Ultrafast laser-material processing in the ablation-cooled regime

Abstract

Recently, a new regime of material ablation using ultrashort laser pulses has been demonstrated. In this regime, thousands of pulses collectively interact and ablate the material, if the time between subsequent pulses is much less than the time for heat diffusion. . Ablation results in the violent ejection from the surface of the material exceeding a critical temperature. As a result, there moval of heat through ablation becomes dominant over thermal diffusion, and this process is called the ablation-cooled laser-material removal. It was shown that ablation efficiency could be significantly increased while simultaneously reducing the pulse energy by several ordersof magnitude if the pulses’ repetition rate is increased to several GHz. This thesis explores the scaling of the repetition rate upto 100 GHz. Our results indicate that with increasing repetition rate, the efficiency gains of this regime can be maintained along, while further decreasing the pulse energy requirements by 1-2 orders of magnitude. Dramatically, we find that few-nanojoule pulses at 50-100 GHz ablate more efficiently than tens of microjoule pulses at sub-MHz repetition rates. We present systematic results on crystalline silicon and exploratory studies on several technical materials of industrial relevance. The presently reported pulse energies could easily be obtained directly from mode-locked lasers, potentially eliminating the need for costly and complicated laser amplifiers. Therefore, our results are suggestive of a radical transformation of the laser technology required for ultrafast ablation.