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Description
Ultrafast laser processing enables highly localized material modification, although the final morphology and the extent of the collateral thermal effects strongly depend on the temporal distribution of the delivered energy. In polymers, their low thermal diffusivity and relatively low phase transition temperatures promote heat accumulation even at repetition rates in the kHz regime. Although this accumulation can assist material removal, it may also induce swelling and extended thermally modified regions around the processed area.
In this work, the transition between thermally dominated and ablation regimes was investigated in poly(vinyl chloride) (PVC) under femtosecond laser irradiation. Experiments were performed at 1030 nm using 220 fs pulses. The fluence was fixed at 0.40 J/cm2, with a focused beam radius of 118 μm, remaining below the single-pulse ablation threshold. Repetition rates ranging between 100 Hz and 60 kHz were investigated for different numbers of delivered pulses.
As shown in Fig.1, irradiation at 1 kHz produced a pronounced thermal halo surrounding the ablation crater, indicating that a significant fraction of the deposited energy was diffused into the surrounding material. In contrast, irradiation at 60 kHz resulted in larger and more clearly defined ablation crater together with a reduced thermally affected region. Quantitative measurements confirmed that increasing the repetition rate promoted more efficient material removal while limiting lateral extent of thermal modification.
This behavior is consistent with the concept of ablation cooling1. At higher repetition rates, the shorter time between pulses enhances heat accumulation and facilitates an earlier onset of ablation. Once efficient material removal is established, part of the deposited energy is carried away by the ejected material, reducing the fraction available for heat diffusion.
These results provide experimental evidence that ablation cooling effects can occur in PVC at repetition rates considerably lower than the GHz regime commonly employed in ultrafast burst processing. The low thermal diffusivity of this material allows the balance between material removal and heat diffusion to be reached in the kHz range2, where shielding effects are expected to be less significant3.
Acknowledgements
This work was supported by PID2024-161610OB-I00 (funded by MCIN/AEI/10.13039/501100011033 and by ERDF/EU, Spain) and by the Generalitat Valenciana, Spain (CIPROM/2024/90). A.P.B. gratefully acknowledges the “Ministerio de Ciencia, Innovación y Universidades” of Spain for the grant PRE2022-105016. The research leading to these results has received funding from the French PACA (Provence-Alpes-Cote d’Azur) Regional Council (2022 Grants: INTENSITY). It has been conducted using LaMP facilities at LP3.