Material response during nanosecond laser induced breakdown inside of the exit surface of fused silica

Abstract

The material response following nanosecond, UV laser induced breakdown inside of the exit surface of fused silica is investigated using multimodal time resolved microscopy. The study spans up to about 75 ns delay from the onset of material modification during the laser pulse through the observation of material ejection. A number of distinct processes were identified, including: a) the onset of optical absorption in the material arising from the buildup of an electronic excitation, b) the expansion of the hot modified region (plasma) along the surface and inside the bulk, c) the formation of radial and circumferential cracks, d) the swelling of the affected region on the surface and, e) the onset of ejection of material clusters at about 30 ns delay and its progression to a well‐defined jet by about 75 ns delay. Limited theoretical modeling is used to aid the interpretation of the data.

Interaction of laser pulse with confined plasma during exit surface nanosecond laser damage

“…This value corresponds to the axial speed of expansion of the modified region in our experiments and is only slightly lower than that observed in the experiments [4].…”

“…The high plasma pressure, causes the material to bulge and eventually break, forming an outgoing shockwave in the air and ejecting a high velocity stream of particles. These features are clearly visualized in the experimental observations [4]. Local energy deposition during laser damage by nanosecond pulses produces high temperature and pressure plasmas, which ultimately determine the extent of damage.…”

“…Recently, the processes accompanying the damage event were observed with high spatial and temporal resolution [4]. In the present paper, we relate a physical picture of the damage process to the experimental observations.…”

“…Specifically, we will discuss experiments [4], where damage was induced by 8 nsec pulses of 355 nm light with fluence ~50 J/cm 2 The calculations demonstrate almost complete absorption of laser radiation by plasma for 355 nm laser light [5]. We approximate the motion of the material and the associated pressure fronts along the axial direction (normal to the surface) as initially one-dimensional when the laser spot size is much larger then the absorption length, which is the case in our experiments.…”

Comparison of material response in fused silica and KDP following exit surface laser- induced breakdown

“…Laser damage on the surface of optical materials is associated with a cascade of material response effects that include plasma formation, expansion and confinement, high pressure and temperature of the affected material, surface swelling, gaseous material ejection, generation and propagation of shockwave and stress waves, particle ejection, mechanical damage of adjacent material including cracking and pulverization, as well as elastic and plastic deformation leading to densification [7]. These effects ultimately lead to the formation of a crater on the surface that involves modified material having a high concentration of defects [8][9][10][11][12].…”