Effects of Repetition Rates, Pulse Number and Laser Fluence on Ablation Efficiency in Metal Processing With Ultrafast Laser Bursts

Ultrafast laser is a promising way for high-precision processing of metals by pulsed laser ablation, however, the machining efficiency remains unsatisfactory to meet the increasing demands on fabrication throughput in manufacturing industry. In single-pulse ablation, the ablation depth is typically restricted by limited heat penetration depth at fluence close to the ablation threshold. Increasing pulse energy (fluence) brings about higher ablation depth, however, decreased ablation efficiency and more thermal damage. Multi-pulse ablation has been proposed to introduce reduced ablation threshold and enhanced laser absorption. Nevertheless, ablation efficiency cannot be effectively improved at repetition rate below megahertz (MHz). Increasing repetition rate (typically above 10-100 MHz) will stimulate thermal accumulation when the pulse time interval can be shortened so that the remaining material after ablation cannot cool down before the arrival of successive laser pulse. In this way, the thermal energy stored in the material can be reactivated to ablate more materials. At increasing repetition rates, thermal accumulation will be stronger and anticipated to increase the ablation efficiency. However, when the time interval of laser pulses is comparable or even shorter than the plasma lifetime (typically in tens to hundreds of nanoseconds), ablation will be suppressed by plasma shielding effect, where successive laser pulse energy cannot be deposited onto the target material but reflected, scattered and absorbed by the plasma. Recent experiments demonstrate that plasma shielding can be effectively diminished by ultrafast burst consisting of a great number (typically in hundreds) of sub-pulses at intra-burst repetition rate in gigahertz (GHz) and the overall ablation efficiency can be significantly improved by thermal accumulation simultaneously. To apply this method, intra-burst repetition rates, pulse number and burst laser fluence are key factors to affect ablation efficiency, however, their impacts on ablation efficiency are still unclear. On this aim, we establish a multi-physics numerical model to perform a comprehensive investigation on the effectiveness and applicability of GHz laser burst ablation in metals. We clearly reveal the dominating mechanisms, such as thermal accumulation and plasma shielding effect, and highlight the combined effects of pulse number, intra-burst repetition rate and burst fluence on ablation efficiency and heat-affected zone thickness We found that at GHz intra-burst repetition rate, splitting laser fluence into a great number of sub-pulses stimulate ablation enhancement by thermal accumulation, however, this is only applicable at high laser fluence (typically over 20 J/cm²). For low laser fluence, if too many pulses are included in the burst, the ablation efficiency will be significantly reduced. This study highlights the combined impacts of repetition rate, pulse number and pulse/burst fluence on ultrafast laser burst ablation and sheds light on the effectiveness and feasibility of enhanced ablation efficiency in high-repetition-rate ultrafast laser burst ablation in metals.