Nonadiabatic nano-optical electron tunneling in the transition region between multiphoton-induced emission and adiabatic tunnel emission is explored in the near-field of plasmonic nanostructures. For Keldysh γ values between ∼1.3 and ∼2.2, measured photoemission spectra show strong-field recollision driven by the nanoscale near-field. At the same time, the photoemission yield shows an intensity scaling with a constant nonlinearity, which is characteristic for multiphoton-induced emission. Our observations in this transition region were well reproduced with the numerical solution of Schrödinger’s equation, mimicking the nanoscale geometry of the field. This way, we determined the boundaries and nature of nonadiabatic tunneling photoemission, building on a key advantage of a nanoplasmonic system, namely, that high-field-driven recollision events and their signature in the photoemission spectrum can be observed more efficiently due to significant nanoplasmonic field enhancement factors.

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Experimental and numerical investigation of the transition region between the multiphoton emission and tunnel emission regimes, where nonadiabatic tunneling occurs, revealed the onset to be around Keldysh ~2, where features of both regimes are present.

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We demonstrate the nanoscale control of field enhancement of nanorods illuminated with linearly or circularly polarized femtosecond laser pulses. Simulations show that excitation and phase relations of different plasmon modes determines the achievable field enhancement.