Interaction of intense ultra-short laser pulses with dielectrics

The interaction of intense ultra-short laser pulses with dielectrics is studied theoretically using a one-dimensional simulation model. The curl Maxwell's equations are solved coupled to the electron continuity, momentum and energy equations. What is new and innovative in our approach is the application of the 'forest fire' method to describe the optical field ionization. The cumulative action of the applied laser field and holes in the dielectric, created during ionization events, enhances the optical field ionization rate by orders of magnitude speeding up the electron avalanche development in the early times of the ionization dynamics. For peak laser intensity of about 10(17) W m(-2) (10(13) W cm(-2)) the 'forest fire' model plays a major role in the ionization dynamics and increases the electron density by two orders of magnitude. The underlying mechanisms leading to electron avalanche in dielectrics are investigated for peak laser intensities between 10(16) and 3 x 10(18) W m(-2) and laser pulse durations between 10 fs and 1 ps. It was established that the dominant process for electron multiplication depends only on the laser fluence: forF < 4 kJ m(-2) (F < 0.4 J cm(-2)) the cumulative optical field ionization rate exceeds the collisional ionization rate, while forF > 4kJ m(-2) (F > 0.4 J cm(-2)) the collisional ionization is the dominant ionization mechanism. Since for laser pulse duration exceeding 20 fs the threshold for dielectric damage of SiO2 is at least F-thr > 15 kJ m(-2) (Fthr > 1.5 J cm(-2)) (Lenzner et al 1998 Phys. Rev. Lett. 80 4076), the primary cause for dielectric damage is an electron avalanche due to collisional ionization.