Limit possible electric field in plasmon nanogap resonator

We propose the theory of the plasmon excited in ultra-narrow plasmonic gap formed by a metal cylinder on a metal surface, that generates a huge resonant local electromagnetic field in optical frequencies. Resonance conditions are found. The maximum possible enhancement of the electric field in the nanogap is estimated as $\left|E_{max} \right|/ \left| E_{0} \right| \propto \left| \varepsilon_m \right|^{3} \varepsilon_{d}^{-1} \left( \Im \varepsilon_{m}\right) ^{-2}$ A simple ultranarrow-gaped resonator can be used for SERS sensing and infrared spectroscopy of adsorbed molecules. We assume that a plasmonic nanogap is simplest and most straightforward way to increase flexibility as well as sensitivity of the plasmon-enhanced spectroscopes.