  Given the quasi-static nature of optical-field emission and the nontrivial
dependence of the emission rate on the instantaneous electric field strength,
the CEP-sensitive component of the emitted photocurrent is highly sensitive to
the energy of the optical pulse, and should carry information about the
underlying sub-cycle dynamics of electron emission. Here we examine
CEP-sensitive photoemission from plasmonic gold nanoantennas excited with
few-cycle optical pulses of increasing energy. We observe antiresonance-like
features in the CEP-sensitive photocurrent; specifically, at a critical pulse
energy, we observe a sharp dip in the magnitude of the CEP-sensitive
photocurrent accompanied by a sudden shift of {\pi}-radians in the phase of the
photocurrent. Using a quasi-static tunneling emission model, we find that these
antiresonance-like features arise due to competition between electron emission
from neighboring optical half-cycles, and that they are highly sensitive to the
precise shape of the driving optical waveform at the surface of the emitter. As
the underlying mechanisms that produce the antiresonance-like features are a
general consequence of nonlinear, field-driven photoemission, the
antiresonance-like features could be used to probe sub-optical-cycle,
sub-femtosecond emission processes, not only from solid-state emitters, but
also from gas-phase atoms and molecules. Beyond applications in the study of
ultrafast, field-driven electron physics, an understanding of these
antiresonance-like features will be critical to the development of novel
photocathodes for future time-domain metrology and microscopy applications that
demand both attosecond temporal and nanometer spatial resolution.
