Plasmon ruler with gold nanorod dimers: utilizing the second-order   resonance
  The idea of utilizing the second-order plasmon resonance of the gold nanorod {\pi}-dimers for plasmon rulers is introduced. We report on a qualitatively different dependence of the plasmon resonance shift on the interparticle distance for the first- and second-order longitudinal modes, extending the working range of plasmon rulers up to the distance values of 400 nm. 
Anton T. Le, Maxim R. Shcherbakov, Natalia Dubrovina, Anatole Lupu,   and Andrey A. Fedyanin
physics.optics cond-mat.mes-hall