Laser-target interactions and its effect in controlling properties of laser-deposited superconducting thin films

摘要

The nature of evaporation/ablation characteristics during pulsed laser deposition strongly controls the quality of laser-deposited films. To understand the origin of particulates in laser deposited films, we have simulated the thermal history of YBa$- 2$/Cu$-3$/O$-7$/ targets under intense nanosecond laser irradiation by numerically solving the heat flow equation with appropriate boundary conditions. During planar surface evaporation of the target material, the sub-surface temperatures were calculated to be higher than the surface temperatures. While the evaporating surface of the target is constantly being cooled due to the latent heat of vaporization, sub-surface superheating occurs due to the finite absorption depth of the laser beam. Sub- surface superheating was found to increase with decreasing absorption coefficient and thermal conductivity of the target, and with increasing energy density. The superheating may lead to sub-surface nucleation and growth of the gaseous phase which can expand rapidly leading to microexplosions and 'volume expulsion' of material from the target. Experiments conducted by us and other research groups suggest a strong relation between degree of sub-surface superheating and particle density in laser-deposited films.