Thermal accumulation at kilohertz repetition rates inside fused silica under ultrafast laser irradiation.

Thermal accumulation effect has proved to reduce ablation threshold and improve the ablation rate during multi-pulse ultrafast laser ablation. It was widely believed that this effect cannot be triggered until the laser repetition rate is raised to the megahertz range. In this Letter, we experimentally discover strong thermal accumulation in fused silica at kilohertz repetition rates and its significant contribution to enhance ablation rate. It is found that the threshold repetition rates to trigger thermal accumulation are intrinsically determined by material thermal diffusivity and insensitive to ambient conditions. We observe two-fold enhancement of the ablation rate and clearly discriminate the contribution from thermal and non-thermal accumulation effects by 35% and 50%-70%, respectively. A multi-physics model is developed to assist the understanding of the process. This Letter promotes the fundamental understanding of thermal/non-thermal accumulation effects and opens the door to low-repetition-rate thermal accumulation for low thermal diffusivity materials.