Transient Coupling Model of Plasma and Laser Field in Water
A nonlinear transient time domain model is presented to reveal some detailed interplay processes between laser and its self-generated plasma in water. The laser-induced breakdown threshold and the plasma absorption coefficient calculated by this model are consistent with the experimental results quantitatively. Moreover, the distribution of free electron density, optical field, and energy losses of laser in the focusing area during whole laser pulse is predicted by the proposed model. Numerical calculation outlines that for the interaction between femtosecond laser and water, there exists a splitted Gaussian pulse along with the conical plasma. On the other hand, the plasma generated by picosecond or longer laser pulse can move in the opposite direction to laser beam propagation, and the velocity of this inverse movement linearly depends on the electric field intensity. Furthermore, the avalanche ionization is also significant for femtosecond laser interaction with water. In addition, the expansion properties of the plasma and the coupled optical field are discussed. The model proposed in this paper could be helpful to laser ophthalmological operation and laser-induced breakdown spectroscopy.