Enhanced two-temperature model and its application in comprehensive analysis of femtosecond laser melting of gold, copper and their alloys

Extensive research on ultrashort laser-induced melting of noble metals like Au, Ag and Cu is available. However, studies on laser energy deposition and thermal damage of their alloys, which are currently attracting interest for energy harvesting and storage devices, are limited. This study investigates the melting damage threshold (DT) of three intermetallic alloys of Au and Cu (Au3Cu, AuCu and AuCu3) subjected to single-pulse femtosecond laser irradiation, comparing them with their constituent metals. This is accomplished by extending an earlier-developed two-temperature model (TTM)-based code with several improvements, including precise modeling of temperature-dependent optical properties and ballistic electron transport. The dynamic optical model inclusive of ballistic effects is demonstrated to reproduce the experimental DT of pure metals with minimal variation and is therefore adopted for further investigation. Our simulations reveal that the alloy films have significantly lower incipient and complete melting thresholds compared to the pure metals due to their low thermal conductivity and high electron-phonon coupling strength. Theoretical studies on varying the thickness of metal and alloy films unveil the usual trend of a rapid increase in DT up to a certain thickness, followed by a saturation region. This universal DT profile is elucidated by proposing a first-of-its-kind analytical function. Excellent agreement between the coefficients of the function with optical and electron diffusion parameters derived from the comprehensive theory proposed here reinforces the robustness of the model. The novelty of this study also lies in introducing the concept of a critical film thickness for which the entire film attains the melting temperature at its complete melting threshold.