| Abstract [eng] |
Milling efficiency can be improved by utilising burst modes consisting of ultrashort pulses with MHz or GHz repetition rates. In these regimes, several lower-energy pulses initially heat the material or create defects, reducing the ablation threshold and allowing subsequent pulses to remove material more efficiently. As the number of pulses in a GHz burst increases, the ablation mechanism in dielectrics shifts from material sublimation to surface cracking and fragmentation, resulting in more efficient material removal. The aim of this work was to investigate bottom-up milling of fused silica using various ultrashort laser burst modes. Cavities with a diameter of 100 µm were milled and analysed. Milling with MHz bursts produced shallow cavities and was therefore inefficient. When more than two pulses were used in a MHz burst, most cavities failed due to fused debris accumulating inside the cavities. For GHz bursts, the milling efficiency increased with the number of pulses in a burst up to 21 pulses, after which it began to decrease. The highest milling efficiency, 201±15 µm³/µJ, was achieved with 21 pulses per burst. However, increasing the number of pulses also reduced processing quality by increasing chipping and sidewall roughness. On the bottom side of the sample, chipping increased from less than 5 µm to approximately 25 µm when the number of pulses in a GHz burst was increased from 4 to 21. Bottom-up processing using nanosecond pulses (4 and 11 ns) resulted in similar surface quality (chipping > 20 µm), a comparable milling rate, and slightly higher milling efficiency than GHz bursts of similar duration. Airflow nozzles were used to improve debris removal, which increased the achievable cavity depth. Debris removal was enhanced the most when using a de Laval air nozzle, enabling the fabrication of cavities with a 100 µm diameter and 8.5 mm depth in fused silica. This depth is more than eight times greater than that achieved without additional airflow. |
