30 November 2010 Imaging the early material response associated with exit surface damage in fused silica
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The processes involved at the onset of damage initiation on the surface of fused silica have been a topic of extensive discussion and thought for more than four decades. Limited experimental results have helped develop models covering specific aspects of the process. In this work we present the results of an experimental study aiming at imaging the material response from the onset of the observation of material modification during exposure to the laser pulse through the time point at which material ejection begins. The system involves damage initiation using a 355 nm pulse, 7.8 ns FWHM in duration and imaging of the affected material volume with spatial resolution on the order of 1 μm using as strobe light a 150 ps laser pulse that is appropriately timed with respect to the pump pulse. The observations reveal that the onset of material modification is associated with regions of increased absorption, i.e., formation of an electronic excitation, leading to a reduction in the probe transmission to only a few percent within a time interval of about 1 ns. This area is subsequently rapidly expanding with a speed of about 1.2 μm/ns and is accompanied by the formation and propagation of radial cracks. These cracks appear to initiate about 2 ns after the start of the expansion of the modified region. The damage sites continue to grow for about 25 ns but the mechanism of expansion after the termination of the laser pulse is via formation and propagation of lateral cracks. During this time, the affected area of the surface appears to expand forming a bulge of about 40 μm in height. The first clear observation of material cluster ejection is noted at about 50 ns delay.
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Stavros G. Demos, Rajesh N. Raman, Raluca A. Negres, "Imaging the early material response associated with exit surface damage in fused silica", Proc. SPIE 7842, Laser-Induced Damage in Optical Materials: 2010, 78420M (30 November 2010); doi: 10.1117/12.867724; https://doi.org/10.1117/12.867724

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