Yusuke Ito

Fine micromachining of sapphire using ultrafast lasers via nonlinear absorptions has been extensively studied. However, severe damage, along with the low material removal rate, limits the precision and machining speed. Here we use transient and selective laser processing for sapphire by coaxially delivering a single femtosecond laser pulse and a single microsecond laser pulse that are temporally synchronized. It shows that the micromachining speed was improved 10⁴ times, and cracks in the base material are mitigated, compared to femtosecond laser percussion processing with a repetition rate of 1 kHz. The ultrafast electron excitation dynamics induced by the femtosecond laser are captured using pump-probe imaging, revealing a rapid relaxation process with a decay time constant on the order of picoseconds. The machining process is further visualized using a high-speed camera and compared with numerical simulations to elucidate the underlying mechanism. We find that the increased absorption coefficient of sapphire for the microsecond laser pulse, owing to the bandgap narrowing and thermal ionizations, along with high-temperature regions expanding from the sample surface into deeper areas along the laser propagation path, drives the machining process. These results advance high-efficiency and precision micromachining for sapphire and other hard and brittle transparent materials.

Transparent dielectrics are essential in advanced photonic, electronic, and medical devices owing to their excellent properties, but these very properties also make efficient micromachining difficult, particularly for high-aspect-ratio deep-hole fabrication. Here, we demonstrate a dual-beam approach that enables 153-µm-deep hole drilling within an effective processing time of 50 ps. In this scheme, a high-angle Bessel beam generates a highly absorptive filament, while a perpendicular, cylindrically focused pulse delivers energy to the filament vicinity. Compared with recent GHz Bessel-burst drilling, which typically relies on multiple pulses separated by several hundred picoseconds and therefore requires a total processing window on the order of nanoseconds, the present approach achieves analogous filament-assisted deep drilling in a compressed temporal window. This result highlights the potential of the proposed scheme as a new strategy for single-shot, high-aspect-ratio micromachining of transparent dielectrics and may provide a basis for future improvements in practical processing throughput through further development of system-level implementation strategies.

Laser powder bed fusion (LPBF) is a key technology for high-precision manufacturing of complex-shaped metallic components. A detailed understanding of the spatter and melt pool behavior during the process is essential for optimizing the process parameters and improving the part quality. This study presents an observation system that combines ultrahigh-speed imaging at 1,000,000 frames per second (fps) with quad-directional (four-direction) laser illumination. This setup facilitates real-time, high-fidelity visualization of dynamic phenomena such as melt pool evolution and spatter generation, which are difficult to observe using conventional single-direction illumination, by reducing self-shadowing and clarifying feature contours. Using this system, transient melt pool behavior with and without spatter was contrasted across four laser powers of 50, 100, 150, and 200 W and four scanning speeds of 50, 100, 150, and 200 mm/s; under spatter-generating conditions, surface protrusions grew more rapidly and detached as spatter once their velocity and geometry were sufficient to overcome surface tension, whereas under nonspattering conditions, they were reabsorbed before detachment. Image analysis quantified frame-to-frame brightness fluctuations on the melt pool surface as a relative indicator of dynamic surface instability and revealed its dependence on process parameters, particularly the volumetric energy density. The results support a mechanism in which dynamic surface instability promotes spatter formation. This approach provides a powerful tool for in-situ analysis of LPBF and contributes to the development of advanced process control and quality assurance in metal additive manufacturing, including real-time spatter control and process-window optimization. Conceptual Quad-directional illumination paired with an ultrahigh-speed camera capturing one million frames per second makes rapid changes on the melt pool surface during laser powder bed fusion clearly observable. A simple measure of surface-brightness fluctuation summarizes the degree of surface instability and links it to the energy input per unit volume. When surface bumps grow quickly and maintain a favorable shape, they detach as spatter; under calmer settings they are reabsorbed. These insights help select more stable process windows and refine scan strategies to reduce spatter in metal additive manufacturing.

Femtosecond lasers with extremely high peak intensity have driven remarkable advancements in manufacturing across science, medicine, and industry. However, the problem of notably low machining speed remains unsolved. Here, we demonstrate that by transiently exciting electrons in a transparent material, the laser drilling speed is increased by a factor of 1 million compared to that in multishot percussion drilling. By irradiating with a single shot of a spatially shaped ultrashort laser pulse, the optical properties are momentarily changed on the picosecond scale, making the material considerably easier to machine by a successive laser pulse. The selective absorption of laser energy in regions with excited electrons leads to the rapid heating and evaporation of material at an extraordinarily high speed. Furthermore, the machining is achieved using a low-power light source, four orders of magnitude lower than conventional femtosecond lasers. The concept of transiently altering material properties is expected to usher in a paradigm shift in research and development for manufacturing.

Laser-induced stress waves play a crucial role in ultrashort pulse laser (USPL) processing such as laser shock peening. However, previous studies have primarily relied on numerical simulations or two-dimensional measurements, lacking experimental validation of the three-dimensional stress distribution. This study presents what we believe to be a novel optical measurement technique to quantify the three-dimensional axisymmetric shear stress distribution of laser-induced stress waves using time-resolved circular polariscope imaging and integrated photoelasticity. Femtosecond laser pulses were used to generate stress waves in silica glass, and the induced birefringence was captured using a polariscope. The slow-axis angle and the optical retardance were extracted and used to reconstruct three-dimensional distribution of the shear stress component. The reconstructed stress distribution agrees with theoretical predictions and numerical simulations of a previous study. Additionally, we found that higher pulse energies resulted in broader stress wavefronts. This behavior is attributed to the expansion of the ablation region, which alters the wavefront shape. The ability to visualize and quantify the shear stress in three dimensions offers a more comprehensive understanding of stress wave interactions with materials. This study lays the foundation for further advancements in USPL processing and experiments utilizing USPL-induced high pressure.