Energy balance of optical breakdown in water

During optical breakdown, the energy delivered to the sample is either transmitted, reflected, scattered, or absorbed. The absorbed energy can be further divided into the energy required to evaporate the focal volume, the energy radiated by the luminescent plasma, and the energy contributing to the mechanical effects such as shock wave emission and cavitation. The partition of the pulse energy between these channels was investigated for 4 selected laser parameters (6 ns pulses of 1 and 10 mJ, 30 ps pulses of 50 (mu) J and 1 mJ, all at 1064 nm). The results indicated that the scattering and reflection by the plasma is small compared to plasma transmission. The plasma absorption can therefore be approximated by A approximately equals (1-T). The ratio of the shock wave energy and cavitation bubble energy was found to be approximately constant (between 1.5:1 and 2:1). For a more comprehensive study of the influence of pulse duration and focusing angle on the energy partition, we therefore restricted our measurements to the plasma transmission and the cavitation bubble energy. The bubble energy was used as an indicator for the total amount of mechanical energy produced. We found that the plasma absorption first decreases strongly with decreasing pulse duration, but increases again for pulses shorter than 3 ps. The conversion of the absorbed energy into mechanical energy is approximately equal to 90% with ns-pulses at large focusing angles. It decreases both with decreasing focusing angle and pulse duration (to less than or equal to 15% for fs-pulses). The disruptive character of plasma-mediated laser surgery is therefore reduced with ultrashort laser pulses.