Nonthermal ablation of crystalline c-cut Sapphire using femtosecond deep UV laser pulses

The demand for precise and high-quality machining of wide-bandgap materials, such as glasses and crystals, is of considerable significance in science and industry. Among these materials, sapphire stands out as an appealing choice due to its exceptional mechanical and optical attributes, high thermal conductivity and stability, low electrical conductivity, and resilience against harsh chemicals. Despite its hardness, sapphire is brittle, making it prone to cracking during conventional machining attempts. Recently, alternative non-contact approaches, like laser ablation, have emerged as potential solutions to improve the machining quality. However research of laser processing of wide-bandgap materials, especially utilizing the high harmonics of femtosecond solid-state laser systems, remains incomplete. Our study focuses on investigating the nonthermal laser ablation of c-cut sapphire crystals using femtosecond (300 fs) deep UV (206 nm) laser pulses and comparing the results with traditional IR femtosecond ablation. The publication encompasses a comprehensive depiction of the ablation process and a review of the various achieved morphologies with accompanying scanning electron microscope images. Our findings indicate that efficient ablation with surface roughness below 100 nm can be achieved through a singlestep process within specific laser processing parameter ranges. The ablation process of sapphire encompasses a strong incubation effect, hence the pulses need to be tightly overlapped. Additionally, we provide a detailed description of methodology used to extract surface roughness which was utilized in all the presented research and offers a practical framework for characterizing ablation results obtained from diverse laser systems.