Tailored adaption of thermal diffusion by ablation cooling on polymers through unique dependence on laser repetition rate

High repetition rate femtosecond laser processing reduces thermal effects through ablation cooling. This often-overlooked effect occurs when ablation is induced in the heat accumulation regime, typically associated with the GHz repetition rate regime, where undesirable shielding effects occur. However, the low thermal diffusivity and phase transition temperatures of polymers enable ablation cooling to be observed at lower repetition rates (<1 MHz), while preventing shielding effects. Separating these phenomena offers insights into ablation cooling effects that are applicable to a broader range of materials in GHz regimes. This work analyzes the thermal and ablative effects on poly(ethylene terephthalate) (PET) surfaces resulting from high repetition rate (10 kHz – 1 MHz) femtosecond (450 fs) laser irradiation at λ = 515 nm, for different fluences (0.91 – 1.68 J/cm²) and pulse numbers (200 – 400). The results evidence that extended thermal diameters decrease at higher repetition rates (∼1 MHz) compared to the 40 – 100 kHz regime due to ablation cooling, resulting in more optimal processing. This effect is independent of fluence and pulse number, being influenced only by repetition rate. A thermal model explains the reduction of thermal effects and saturation of ablation diameters, serving as a predictive tool for modifications induced at high repetition rates.