[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Laser-Induced Breakdown in Dielectrics: Strong Electron Superheating

摘要： Laser-induced breakdown hinders the operation of modern lasers, photonic elements and devices, and can be also employed as an important operational regime for high-intensity laser technology of new materials. In this report we generalize the two-temperature model of laser-matter interaction to simulate the regimes of laser-induced breakdown in dielectrics. This generalization allow us to associate the onset of breakdown with a sequence of the step-wise increase and decrease of the mean electron energy, (cid:2013)(cid:3032), and related evolution of the free electron density, (cid:1866)(cid:3032). The model set of the rate equations includes photo-ionization and recombination kinetics, radiation absorption, energy release and exchange effects defining the time evolution of (cid:2013)(cid:3032) to the band gap energy (cid:1831)(cid:3008), and critical energy of ionization, (cid:1831)(cid:3030)(cid:3045)(cid:124)1.5(cid:1831)(cid:3008). Additionally, our model includes an effect of a strong electron superheating, Δ(cid:2013)(cid:3032), above (cid:1831)(cid:3030)(cid:3045). Namely, in treating the onset of impact ionization by generated free electrons the related impact ionization rate, (cid:1875)(cid:3036)(cid:3040)(cid:3043) = (cid:2028)(cid:3035)(cid:2879)(cid:2869), is associated with the time of the electron heating to the critical energy, (cid:2028)(cid:3035) = (cid:1831)(cid:3030)(cid:3045)((cid:1856)(cid:2013)(cid:3032) (cid:1856)(cid:1872)? )(cid:2879)(cid:2869) ≈ 1 ((cid:2009)(cid:1835))(cid:2879)(cid:2869), where (cid:2009) = (cid:2026)(cid:3090) (cid:1831)(cid:3030)(cid:3045)? is the avalanch coefficient, (cid:2026)(cid:3090) is the related absorption cross-section of free electrons and I is the radiation intensity). However, when the neutral atoms start to deplet the impact ionization can be controlled by an additional collision time, (cid:2028)(cid:3030) = (cid:1856)(cid:3028)((cid:1865)(cid:3032) (cid:1831)(cid:3030)(cid:3045)? )(cid:3117)/(cid:3118)((cid:2869)(cid:2879)(cid:3041)(cid:3280) (cid:3041)(cid:3276)? )(cid:2870)(cid:3117)/(cid:3118) (where (cid:1856)(cid:3028) is the interatomic distance, (cid:1865)(cid:3032) is the electron mass and (cid:1866)(cid:3028) is the atomic density). That is, if (cid:1866)(cid:3032) ? (cid:1866)(cid:3028) and (cid:2028)(cid:3030) ? (cid:2028)(cid:3035) the impact ionization is controlled by (cid:2028)(cid:3035) and the ionizing impacts can occur at (cid:2013)(cid:3032) > (cid:1831)(cid:3030)(cid:3045)within the time (cid:2028)(cid:3030)(cid:124) 0.1 fs. However, when (cid:1866)(cid:3032) tends to (cid:1866)(cid:3028) by reaching the level of (cid:2013)(cid:3032) > (cid:1831)(cid:3030)(cid:3045) and seeking new neutral atoms the free electrons continue to experience the impacts with the ionized atoms and recombining electron-ion pairs. By making these interactions the free electrons can acquire an additional energy of Δ(cid:2013)(cid:3032) ≈ (cid:2028)(cid:3030) before producing the final ionizing impacts and loosing the energy of (cid:1831)(cid:3008).