Quartz glass is extensively utilized in various industries and applications owing to its high-temperature tolerance, corrosion resistance, excellent light transmission, and superior electrical insulation. However, the traditional mechanical processing of quartz glass causes chipping and cracking because of its high hardness and brittleness. Femtosecond laser ablation, which involves the ionization of materials in the irradiated area, enables precise material removal or modification at the nanometer or micrometer scale. High repetition frequency and high average power femtosecond lasers are commonly employed for high-fluence irradiation to enhance material removal efficiency in femtosecond laser ablation. Nevertheless, thermal accumulation caused by high-fluence irradiation can lead to the formation of microcracks, recast layers, and debris redeposition, adversely affecting the processing efficiency and quality. Hence, to achieve high-quality precision processing by high-fluence femtosecond laser ablation, a frost-assisted femtosecond laser processing method with controlled frost layer thickness is proposed in this study, and the optimized processing parameters are obtained.

A frost-assisted femtosecond laser processing platform is built for a high-quality debris-free engraving process, in which a thermoelectric cooler (TEC) mounted onto a translation stage is used to lower the sample temperature below its freezing point. The processing platform is enclosed by a polymethyl methacrylate (PMMA) cover to control environmental humidity. Firstly, a quartz glass is placed in a plasma cleaner to remove surface contaminants and enhance its hydrophilicity. Dry air is then injected into the PMMA cover to reduce its internal relative humidity to 20%. Subsequently, the quartz glass is fixed on the TEC with a surface temperature of -5 ℃, facilitating the gradual formation of a frost layer with increasing thickness. Once a certain frost layer thickness is reached, the dry air flow is increased to decrease the relative humidity further to 12%. After the frost layer stabilizes, a tightly focused femtosecond laser with a wavelength of 1030 nm and pulse width of 266 fs is utilized to execute single-pass micro-grooving on the frost layer surface at a constant velocity.

The variation in frost layer thickness with respect to time, under different relative humidity values, is shown in Fig. 3. The frost layer reaches a stable thickness when the environment relative humidity is kept at 12%, and stable frost layers with different thicknesses can be obtained by controlling the time of frost growth. Frost growth is dependent on both the environmental humidity and surface temperature of the frost layer. When the environmental humidity is kept constant, the growth rate of frost gradually decreases owing to an increase in the surface temperature of the frost layer. To achieve a high-quality and high-precision machining of microgrooves, the effects of laser power, frost layer thickness, scanning speed, and repetition frequency on the surface morphology are investigated in detail. The optimized processing parameters are as follows: frost layer thickness of 4?10 μm, average power of 0.3?0.7 W, pulse repetition frequency of 25?5000 kHz, and scanning speed of 0.05?1.00 mm/s. The experimental results show that the frost layer does not completely melt if the laser power is too low, whereas the heat-affected zone and recast layer are significantly increased if the laser power is too high (Fig. 4). If the frost layer is too thin, it quickly melts and evaporates under laser irradiation, resulting in increased debris repositioning. If the frost layer is too thick, the laser energy is blocked or attenuated, resulting in shallower microgrooves (Fig. 5). Moreover, the optimal frost layer thickness differs under different processing parameters (Fig. 6). The reason for achieving high quality via frost-assisted processing can be explained as follows: 1) The thin frost layer rapidly melts under laser irradiation, forming a water film near the laser focus (white bright spot in Fig. 8). 2) Some of the ablation debris is dispersed in the water, reducing the occurrence of splashing. The presence of water film allows for rapid cooling of the splashed debris, reducing debris adhesion and facilitating debris removal from the glass surface via a subsequent ultrasonic cleaning process. 3) In a zone far from the focal spot, the frost layer itself provides substantial protection against debris adhesion on the glass surface.

Through closed-loop control of the TEC surface temperature and fine adjustment of the relative humidity of the processing environment, precise control of the frost layer thickness on a quartz glass surface is achieved at the micron level such that the thickness of the frost layer remains stable over the long term. The experimental results indicate that the laser processing parameters and frost layer thickness significantly influence the processing quality, and different optimal frost layer thicknesses exist for different laser processing parameters. By single-step direct writing with appropriate processing parameters and frost layer thickness, frost layer melting into water film is faster than that by laser direct-writing, and the high-quality and high-precision processing of a surface groove with width of ~2 μm can be realized. Compared with other processing methods, thin frost-assisted laser processing not only effectively suppresses the generation of the recast layer and adhesion of surface debris but also avoids the adverse effects caused by cavitation bubbles when the liquid layer is too thick. Because the frost layer is homogeneously deposited on the material surface by the condensation of water vapor in the processing atmosphere, the proposed precise control method for frost layer thickness is suitable for flat surfaces as well as for surfaces with certain curvatures. Since high-precision frost-assisted femtosecond laser ablation is relatively sensitive to frost layer thickness, the proposed method is expected to significantly improve the large-area uniformity and stability of frost-assisted processing, thus presenting new opportunities for the high-quality processing of various surface functional micro/nanostructures.