Nonadiabatic nano-optical tunneling of photoelectrons in plasmonic near-fields

Our work demonstrates nonadiabatic tunneling of photoelectrons in the near-field of gold nanostructures, which occurs in the transition region between the multi-photon-induced photoemission and tunneling emission regimes. Measured kinetic energy spectra at higher laser intensities indicates strong-field electron accelaration and recollision, characteristic for the tunneling emission regime. At the same time, constant scaling of the photoelectron current with the intensity has been measured, a trait of the multi-photon-induced photoemission regime. The Keldysh value of gamma approximate to 2 for the transition was determined by analyzing the measured photoemission spectra. This value is in good agreement with the results acquired by the numerical solution of the Time-Dependent Schrodinger Equation.