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Unusual optical phenomena arise when light interacts with time-varying dielectric media. Here, relativistic propagation of a photoconductivity front in silicon is exploited to temporally stretch and time-reverse terahertz pulses.

We report on subcycle terahertz light-field emission of electrons from tungsten nanotips under extreme conditions corresponding to a Keldysh parameter γ_{K}≈10^{−4}. Local peak THz fields up to 40 GV/m are achieved at the apex of an illuminated nanotip, causing subcycle cold-field electron emission and acceleration in the quasistatic field. By simultaneous measurement of the electron bunch charge and energy distribution, we perform a quantitative test of quasistatic Fowler-Nordheim tunneling theory under field conditions that completely suppress the tunnel barrier. Very high bunch charges of ∼10^{6} electrons/pulse are observed, reaching maximum energies of 3.5 keV after acceleration in the local field. The energy distribution and emission current show good agreement with Fowler-Nordheim theory even in this extreme field regime. Application of this theory under such extreme THz field conditions predicts a single-shot electron energy distribution with a spectral purity reaching 10^{−4}. THz field-induced reshaping and sharpening of the nanotip is observed, reducing the tip radius from 120 to 35 nm over roughly 10^{9} THz shots. These results indicate THz-driven nanotips in the extreme field limit are promising electron sources for ultrafast electron diffraction and microscopy.