Ultrafast laser drilling has become an essential technology for creating micro-holes in various materials as it offers unparalleled precision and minimal thermal damage. The quality and efficiency of this process are primarily determined by the selection of optimal process parameters. The effects of these key parameters on micro-hole drilling must be elucidated to enhance both quality and efficiency. Typically, the effects of these parameters are investigated indirectly by analyzing the cross-sections and surfaces of micro-holes after drilling. However, ultrafast laser drilling is an inherently dynamic process, and certain transient intermediate stages are critical for clarifying the effects of these process parameters on the outcomes. Examining these stages via the conventional cross-sectional analysis after the completion of drilling is challenging. Therefore, this study focuses on the in-situ observation of micro-hole drilling in metal and diamond materials using a 900-fs ultrafast laser.

In this study, micro-holes were drilled into both metal and diamond using a 900-fs ultrafast laser. The effects of varying the laser pulse energy, frequency, and focal position on the drilling speed and maximum hole depth were investigated. In-situ observation was performed to monitor the real-time drilling process, with emphasis on the dynamic changes in the drilling speed at different hole depths. The dynamic evolution of the micro-hole morphology was observed in-situ, which involved several steps. First, the side of the sample was roughly ground and finely polished to obtain a smooth and vertical cross-section. Subsequently, the laser-processing position was precisely controlled to ensure that it was exactly at the edge of the prepared sample, and the laser-spot scanning position was adjusted to create a semi-hole at the sample edge. Finally, the processed area was adjusted to the center of the field-of-view of the in-situ imaging system. The laser power was configured as required, and processing was commenced. The dynamic evolution of the micro-hole morphology was magnified 50 times using an imaging system and recorded using a camera.

The results show that, as the depth of the hole in the metal increases, the hole walls exhibit lateral contraction and expansion, thus resulting in various average drilling speeds. Specifically, the drilling speed decreases initially and then increases, before it finally decreases as the hole depth increases (Fig. 3). By contrast, the hole walls in the diamond maintains a near-vertical orientation, thereby resulting in a consistent and gradual decrease in the drilling speed (Fig. 4). Additionally, increasing the pulse frequency while maintaining a constant pulse energy results in a corresponding increase in the average drilling speed. However, a saturation effect is observed on the maximum hole depth due to the pulse frequency. After the frequency threshold is exceeded, the maximum hole depth no longer changes significantly (Fig. 5). This saturation effect suggests that an optimal range of pulse frequencies exists that maximizes the drilling efficiency without compromising the hole depth.

Furthermore, the pulse energy and frequency are identified as the primary factors influencing the maximum hole depth, with the focal position having a negligible effect (Fig. 8). The increase in pulse energy and frequency enhances the maximum hole depth; however, these parameters must be balanced to avoid potential drawbacks, such as material damage or inefficient energy usage.

This study provides a comprehensive analysis of the effects of laser pulse energy, frequency, and focal position on micro-hole drilling in metals and diamond using a 900-fs ultrafast laser. In-situ observations reveals significant differences in the drilling dynamics between metals and diamonds. The findings indicate that, whereas the average drilling speed in metals exhibits a nonlinear pattern owing to changes in the hole-wall dynamics, the diamonds result in steady drilling. Additionally, this study confirms that pulse energy and frequency are the most critical factors affecting the maximum hole depth, with the pulse frequency showing a saturation effect. These insights can guide the optimization of ultrafast laser-drilling parameters for different materials, thereby enhancing the efficiency and quality of the process.