Production of permanent optical in fused silica by propagation of Ir femtosecond laser pulses

Intense ultra-short laser pulses propagating in the atmosphere undergo important changes in their spatial and spectral characteristics they self-organise in the form of narrow filaments with high peak intensity, which persists over exceptionally long distances. This particular mode of propagation is understood as a balance between two effects. At first, we have the optical Kerr effect acting as a convergent lens leading to a self-focussing of the beam with a catastrophic collapse on axis. Near the point of collapse, the intensity becomes high enough that it starts ionising the molecules of air, liberating free electrons. This process is known as photoionisation (PI) and acts as a divergent lens. In the article we extend the concepts described above to the case of propagation through silica plates and try to connect filamentary propagation and generation of damage. Additional physical contributions to the effect will be discussed such as electron multiplication by avalanche processes, group velocity dispersion and space time focusing. Two cases will be examined: The case of weak focusing and the case of strong focusing. For both cases, numerical simulations will be compared with experimental data. In the case of weak focusing, despite drastic pulse reshaping leading to local intensities in excess of 1012 w/cm2, little or no permanent damage is observed. By contrast, in a strong focusing geometry, extensive damage is produced due to avalanche processes. (author)