Optical tracing of Q-switched mid-IR laser induced bubble formation

Q-switched TEM₀₀ thulium laser (λ = 2.01 μm, 300 ns FWHM) induced bubble dynamics at a silica-water interface were investigated. In particular, bubble dynamics far above the Threshold Radiant Exposure (TRE) for bubble formation were studied. At this, different silica surfaces (all of standard optical quality) were used, at which particular TREs were measured. Irradiation parameters like pulse duration or beam diameter were kept constant. As reported recently, every silica surface provides different nucleation sites for bubble formation and thus shows a specific TRE. By means of a sensitive optical detection device bubble lifetimes at the interface were monitored as a function of radiant exposure. Measurements were performed with maximum radiant exposures of 330 mJ/mm² on four different silica surfaces. The TREs of the surfaces were preliminarily determined to lie between 38 mJ/mm² and 95 mJ/mm². Above 150 mJ/mm², bubble lifetimes were measured to be independent of the surface used and its particular TRE. An explanation for this behavior was found by fast flash photographs of the onset of bubble formation, revealing vanishing differences in the heterogeneous nucleation with increasing radiant exposure.