Experimental and theoretical progress on subpicosecond laser pulse breakdown in dielectric films is reviewed. The
single pulse threshold fluences can be related to fundamental material properties and scaling laws with respect to pulse
duration and material bandgap. Multiple pulse thresholds are controlled by native and laser-induced defects. A
phenomenological model is introduced which describes the accumulation and relaxation of such defects. The model is
able to explain the experiments and can be used to assess relevant defect parameters. Experimental results are presented
that exemplify how the ambient atmosphere affects the multiple-pulse laser damage thresholds.
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).
We report on femtosecond (fs) laser experiments yielding the time constants τrel for the non-radiative relaxation from
optically excited high energy MNa** states to the fluorescent MNa* state in CaF2 samples. The values obtained with the
third and second harmonics of the fs laser amount to τrel (262 nm) = (3.0 ± 0.3) ps and to τrel (392 nm) = (1.0 ± 0.1) ps for
the two selected MNa** states at 4.7 eV (262 nm) and 3.2 eV (392 nm) excitation energy, respectively. These time
constants were derived from depletion processes of the fluorescence at 740 nm (MNa* state) using fs laser pulses of the
NIR fundamental wavelength (785 nm) at variable delay relative to the UV fs laser pulses. In addition, photobleaching of
the MNa centers upon UV fs laser irradiation is observed and simulated by assuming a constant fraction of MNa bleaching
per pulse for a given laser fluence. This fraction ranges from 0.14% per pulse at 392 nm and 0.28mJ/cm2 to about 1% per
pulse at about 6 mJ/cm2.
The influence of Na stabilized F and M centers on the DUV absorption behavior of CaF2 is comparatively studied for
nanosecond and femtosecond laser pulses by in-situ transmission and laser induced fluorescence measurements.
For 193 nm nanosecond pulses the steady state transmission of ArF laser pulses through CaF2 is measured in dependence
on the incident fluence H ≤ 10 mJ cm-2 pulse-1. The related absorption coefficients αst(H) are proportional to H and
rationalized by effective 1- and 2-photon absorption coefficients αeff and βeff, respectively. The αeff and βeff values
increase with the Na content of the CaF2 samples as identified by the fluorescence of Na related MNa centers at 740 nm.
This relation is simulated by a complex rate equation model describing the ArF laser induced MNa generation and
annealing. MNa generation starts with intrinsic 2-photon absorption in CaF2 yielding self-trapped excitons (STE). These
pairs of F and H centers can separate upon thermal activation and the F centers combine with FNa to form MNa centers.
MNa annealing occurs by its photo dissociation into a pair of F and FNa centers.
Comparative transmission measurements with DUV femtosecond pulses are done using the fourth harmonic of a Ti:Safs-
laser at 197 nm. The resulting βeff values virtually show no dependence on the MNa center concentration. Furthermore,
the absolute βeff values are lower by a factor of three compared to those obtained for nanosecond pulses. This is
explained by additional two-step absorption for nanosecond pulses after formation of self-trapped excitons (STE).
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