We
demonstrate that prestructured metal nanogaps can be shaped
on-chip to below 10 nm by femtosecond laser ablation. We explore the
plasmonic properties and the nonlinear photocurrent characteristics
of the formed tunnel junctions. The photocurrent can be tuned from
multiphoton absorption toward the laser-induced strong-field tunneling
regime in the nanogaps. We demonstrate that a unipolar ballistic electron
current is achieved by designing the plasmonic junctions to be asymmetric,
which allows ultrafast electronics on the nanometer scale.
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