Transient plasma shielding effects during pulsed laser ablation of materials

The absorption of laser energy by the plasma during pulsed laser deposition of thin films has been analyzed theoretically. The amount of laser energy absorbed in the plasma termed as the “plasma shielding factor” is a function of the incident laser wavelength, and time dependent plasma dimensions and electron density. Due to time varying parameters, a quantitative analysis of the plasma absorption is difficult. A model which takes into account the absorption of laser energy by the plasma has been developed. In this model, the time-dependent plasma dimension is replaced by the time dependent ablation depth. Using simulated absorption coefficient values, the ablation characteristics of silicon and high T_c superconductors are computed and compared with experimental results. The plasma shielding factor was found to vary approximately linearly with absorbed laser energy. The calculations also showed that the plasma shielding was strongly dependent on the laser fluence but varies very weakly with the simulated plasma absorption coefficient values. Experimental results on plume shielding showed good agreement with the calculations.