In this presentation, we will discuss the emission mechanism and ultrafast electron dynamics of laser-induced electron emission from a tungsten tip for weak and strong optical fields based on measured electron energy spectra and simulations.

Illuminating a sharp metallic tip with femtosecond laser pulses produces spatially and temporally confined electron pulses by plasmonic effects at the tip apex [1], and created coherent pulsed field emission with spatio-temporal control with femtosecond and nanometer resolution [2, 3]. The emission mechanism depends on the strength of the laser field. For relatively weak fields, single-electron excitations by single- and multi-photon absorption are prevalent, and photo-excited electrons are either tunneling through the surface potential barrier or being emitted above the barrier [4]. On the other hand, very strong fields largely modify the surface barrier and drive direct tunneling emission from the Fermi energy through the barrier, what is termed optical field emission [5, 6].

Here, we have investigated electron energy spectra of the electron emission induced by 7 fs laser pulses from a clean tungsten tip apex. By measuring energy spectra with varying laser power, a smooth transition of emission mechanisms from the weak-field regime to the strong-field regime was observed. In strong laser fields, we confirmed the appearance of optical field emission. It is characterized by the opening of a peculiar emission channel. This channel involves prompt laser-driven tunneling emission and subsequent laser-driven electron re-scattering off the surface, delayed by the electrons traveling far inside the metal before scattering [6].
 

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3. H. Yanagisawa, et al. Sci. Rep. 7, 12661 (2017).
4. H. Yanagisawa, et al. Phys. Rev. Lett. 107, 087601 (2011).
5. M. Kruger, et. al., Nature 475, 78 (2011).
6. H. Yanagisawa, et al. Sci. Rep. 6, 35877 (2016).