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Characterization of the laser induced damage threshold of mirrors in ...

Characterization of the laser induced damage threshold of mirrors in ...

Niemz developed a model

Niemz developed a model for the optical breakdown in dielectrics that takes into account avalanche ionization, electron ion recombination and electron diffusion.[51] His model is able to reproduce FT h ∝ √ τP in a range from 4ps to 8µs and it also reproduces the deviation from that proportionality. 2.3.2 Pulse frequency dependence The dependence of the damage threshold on the pulse frequency has been investigated for ion-doped glass [9], for single dielectric layers on bulk material [6] and for actual dielectric mirrors with multiple layers.[7] It is quite striking, that no dependence for a T a2O5 single layer on a fused silica (SiO2) substrate was found by Mero et al.[6] while Bonse et al. reported a strong dependence for a T a2O5 / SiO2 dielectric high reflector.[7] The data of both experiments are shown in figure 2.9. (a) Data from Mero et al. (b) Data from Bonse et al. Figure 2.9: Damage threshold fluence as a function of pulse repetition rate a)for 100 pulses with a duration of 30 fs on a T a2O5 single layer (d = 2 · λ) on fused silica 4 substrate[6] b) 1000 pulses with a duration of 130 fs on a multilayer T a2O5 / SiO2 high reflector (each ). [7] layer, d = λ 4 Utilizing the mathematical model presented earlier, one would expect a pulse frequency dependent behavior when the temporal spacing between two pulses comes into the range of the lifetime of intermediate decay states. The resulting accumulation of electrons in states closer to the CB would most probably reduce the damage threshold as less photons are required to reach a critical electron density in the CB. Now both sample types have been manufactured using ion beam sputtering, a strong difference in the 14

lattice 8 or its deformations should not be the case. The question is if one would expect different possible decay states in a multilayer structure compared to the single layer case. As far as we know no investigations in this direction have been done yet. 2.3.3 Influence of the center wavelength Due to its wide availability, stability and ease in handling Kerr-lens mode locked Ti:Sapphire oscillators have been the main source for fs pulses almost since its discovery by Spence et al. 1991.[57] Probably for that reason the first studies on fs damage threshold were done with chirped pulse amplified 9 Ti:Sa systems.[2, 28] Until today, the short-pulse laser-induced breakdown studies are concentrated on the Ti:Sa reachable spectral range from ≈ 500 − 1100nm with an emphasis on wavelengths around 800nm. Simanovskii et al. have investigated the damage threshold for 400nm, 800nm and in the interval between 4µm and 8µm for a pulse duration of 1ps.[8] Their data is shown in figure 2.10. One can see first an increase of damage threshold with increasing wavelength, but from a certain point on a distinct decrease. Now only thinking about the wavelength dependence of the multi photon ionization (MPI) one would expect a steady increase of the damage threshold with the wavelength. Figure 2.10: Wavelength dependence of the breakdown threshold for wide band gap materials. Solid lines show general trend as guide for the eye. [8] The qualitative explanation for this lies in the avalanche ionization absorption crossection σ from eq 2.5. From the Drude model one can deduce an expression for σ [4]: 8 strictly speaking there is no real lattice, these thin layers are to large extents amorphous or in some intermediate configuration. This term is only used for simplicity here. 9 for a description of CPA see [58, 59] 15

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