Transition from non-thermal to thermal ablation driven by energy accumulation in multi-pulse femtosecond laser processing of 4H-SiC

Femtosecond laser processing offers high precision and limited thermal damage for silicon carbide (SiC) machining. Although the transition in ablation mechanism under multi-pulse irradiation has been inferred from ablation morphology evolution, effective approaches are still lacking for quantitatively identifying the dominant ablation mechanism and clarifying its underlying physical origin. In this study, an extended multi-pulse two-temperature model (eTTM) incorporating carrier density evolution was established to investigate transient energy evolution and multi-pulse accumulation in 4H-SiC. Experimentally, the surface morphology evolved from flat-bottomed LIPSS to deep V-shaped craters accompanied by a heat-affected zone as the pulse number ( N ) increased. Simulations showed that the dominant removal mechanism gradually shifted from carrier-driven non-thermal ablation to lattice-temperature-dominated thermal ablation, with a critical transition at approximately N = 100. The predicted ablation profiles agree well with the experiments, suggesting that material removal in the LIPSS-containing and deep-crater regions is predominantly associated with non-thermal and thermal ablation, respectively. This mechanism transition originates from the pronounced asymmetry of energy accumulation, where the lattice temperature continuously accumulates with pulse number and eventually exceeds the critical thermodynamic temperature. Overall, these results clarify how multi-pulse energy accumulation governs the transition in the dominant ablation mechanism and provide a theoretical basis for controllable femtosecond laser processing of semiconductor materials.