This study investigates the prevalent process problems, such as “microcracking” and “induced streaking,” in the femtosecond laser processing of hard and brittle transparent materials. The study employs the femtosecond time-resolved pump-probe shadow imaging technique to visualize the electron dynamics during the femtosecond laser multi-pulse ablation of quartz glass. Particularly, the plasma filament evolution at the early stage of laser pulse ionization (before 700 fs) is analyzed. The multi-pulse-induced microstructures distribute the filament formation regions on both sides of the microstructure with respect to the axial direction of the light pulse propagation. The distribution on both sides is primarily due to the refraction of the light pulse by the sidewalls of the microstructure, while that on the axis is caused by the difference in the shape of the bottom and sidewalls of the microstructure, creating the light range difference. The empirical results show that the pulse train induces a remodeling effect of the microstructure on the subsequent light field during multi-pulse processing, affecting the distribution of the plasma filament formation region and energy deposition—the core mechanism responsible for common process problems.

A femtosecond time-resolved pump-probe shadow imaging setup was built to capture the propagation and ionization process of a single subsequent pulse beneath the microstructure induced by irradiating the material with 219 fs pulses. First, the actual spatial location of the focus was determined by imaging the shadow of the air-ionized plasma at the focus. After that, the power density at the material surface was obtained for different focus positions. The distinctive “V” and “inverted trapezoid” shapes were obtained after controlling the relative positions of the laser focus and the material. Second, the ionization process of femtosecond time-resolved propagation of the 220th pulse under different microstructures was obtained by modulating the time delay between pump and probe beams. Finally, the ionized filament-forming regions in the transient ionization images were compared with the process defects to reveal the formation mechanism of the process defects.

The propagation and ionization process of the 220th pulse is observed using femtosecond time-resolved pump-probe shadow imaging (Figs.5.6.9.10). The physical mechanisms governing process problems such as “microcracking” in micromachining of hard and brittle materials are revealed. The light-field remodeling effect, guided by various morphology microstructures, leads to energy deposition and the mechanism of generating common process problems. In the context of multi-pulse processing, the influence of energy deposition (propagation and ionization) is determined by the linear refractive index of the material, a nonlinear refractive index that varies with the light intensity, plasma defocusing effect, microstructure morphology, and the focusing conditions in conjunction with the laser fluence on the material surface. Moreover, the relaxation time of ionized free electron number density during light field propagation is determined to be less than 300 fs across diverse microstructures.

Under multi-pulse irradiation, the remodeling effect of different microstructural morphologies on the subsequent light field orchestrates the nonlinear ionization process. In the case of the V-shaped structure, the formation process is accompanied by decreasing tilt angle of the sidewalls, guiding the ionization filamentation direction of the subsequent light field and sweeping across the sidewall region. It corresponds to the areas of “microcracks” and “induced stripes” on both sides of the microstructure. Conversely, the “inverted trapezoidal” structure yields strong ionization filament formation at its relatively flat bottom center region owing to the ionization effect at the bottom of the structure. For the “inverted trapezoidal” structure, a strong ionization effect occurs at the center of the relatively flat bottom, which is the root cause of the fragmentation at the bottom. Overall, the light-field remodeling effect, facilitated by different morphology microstructures, plays a crucial role in energy deposition and governing the common process problems. This result offers directions for optimizing machining processes, such as selecting the number of pulses and controlling the focal feed. Additionally, the high-temporal-resolution pump-probe shadow imaging technique holds promise for predicting fragmentation regions in different morphologies of hard and brittle transparent materials, serving as a powerful tool for online monitoring of high-end processing equipment.