Theory of photon emission in electron tunneling to metallic particles

We consider photon emission in electron tunneling to a small metal particle. The radiative (dipole) plasmon mode of the particle can be excited resonantly by a tunneling electron either when it is in the barrier or else as a decay channel of the hot electrons injected into the particle via elastic tunneling. Analytical estimates show that for a particle with a few hundred angstrom radius the former process dominates over the latter by a factor of 10^3, and that the integrated light intensity is 10^-3 photon per tunneling electron under optimum conditions, in agreement with recent observations.