Mass nature of heat and its applications IV: Thermal wave and periodic temperature oscillation in metallic films heated by ultra-short pulsed lasers

The ultra-fast heat transfer in metallic films has attracted great interest in modern femtosecond laser processing and metallic film manufacturing. Considering the unphysical infinite propagation speed of heat disturbances based on Fourier's model, some hyperbolic heat transfer models have been developed in the past decades, leading to the character of thermal wave in metallic films under ultra-fast laser heating conditions. In this paper the thermomass model is applied to obtain the governing equation for heat conduction in the thin films under pulsed laser heating, which is a damped wave equation and identical with that based on the Hyperbolic Two-Step (HTS) model. The semi-implicit Crank-Nicholson scheme is used to solve governing equations. Numerical results show that there may be two kinds of temperature oscillations existed in metallic films heated by ultra-short pulsed lasers, and the thermally oscillating boundary condition usually dominates over that caused by thermal wave induced oscillation of the electron temperature, which is validated by the measurement of the temperature response at the rear surface using a femtosecond laser pump-probe system. The measured electron temperature curve agrees well with the theoretically predicted one.