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

During optical breakdown, the energy delivered to the sample is either transmitted, reflected, scattered, or absorbed. Pathways for the division of the absorbed energy are the evaporation of the focal volume, the plasma radiation, 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 intraocular microsurgery (6-ns pulses of 1 and 10 mJ focused at an angle of 22°, and 30-ps pulses of 50 μJ and 1 mJ focused at 14°, all at 1064 nm). Scattering and reflection were found to be small compared to transmission and absorption during optical breakdown. The ratio of the shock wave energy and cavitation bubble energy was approximately constant (between 1.5:1 and 2:1). These results allowed us to perform a more comprehensive study of the influence of pulse duration (100 fs-76 ns) and focusing angle (4°-32°) on the energy partition by measuring only the plasma transmission 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 pulse duration, but increases again for τ < 3 ps. The conversion of the absorbed energy into mechanical energy is ≈ 92% with ns pulses at large focusing angles. It decreases both with decreasing focusing angle and pulse duration (to < 159% for fs pulses). The disruptive character of plasma-mediated laser effects is therefore strongly reduced when ultrashort laser pulses are used.