Cavitation and shock waves emission on the rigid boundary of water under mid-IR nanosecond laser pulse excitation

The processes of conversion of light energy into mechanical energy under mid-IR nanosecond laser excitation on a rigid boundary of water are investigated. Strong water absorption of Q-switched Cr:Yb:Ho:YSGG (2.85 µ m, 6 mJ, 45 ns) laser radiation provides rapid energy deposition of ~8 kJ cm −3 accompanied with strong mechanical transients. The evolution of shock waves and cavitation bubbles is studied using the technique of shadowgraphy and acoustic measurements, and the conversion efficiency into these energy channels for various laser fluence (0.75–2.0 J cm −2 ) is calculated. For 6 mJ laser pulse with fluence of 2.0 J cm −2 , the conversion into shock wave energy reaches 67%. The major part of the shock wave energy (92%) is dissipated when the shock front travels the first 250 µ m, and the remaining 8% is transferred to the acoustic far field. The calculated pressure in the vicinity of water-silicon interface is 0.9 GPa. Cavitation efficiency is significantly less and reaches up to 5% of the light energy. The results of the current study could be used in laser parameters optimization for micromachining and biological tissue ablation.