds of structures, studies were mostly focused on the dipole resonance mode because the LFE factor is higher than that of the quadrupole one. 

Researchers at East China Normal University and Shanghai Institute of Optics and Fines Mechanics (SIOM/China) propose a metallic nanostructure consisting of a nanodisk in a nanocrescent.[APPLIED PHYSICS LETTERS 98, 163110,2011]

In the paper, they propos a metallic NCND structure.The LFE factors of the quadrupole peaks are 15-fold higher than that of the single nanocrescent, and the resonance wavelengths are tunable in the whole visible region. They studied the enhancement mechanisms of the quadrupole mode, and found that when the plasmonic modes of the nanocrescent and the nanogap matched well, the LFE factors as high as 700 could are acquired as the radius of the nanotips keep as 1 nm. The special properties make the NCND structures a good candidate for the localized surface plasmon spectroscopy and the plasmon resonance sensing.