Strong-field photoemitted electrons from metallic tips show carrier-envelope phase effects

Summary form only given. Sharp nanometer-sized metallic tips recently emerged as a test bed for exploring strong-field phenomena such as high-harmonic generation and photoemission [1-4]. When being illuminated with few-cycle laser pulses of sufficient field strength, optical field enhancement at the tip apex results in tunnelling of electrons out of the tip. The acceleration of these electrons within the local near-field gradient can be so strong that the typical quiver motion of the electrons in an oscillating laser field is fully suppressed [3,4]. These sub-cycle electrons traverse the near field with a decay length of only a few nm within less than one half cycle of the laser field. Hence their motion is expected to sensitively depend on the carrier-envelope phase (CEP) of few-cycle driving pulses, enabling steering and controlling of electron motion around metallic nanoparticles by CEP variation.Here, we show for the first time how the CEP of such few-cylce pulses affects the acceleration of strong-field emitted electrons in the near-field of sharp nanometer-sized gold tips. Gold tips, etched to an apex radius of down to 5 nm are irradiated with 16-fs pulses (2.6 cycles) at a wavelength of 1.65 μm from a noncollinear optical parametric amplifier (NOPA) system followed by difference frequency generation (Fig. 1a). The combination of frequency conversion stages ensures that the pulses have a highly stable CEP with residual phase fluctuations of ~66 mrad as measured in an f-to-2f interferometer over a time span of 10 min. The CEP is controlled via a pair of fused silica wedges, and the energy spectra of the emitted and accelerated electrons are measured as a function of CEP using a photo-electron spectrometer (PES).The recorded kinetic energy spectra (Fig. 1b) show a clear modulation of the spectral width with the CEP. The red and black circles indicate the highand low energy cutoff and are plotted in Fig. 1c together with fitted sinecurves. They display a- inversely phased 2π-periodicity, leading to a periodic narrowing and broadening of the spectra. The same periodicity is found in the total electron yield (Fig. 1d). The measurements agree well with simulations, tracing the marked periodic modulation of the high-energy cutoff to the variation of the maximum field amplitude with CEP and its effect on the near-field acceleration. We believe that such a field-driven control of the electron motion in the near field of solid state nanostructures can be seen as a new form of quantum electronics, paving the way towards the generation, measurement, and application of attosecond electron pulses.