Heat transfer mechanisms during short-duration laser heating of thin metal films

A perturbation technique is used to simplify the generalized governing equations of the parabolic two-step model. The generalized form of the two-step model contains diffusion terms in both electron and lattice energy equations and assumes that incident laser radiation is absorbed by both electron gas and solid lattice to account for the thermal behavior of semiconducting and impure materials. The simplified perturbation technique is used to eliminate the coupling between the electron and the lattice energy equations when the temperature difference between the electron and the lattice is a small perturbed quantity, which is true in materials exhibiting high coupling factors. A mathematical criterion is derived to determine the conditions under which electron and lattice are in thermal equilibrium. It is found that five dimensionless parameters control the state of thermal equilibrium between the lattice and the electron.