Subsurface heating effects during pulsed laser evaporation of materials

Abstract

We have theoretically and experimentally investigated the thermal effects of targets evaporated by nanosecond laser pulses. The subsurface temperatures were calculated to be higher than the surface temperatures during planar surface evaporation of the target material. While the evaporating surface is being cooled due to the latent heat of vaporization, subsurface superheating occurs due to a finite absorption depth of the laser beam. The temperature profiles of silicon targets irradiated by nanosecond laser pulses were determined by solving the one-dimensional heat flow equation using an implicit finite difference method. The subsurface superheating increased at higher energy densities, and decreased with increasing absorption coefficient of the material. This internal heating of the target during pulsed laser irradiation can be correlated with the explosive removal of material from the target. This may lead to deposition of small particles on films fabricated by the pulsed laser evaporation technique.