Energy balance of optical breakdown in water at nanosecond to femtosecond time scales

During optical breakdown, the energy delivered to the sample is either tran smitted, reflected, scattered, or absorbed. Pathways for the division of th e absorbed energy are the evaporation of the focal volume, the plasma radia tion, and the mechanical effects such as shock wave emission and cavitation . The partition of laser energy between these channels during breakdown in water was investigated for four selected laser parameters typical for intra ocular microsurgery (6-ns pulses of 1 and 10 mJ focused at an angle of 22 d egrees, and 30-ps pulses of 50 mu J and 1 mJ focused at 14 degrees, all at 1064 nm). Scattering and reflection were found to be small compared to tran smission and absorption during optical breakdown. The ratio of the shock wa ve energy and cavitation bubble energy was approximately constant (between 1.5:1 and 2:1). These results allowed us to perform a more comprehensive st udy of the influence of pulse duration (100 fs-76 ns) and focusing angle (4 degrees-32 degrees) on the energy partition by measuring only the plasma t ransmission and the cavitation bubble energy. The bubble energy was used as an indicator for the total amount of mechanical energy. We found that the absorption at the breakdown site first decreases strongly with decreasing p ulse duration, but increases again for tau < 3 ps. The conversion of the ab sorbed energy into mechanical energy is approximate to 90% with ns pulses a t large focusing angles. It decreases both with decreasing focusing angle a nd pulse duration (to < 15% for fs pulses). The disruptive character of pla sma-mediated laser effects is therefore strongly reduced when ultrashort la ser pulses are used.