UV nanosecond laser optical breakdown and filamentation in bulk water

This research observes and studies nanosecond laser filaments in water through cavitation bubbles. The formation of bubbles would represent an area with significant energy intensity as filamentation. At the same time, the appearance of bubbles affects the travel of the laser beam. The 266 nm laser is focused below the surface with an 80 mm focal length. The cavitation bubbles are captured using a CCD camera. The focal point is shifted from surface to underwater in the experiment with different average energy per pulse. Filamentation can be produced using a low repetition rate by placing the focal point more than 2.5 mm below the surface. The cavitation bubbles diameter in the laser ablation is proportional to the intensity of the applied energy, where the higher laser energy intensity has a larger bubbles diameter. At the same time, the more significant energy intensity the bubbles nucleus initiates near the surface. An optical breakdown can be seen with bubbles created near the surface and the speed moving far away from the surface. The observations show that the filament morphology in water is the same as that in air.