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28 November 2011 The impact ionization coefficient in dielectric materials revisited
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Avalanche ionization plays a crucial role in the photoionization of dielectric materials and as such is important for optical damage. Although it has been investigated closely during the last years, there is little experimental evidence of how the impact ionization parameter changes with electron density and/or incident pulse intensity. One reason is that in most dielectric materials there are several competing ionization and relaxation processes. Here we present an UV-pump IR-probe experiment that allowed us to isolate the avalanche ionization from other major ionization processes, especially multiphoton ionization, electron tunneling, and relaxation into traps and their re-excitation. We have measured the intensity dependence of a transmitted IR pulse, propagating through a thin sample of UV-grade sapphire (α-Al2O3), after seeding electrons in the conduction band with a UV pulse. We show that the assumption of an intensity independent impact ionization factor α cannot explain the results. Application of a simple avalanche ionization model within the flux-doubling approximation requires an intensity dependent coefficient a(I) to explain the data. We also determined the two photon absorption coefficient of sapphire at 266 nm (β(2) = (2.7 ± 0.1) • 10-11cm/W) as well as the "free" electron absorption cross section for 800 nm of conduction band electrons in sapphire (δ0 = (12.5 ± 0.2) •10-18 cm2).
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C. Karras, Z. Sun, D. N. Nguyen, L. A. Emmert, and W. Rudolph "The impact ionization coefficient in dielectric materials revisited", Proc. SPIE 8190, Laser-Induced Damage in Optical Materials: 2011, 819028 (28 November 2011);

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