Nonlinear scaling of strong field-driven multielectron emission

When strong laser pulses in the near-infrared are focused on sharp needle tips, electrons can be emitted via multiphoton or optical tunneling emission. The emission yield scales nonlinearly. When emission yield and, in particular, its nonlinearity are plotted vs the laser intensity, the photo-emission regime can be identified: the nonlinearity decreases when going from the multiphoton to the optical tunneling regime. Such measurements are hampered by the fact that the transition from multiphoton to optical tunneling emission typically coincides with the transition from less than one electron emitted per laser pulse to more than one, meaning that space charge effects arise. We use a multi hit-capable detector to investigate these effects and find that the measured electron yield as a function of the intensity agrees perfectly with theoretical one-electron emission models, near field intensities up to 2.6×1013Wcm−2 (Keldysh-γ=0.7), even though up to four electrons per laser pulse are detected. In particular, we observe a slope of over six in the multiphoton emission regime (γ≈1.6) and a slope as small as 2.8 for γ≤1. Our results demonstrate that the electron emission is uncorrelated even in the tunneling regime with up to four electrons per two-cycle laser pulse and dominant contribution of two-electron events of 30%. We also discuss how these data can be used as a valuable tool to determine the emission regime in which space charge effects can still be excluded. In the future, we expect such measurements to yield insights into emission beyond one electron models including correlation effects.