At the surfaces of nanostructures, enhanced electric fields can drive optical-field photoemission and thereby generate and control electrical currents at frequencies exceeding 100 THz (refs. 1,2,3,4,5,6,7,8,9,10,11). A hallmark of such optical-field photoemission is the sensitivity of the total emitted current to the carrier-envelope phase (CEP)1,2,3,7,11,12,13,14,15,16,17. Here, we examine CEP-sensitive photoemission from plasmonic gold nanoantennas excited with few-cycle optical pulses. At a critical pulse energy, which we call a vanishing point, we observe a pronounced dip in the magnitude of the CEP-sensitive photocurrent accompanied by a sudden shift of π radians in the photocurrent phase. Analysis shows that this vanishing behaviour arises due to competition between sub-optical-cycle electron emission events from neighbouring optical half-cycles and that both the dip and phase shift are highly sensitive to the precise shape of the driving optical waveform at the surface of the emitter. As the mechanisms underlying the dip and phase shift are a general consequence of nonlinear, field-driven photoemission, they may be used to probe sub-optical-cycle emission processes from solid-state emitters, atoms and molecules. Improved understanding of these CEP-sensitive photocurrent features will be critical to the development of optical-field-driven photocathodes for time-domain metrology and microscopy applications demanding attosecond temporal and nanometre spatial resolution.
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The data that support the plots within this paper and other findings of this study are included in this article and its Supplementary Information and are available from the corresponding authors upon reasonable request.
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We thank J. Daley for assistance with device fabrication. This work was supported by the US Air Force Office of Scientific Research (AFOSR) through grants FA9550-12-1-0499 and FA9550-19-1-0065, the Center for Free-Electron Laser Science at DESY, the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007-2013) through the Synergy Grant ‘Frontiers in Attosecond X-ray Science: Imaging and Spectroscopy’ (AXSIS) (609920) and The Hamburg Center for Ultrafast Imaging: Structure, Dynamics and Control of Matter at the Atomic Scale, an excellence cluster of the Deutsche Forschungsgemeinschaft. R.G.H. acknowledges support for the device fabrication work from the Center for Excitonics, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under award DE-SC0001088, and support provided by the Royal Society–Science Foundation Ireland University Research Fellowship.
The authors declare no competing interests.
Peer review information: Nature Physics thanks Matthias Kling, Christoph Lienau, Johannes Schötz and the other, anonymous, reviewer(s) for their contribution to the peer review of this work
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Supplementary Figs. 1–9, Table 1, refs. 1–12 and additional discussion (Sections I–IX).