Ionization clamping in ultrafast optical breakdown of transparent solids

We formulate a multiphysics model describing the nonlinear propagation of a femtosecond, near-infrared, tightly focused laser pulse in a wide-band dielectric. The application of our model to the case of bulk sapphire shows that even under extreme excitation conditions, ionization is rigidly clamped at about one tenth of the electron density in the upper valence band. The earlier estimate of approximately 10-TPa pressure that could be attainable through the internal excitation of transparent dielectrics by tightly focused ultrafast laser beams is shown to be off by 2 orders of magnitude. We outline the potential routes towards overcoming the clamping limit. The computed distribution of the absorbed electromagnetic energy, combined with the appropriate equation of state, can be further used as an input to a hydrodynamics code to simulate the dynamics of the void formation inside the bulk of the solid.