Coherent Field Emission by Superfocused Plasmons*

Field emission takes place upon focusing propagating surface plasmon polaritons (SPP) at the apex of a sharp metal tip. The effect is demonstrated with remotely launched SPPs on a silver probe tip. We couple femtosecond laser pulses through a grating inscribed on the taper of a smooth, silver wire 30um from the apex. Field-emitted current is directly correlated with the radiation of the super-focused SPP at the apex. Both current and radiation at the apex are measured as a function of incident polarization on the grating. In the absence of incident light at the apex, the local field of the ``naked'' surface plasmon modulates the tunneling barrier that drives the field emission. We give a detailed analysis of the governing dynamics in the presence and absence of an applied extractor field, and clearly distinguish contributions to tunneling current from Fermi electrons, athermal electron-hole pair distribution created by Landau damping of the plasmon, and the thermalized electrons. Independent of the distribution of the electrons in the metal half-space the emission acquires coherence by the time-dependent field of the plasmon in the vacuum half-space.