Time-resolved studies of short pulse laser-produced plasmas in silicon dioxide near breakdown threshold

Using the technique of frequency-domain interferometry, we demonstrate a new way of studying laser-induced breakdown at the surface of dielectric materials. A theoretical model based on electron production by multiphoton ionisation, inverse bremsstrahlung heating, and collisional ionisation is in quantitative agreement with both the detailed time variation of the dielectric constant and the pulse width variation of the fluence threshold. From the complex reflection coefficient measured with the two probe pulse polarisations in quadrature, we deduce the time variation of the dielectric constant of silica during breakdown.