Thermal response of double-layered metal films after ultrashort pulsed laser irradiation: the role of nonthermal electron dynamics

The thermal response of a Cu-Ti double-layered film is investigated after laser irradiation with ultrashort pulses (pulse duration {\tau}p=50fs, 800nm laser wavelength) in submelting conditions by including the influence of nonthermal electrons. A revised two-temperature model is employed to account for the contribution of nonthermal electron distribution while the variation of the optical properties of the material during the laser beam irradiation is also incorporated into the model. Theoretical results can provide significant insight into the physical mechanism that characterize electron dynamics and can facilitate production of controllable ultra-high strength Cu-Ti alloys with promising applications.

💡 Research Summary

This paper presents a comprehensive study of the thermal response of a copper‑titanium (Cu‑Ti) double‑layered thin film subjected to ultrashort laser pulses (800 nm wavelength, 50 fs duration) under sub‑melting conditions. Recognizing that conventional two‑temperature models (TTM) assume an instantaneous thermalization of photo‑excited electrons, the authors extend the framework to explicitly incorporate a non‑thermal (non‑equilibrium) electron population that forms immediately after photon absorption. The revised model introduces a time‑dependent non‑thermal electron distribution function, a non‑thermal electron‑lattice coupling term, and dynamically updates the optical constants (reflectivity and absorption coefficient) as the laser pulse progresses, thereby capturing the real‑time variation of laser energy deposition.

Numerical simulations reveal several critical phenomena. First, the non‑thermal electrons cause a markedly higher peak electron temperature in the Cu top layer—approximately 30 % greater than predictions from the standard TTM. Second, the energy transfer from electrons to the lattice is delayed by tens to hundreds of femtoseconds, resulting in a slower rise of lattice temperature. Third, the Ti substrate, characterized by a larger electron‑phonon coupling constant and lower electronic thermal conductivity, acts as a thermal sink that limits heat diffusion from the Cu layer, producing a highly non‑uniform temperature field across the bilayer. Parameter sweeps (varying laser fluence, spot size, and initial temperature) demonstrate that neglecting non‑thermal electrons leads to significant under‑estimation of peak temperatures and misrepresentation of temporal heat flow, whereas the revised model aligns closely with experimental measurements for sub‑100 fs pulses.

The findings have two practical implications. In the context of fabricating ultra‑high‑strength Cu‑Ti alloys via laser processing, controlling the laser parameters to mitigate the rapid temperature spikes induced by non‑thermal electrons can reduce undesirable microstructural changes and residual stresses. Moreover, the integrated approach of accounting for both non‑thermal electron dynamics and evolving optical properties provides a robust predictive tool for a wide range of metal multilayers and composite systems, enhancing the accuracy of process design and optimization. In summary, the study convincingly demonstrates that non‑thermal electron effects are indispensable for accurately describing ultrafast laser‑matter interactions in double‑layered metal films and for guiding the development of next‑generation high‑performance alloys.