Electron photoemission is a physical phenomenon related to the interaction between light and matter. When an electromagnetic wave impinges on a material, it can transfer its energy to one of the electrons bound within the target. Considering light as a set of discrete energy packets called photons, if more than one photon is involved in the energy transfer process, the electron photoemission is said to be non-linear. In the non-linear regime, if the optical field is strong enough to bend the energy barrier between the material and the vacuum and to let the electron tunnel out, it is possible to generate sub-optical-cycle current pulses that are synchronized with the optical field waveform.