Convective Onset in Thermal Lensing: Systematic Pulse-Number-Dependent Experiments and Theoretical Insights into the Femtosecond Regime

This study investigates thermal lensing induced by femtosecond-laser pulses, a key technique for examining the thermo-optical response of materials under high-repetition-rate irradiation. Using both experimental and theoretical approaches, this work examines how thermal lensing develops and evolves over time with a focus on the effects of pulse count, pump-on duration, and pump-off duration. The study examines how conduction and convection mechanisms compete during heat dissipation, with conduction dominating at a lower number of pulses. The theoretical approach emphasizes heat generation from single-pulse interactions, which is a central aspect of this research. Importantly, the study also shows the potential thermal lens (TL) amplitude from a single femtosecond pulse, providing insights into the early stages of heat buildup. These results enhance our understanding of heat transfer in ultrafast laser-matter interactions, providing more detailed insight into how thermal lenses form and relax.