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We study numerically the propagation of an ultrashort laser pulse (sub-100 fs) ionizing a dielectric material (e.g. fused silica). Helmholtz equation for the electric field is integrated in time, coupled to a plasma model taking into account basic processes in the conduction band like nonlinear photoionization, impact ionization, self-trapped exciton relaxation, electron collisions in the conduction band and inverse bremsstrahlung. A detailed model of the critical surface (ne = ncr) reflecting a high intensity laser pulse is crucial for understanding the excitation of the material, and thus the absorption of the radiation, potentially leading to optical breakdown. Finally, we link our results to recent experimental findings about ultrafast ablation of fused silica in order to optimize the energy deposition for micromachining processes. [ABSTRACT FROM AUTHOR]