Understanding nonlinear optical phenomenon for underwater material ablation by ultrafast laser with high pulse energy

Abstract A variety of complex nonlinear optical effects are emerged when water is radiated by ultrafast laser with high pulse energy, which induces different underwater ablation morphologies compared with those ablated in air. A numerical model describing the free electron evolution of water medium during one pulse cycle is established to fully understand the plasmas defocusing effect in multiple pulses. Combining with the Kerr self-focusing and laser diffraction effects, the refractive index change of water medium under different pulse energies and repetition rates can be quantitatively analyzed. Three different experimental phenomena between ultrafast laser and water (plasma ball, laser filamentation, beam diffusion) are well explained by the numerical results. These three nonlinear optical phenomena demonstrate the difference of the material removal under water and in air.