  In this work, we investigate multiphoton and optical-field tunneling emission
from metallic surfaces with nanoscale vacuum gaps. Using time-dependent
Schrodinger equation (TDSE) simulations, we find that the properties of the
emitted photocurrent in such systems can be greatly altered by the application
of only a few-volt DC bias. We find that when coupled with expected plasmonic
enhancements within the nanometer-scale metallic gaps, the application of this
DC bias significantly reduces the threshold for the transition to
optical-field-driven tunneling from the metal surface, and could sufficiently
enhance the emitted photocurrents, to make it feasible to electronically tag fJ
ultrafast pulses at room temperature. Given the petahertz-scale instantaneous
response of the photocurrents, and the low effective capacitance of thin-film
nanoantenna devices that enables < 1 fs response time, detectors that exploit
this bias-enhanced surface emission from nanoscale vacuum gaps could prove to
be useful for communication, petahertz electronics, and ultrafast
optical-field-resolved metrology.
