The interaction between an ultrashort laser pulse and a sub-wavelength silicon tip under a high static electric field is investigated numerically and experimentally. Using an original autocorrelation setup of the laser-assisted atom probe tomography, the temporal evolution of the lattice temperature at the tip apex is experimentally monitored. An ultrafast cooling process, related to a confinement of the heating at the surface, is reported. This confinement is well predicted by a new model taking into account the free charges generation by photon absorption, their drift-diffusion motion under the electric field and their energy relaxation to the lattice.