The active control of matter by strong electromagnetic fields is of growing importance, with applications all across the optical spectrum from the extreme-ultraviolet to the far-infrared. In recent years, phase-stable terahertz fields have shown tremendous potential for observing and manipulating elementary excitations in solids 1-3. In the gas phase, on the other hand, driving free charges with terahertz transients provides insight into ultrafast ionization dynamics 4,5. Developing such approaches for locally enhanced terahertz fields in nanostructures will create new means to govern electron currents on the nanoscale. Here, we use single-cycle terahertz transients to demonstrate extensive control over nanotip photoelectron emission. The terahertz near-field is shown to either enhance or suppress photocurrents, with the tip acting as an ultrafast rectifying diode 6. We record phase-resolved sub-cycle dynamics and find spectral compression and expansion arising from electron propagation within the terahertz near-field. These interactions produce rich spectro-temporal features and o er unprecedented control over ultrashort free electron pulses for imaging and di raction. Controlling electric charges with external fields is at the heart of modern information technology, with ultimate bandwidths limited by switching speeds in nanoscopic devices. The term light-wave electronics illustrates the anticipated application of optically fielddriven processes to solids, starting from schemes initially developed for atoms and molecules 7,8. In the terahertz range, strong tabletop sources have opened up the field of nonlinear terahertz optics and are enabling comprehensive control over electronic or structural dynamics, for example, in the manipulation of spin waves, the triggering of phase transitions, and the implementation of terahertz-driven scanning tunnelling microscopy 1-3,9-11. Completely new degrees of freedom are added by employing the localization of optical fields within nanostructures 12-15. Specifically, at metallic nanotips, photoelectron emission 16-18 with characteristic strongfield features is observed 19-21 , including carrier-envelope-phase sensitivity 22,23 , and sub-cycle electron acceleration at mid-infrared frequencies 12. A phase-resolved sampling of such processes may be achieved by so-called streaking spectroscopy, a method commonly applied in attosecond science, in which transient fields translate temporal information, for example, instances of ionization, into photoelectron energy or other degrees of freedom 24-26. The application of streaking spectroscopy to metallic nanostructures has been theoretically studied in-depth, aiming primarily at the full temporal characterization of near-infrared plasmonic fields 27-31. However, the prospects of transferring these concepts to the terahertz domain have not yet been investigated. Here, we show that the enhancement of terahertz fields in nanostructures allows for far-reaching electron trajectory control, spanning from phase-resolved streaking governed by...

We demonstrate ultrafast terahertz (THz) field emission from a tungsten nanotip enabled by local field enhancement. Characteristic electron spectra which result from acceleration in the THz near-field are found. Employing a dual frequency pump-probe scheme, we temporally resolve different nonlinear photoemission processes induced by coupling near-infrared (NIR) and THz pulses. In the order of increasing THz field strength, we observe THz streaking, THz-induced barrier reduction (Schottky effect) and THz field emission. At intense NIR-excitation, the THz field emission is used as an ultrashort, local probe of hot electron dynamics in the apex. A first application of this scheme indicates a decreased carrier cooling rate in the confined tip geometry. Summarizing the results at various excitation conditions, we present a comprehensive picture of the distinct regimes in ultrafast photoemission in the near-and far-infrared.

We investigate photocurrents driven by femtosecond laser excitation of a (sub)-nanometer tunnel junction in an ultrahigh vacuum low-temperature scanning tunneling microscope (STM). The optically driven charge transfer is revealed by tip retraction curves showing a current contribution for exceptionally large tip-sample distances, evidencing a strongly reduced effective barrier height for photoexcited electrons at higher energies. Our measurements demonstrate that the magnitude of the photo-induced electron transport can be controlled by the laser power as well as the applied bias voltage. In contrast, the decay constant of the photocurrent is only weakly affected by these parameters. Stable STM operation with photoelectrons is demonstrated by acquiring constant current topographies. An effective non-equilibrium electron distribution as a consequence of multiphoton absorption is deduced by the analysis of the photocurrent using a one-dimensional potential barrier model.

We present a comprehensive experimental and numerical study of photoelectron streaking at metallic nanotips using single-cycle Terahertz (THz) transients and a static bias voltage as an external control parameter. Analyzing bias voltage dependent streaking spectrograms, we explore the THz-induced reshaping of photoelectron energy spectra, governed by the superimposed static field. Numerical simulations are employed to determine the local field strengths and spatial decay lengths of the field contributions, demonstrating electron trajectory control and the manipulation of the phase space distributions in confined fields with both dynamic and static components.

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