## 3 Experimental Methods and Analysis  
An HGL-LMY500 solid laser cutting and welding processing system (Wuhan Huagong Laser Engineering Co., Ltd., Wuhan, China) was used in the test. The HGL-LMY500 comprises an Nd:YAG solid-state pulsed laser (average output: 500 W), a power supply system, an optical system, a control system, a cooling system, a numerical control table, and a computer (Figure 3). During processing, this system produces a laser with a wavelength of 1064 nm, a multimode circular spot, and a spot diameter of 0.2 mm. By adjusting the current, pulse width, and repetition frequency, the laser output energy, laser light intensity, beam quality, etc. satisfy processing requirements. The main parameters of this laser system are shown in Table 1. Here, the water jet device is a water jet processing system designed  
Figure 2: Impact force of a water jet striking a material at different velocities and impact angles. by our team. Note that the water-jet velocity and angle be adjusted. In addition, the water jet nozzle (water jet nozzle diameter is 0.7 mm) can be fixed to ensure that the water jet can impact and cool the material in the laser processing area. According to the actual work requirement, the motor, liquid tank, filter device, inlet pipe, plunger pump, overflow return control valve, pressure gauge, accumulator, one-way control valve, and jet nozzle and its fixing device. The experimental sample was a polycrystalline silicon plate (length \(\times\) width \(\times\) height: \(20\times 20\times 2\) mm\({}^{3}\)). The effect of different water jet angles and velocities on the depth and widths of the processing grooves in water-jet-assisted laser etching was investigated. Prior to experiment, it is necessary to ensure that the following laser parameters remain unchanged: 40-Hz laser repetition frequency, 0.6-ms pulse width, 1-mm/s scanning speed, and 140-A current. Under this processing parameter, the laser irradiation intensity reaches \(5.5\times 10^{5}\) W/cm\({}^{2}\). When the irradiation intensity is \(5.5\times 10^{5}\) W/cm\({}^{2}\), it is sufficient to achieve the energy density required for irreversible damage to the surface material, and the material is removed by the action of the laser. In water-jet-assisted laser processing, the jet is impacted slightly behind the laser beam (1 mm behind processing area) to avoid direct contact between the jet and laser beam, avoid excessive loss of laser energy, and boost cooling and impact performance. The cross-sections and widths of the resulting grooves are shown in Figures 4 and 5, respectively. Figures 6 and 7 show the groove depths and widths under different water-jet velocity (horizontal axis) and impact angle (colors), respectively. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Technical** & & **Pulse Width** & **Repeat Frequency** & **Single Pulse** & **Processing Speed** \\
**Parameters** & **Current (A)** & **(ms)** & **(Hz)** & **Energy (J)** & **(mm\(\cdot\)s\({}^{-1}\))** \\ \hline Adjustment range & 100–400 & 0.1–20 & 0–150 & 0–90 & \(>\)0.1 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Laser system processing parameters. Figure 3: Schematic diagram of the experimental Nd\({}^{3+}\): YAG laser etching apparatus. Figure 4: Cross-sections of grooves obtained by etching polycrystalline silicon at different impact angles (top row: 30\({}^{\circ}\); bottom row: 60\({}^{\circ}\)) and jet velocities (left to right: 16, 20, 24, and 28 m/s) (25\(\times\)). Figure 5: Widths of grooves obtained by laser etching at different impact angles (top row: 30\({}^{\circ}\); bottom row: 60\({}^{\circ}\)) and jet velocities (left to right: 16, 20, 24, and 28 m/s) (30\(\times\)). As shown in Figures 4-7, the depth and width of the laser-etched grooves in the polysilicon material are dependent on water-jet velocity but not impact angle (the water-jet velocity trends were very similar at both 30\({}^{\circ}\) and 60\({}^{\circ}\)).