Mechanism and performance evaluation of transient and selective laser processing of glass based on optical monitoring.

Femtosecond laser processing has been widely applied in glass processing owing to its ability to fabricate microscale components. To improve processing efficiency, a transient and selective laser (TSL) processing technique was previously developed, in which electron excitation was induced inside a transparent medium by a single pulse of femtosecond (fs) laser, and a single pulse of microsecond (µs) laser can be selectively absorbed in this excited region to heat and remove the material. However, because of its high speed removal process, the unclear mechanism and inefficient evaluation of its processing performance limit its further application. This study analyzes the transient spatiotemporal evolution of the induced plasma and the related material removal mechanism of the TSL processing using a side high-speed monitoring method. To achieve a rapid performance evaluation, a quantitative analysis of the optical plasma signals (on a microsecond timescale) generated in TSL processing was performed by employing a developed coaxial high-speed monitoring method using a photodetector. The variations in the shapes, intensity distribution, and dimensions of the plasma were quantitatively investigated. In addition, the relation between the plasma signal and drilling performance under different laser parameters, including hole depth, hole types, and cracks, was explored and quantitatively analyzed. The revealed mechanism is expected to contribute to the broadening of the application of TSL processing in microfabrication. Furthermore, the developed high-speed and precision monitoring technology can be utilized for high-speed evaluation and precision control of machining quality in real time during ultrahigh-speed laser machining, without time-consuming camera observations.

High-speed microgroove processing of glass by expanding the high-temperature region formed by transient and selective laser absorption.

Microgroove processing of glass is important in many fields, however, it is difficult to achieve the processing with a high speed. In this study, we developed a novel method for the high-speed microgroove processing of glass using two types of lasers, namely a femtosecond laser and a near-infrared continuous-wave (CW) laser. A single femtosecond laser pulse was initially focused on the surface of the material, enabling the area to absorb the CW laser, which is otherwise not absorbed by the glass. The CW laser was then scanned along the material surface, expanding the machined hole to form a groove. The resulting grooves, with a width of approximately 10 µm and depths of up to 350 µm, can be machined with a scanning speed of up to 200 mm/s, 25 times faster than conventional methods. This method exhibits the potential to improve the industrial application of fast laser microprocessing of glass.

Multiscale finite element musculoskeletal model for intact knee dynamics.

BACKGROUND AND OBJECTIVE: The dynamic characteristics of the intact knee joint are valuable for treating knee osteoarthritis and designing knee prostheses. However, it remains a challenge to elucidate the detailed dynamics of the knee due to its complexity of anatomical structure and complex interaction with body dynamics. METHODS: In this study, a unique subject-specific musculoskeletal model with a concurrent high-accuracy intact finite element knee model was created and used to simultaneously evaluate the kinematics and mechanics of an intact knee joint during the gait cycle. RESULTS: A medial pivot motion with external rotation, and a large parallel anterior translation were observed in the stance and swing phases, respectively, which is consistent with the in vivo fluoroscopy measurements. The maximum axial contact force on the knee joint, observed at 45% of the gait cycle, is approximately 2.89 times the body weight. The medial cartilage bears 65.7% of the total axial contact force. The results demonstrate that the cartilage-cartilage contact bears most of the joint load (62.5%) compared to the cartilage-meniscus-cartilage contact (37.5%). Regarding contact mechanics, the maximum contact pressure on both sides of the tibial cartilage (8.2 MPa) is almost similar to the first axial loading peak (14%) of the gait cycle. Additionally, the maximum contact pressure (6.01 MPa) was observed during the stance phase of the gait cycle on the patellofemoral joint. CONCLUSIONS: The predicted results on the tibiofemoral and patellofemoral joints provide a theoretical basis for the treatment of knee joint diseases and knee prosthesis design. Moreover, this approach presents a comprehensive tool to evaluate the mechanics at both the body and tissue levels. Therefore, it has a high potential for application in human biomechanics.

Ultrafast internal modification of glass by selective absorption of a continuous-wave laser into excited electrons.

The internal modification of glass using ultrashort pulse lasers has been attracting attention in a wide range of applications. However, the remarkably low processing speed has impeded its use in the industry. In this study, we achieved ultrafast internal modification of glass by coaxially focusing a single-pulse femtosecond laser and continuous-wave (CW) laser with the wavelength that is transparent to the glass. Compared with the conventional method, the processing speed increased by a factor of 500. The observation of high-speed phenomena revealed that the CW laser was absorbed by the seed electrons that were generated by the femtosecond laser pulse. This technique may help expand the applications of femtosecond lasers in the industry.