Dielectric breakdown of solids and liquids at optical frequencies

Abstract

Recently the application of high power lasers to nuclear fusion, isotope separation, etc. necessitates clarification of the mechanism of optical breakdown as a limiting factor of laser materials. Moreover, there is a hope of investigating the intrinsic breakdown free from secondary effects such as electrode, space charge voids, etc. by using lasers. The authors^1–14 have investigated dielectric breakdown of various dielectrics at optical frequencies by using Q-switched ruby and Nd-glass lasers. Several examples of optical breakdown fields (rms) by ruby laser are shown in Table 1 together with dc impulse values²»⁴ and energies of the band gap or the absorption edge. In many dielectrics except gases the optical breakdown field coincides fairly well with the dc value and is rather insensitive to the band gap. Based on these facts, we have proposed^1–3 that impact ionization due to free electron acceleration by optical field (inverse Bremsstrahlung) is the predominant factor. This conclusion has been confirmed by several other authors.^5,6 From the microwave breakdown theory,⁷ the energy gain rate of a free electron at the optical field Eo (rms) with the angular frequency ω is given by (dε/dt)E = e²E²o/m∗(ω² + ν²), (1) where ε, e, m∗, and ν are electron average energy, charge, effective mass, and collision frequency respectively. The rate of energy gain decreases with increasing ω in the region ω >> ν. The average ionization frequency can be given by the occurrence of the TSD peak can be attributed to a relaxation process or to a thermodynamic phase transformation.