Surface defects introduced by conventional mechanical processing methods can induce irreversible damage and reduce the service life of optics applied in high-power lasers. Compared to mechanical processing, laser polishing with moving beam spot is a noncontact processing method, which is able to form a defect-free surface. This work aims to explore the mechanism of forming a smooth, defect-free fused silica surface by high-power density laser polishing with coupled multiple beams. The underlying mechanisms of laser polishing was revealed by numerical simulations and the theoretical results were verified by experiments. The simulated polishing depth and machined surface morphology were in close agreement with the experimental results. To obtain the optimized polishing quality, the effects of laser polishing parameters (e.g. overlap rate, pulse width and polishing times) on the polishing quality were experimentally investigated. It was found that the processing efficiency of fused silica materials by carbon dioxide (CO2) laser polishing could reach 8.68 mm2 s-1, and the surface roughness (Ra) was better than 25 nm. Besides, the cracks on pristine fused silica surfaces introduced by initial grinding process were completely removed by laser polishing to achieve a defect-free surface. The maximum laser polishing rate can reach 3.88 μm s-1, much higher than that of the traditional mechanical polishing methods. The rapid CO2 laser polishing can effectively achieve smooth, defect-free surface, which is of great significance to improve the surface quality of fused silica optics applied in high-power laser facilities.

Within this work, we present a system for the measurement of the three-dimensional (3D) trajectories of spatters and entrained particles during laser powder bed fusion (L-PBF) of metals. It is comprised of two ultrahigh-speed cameras and a reconstruction task specific processing reconstruction algorithm. The system enables an automated determination of 3D measures from the trajectories of a large number of tracked particles. Ambiguity evolving from an underdetermined geometrical situation induced by a two-camera setup is resolved within the tracking using a priori knowledge of L-PBF of metals. All processing steps were optimized to run on a graphics processing unit to allow the processing of large amounts of data within an appropriate time frame. The overall approach was validated by a comparison of the measurement results to synthetic images with a known 3D ground truth.

In this review, we describe our research on the development of the 13.5 nm coherent microscope using high-order harmonics for the mask inspection of extreme ultraviolet (EUV) lithography. EUV lithography is a game-changing piece of technology for high-volume manufacturing of commercial semiconductors. Many top manufacturers apply EUV technology for fabricating the most critical layers of 7 nm chips. Fabrication and inspection of defect-free masks, however, still remain critical issues in EUV technology. Thus, in our pursuit for a resolution, we have developed the coherent EUV scatterometry microscope (CSM) system with a synchrotron radiation (SR) source to establish the actinic metrology, along with inspection algorithms. The intensity and phase images of patterned EUV masks were reconstructed from diffraction patterns using ptychography algorithms. To expedite the practical application of the CSM, we have also developed a standalone CSM, based on high-order harmonic generation, as an alternative to the SR-CSM. Since the application of a coherent 13.5 nm harmonic enabled the production of a high contrast diffraction pattern, diffraction patterns of sub-100 ns size defects in a 2D periodic pattern mask could be observed. Reconstruction of intensity and phase images from diffraction patterns were also performed for a periodic line-and-space structure, an aperiodic angle edge structure, as well as a cross pattern in an EUV mask.

The importance to industry of non-contact bearings is growing rapidly as the demand for highspeed and high-precision manufacturing equipment increases. As a recently developed non-contact technology, near-field acoustic levitation (NFAL) has drawn much attention for the advantages it offers, including no requirement for an external pressurized air supply, its compact structure, and its ability to adapt to its environment. In this paper, the working mechanism of NFAL is introduced in detail and compared to all existing non-contact technologies to demonstrate its versatility and potential for practical applications in industry. The fundamental theory of NFAL, including gas film lubrication theory and acoustic radiation pressure theory is presented. Then, the current stateof- the-art of the design and development of squeeze film air bearings based on NFAL is reviewed. Finally, future trends and obstacles to more widespread use are discussed.

Microcutting is a precision technology that offers flexible fabrication of microfeatures or complex three-dimensional components with high machining accuracy and superior surface quality. This technology may offer great potential as well as advantageous process capabilities for the machining of hard-to-cut materials, such as tungsten carbide. The geometrical design and dimension of the tool cutting edge is a key factor that determines the size and form accuracy possible in the machined workpiece. Currently, the majority of commercial microtools are scaled-down versions of conventional macrotool designs. This approach does not impart optimal performance due to size effects and associated phenomena. Consequently, in-depth analysis and implementation of microcutting mechanics and fundamentals are required to enable successful industrial adaptation in microtool design and fabrication methods. This paper serves as a review of recent microtool designs, materials, and fabrication methods. Analysis of tool performance is discussed, and new approaches and techniques are examined. Of particular focus is tool wear suppression in the machining of hard materials and associated process parameters, including internal cooling and surface patterning techniques. The review concludes with suggestions for an integrated design and fabrication process chain which can aid industrial microtool manufacture.

Femtosecond laser technology has attracted significant attention from the viewpoints of fundamental and application; especially femtosecond laser processing materials present the unique mechanism of laser-material interaction. Under the extreme nonequilibrium conditions imposed by femtosecond laser irradiation, many fundamental questions concerning the physical origin of the material removal process remain unanswered. In this review, cutting-edge ultrafast dynamic observation techniques for investigating the fundamental questions, including timeresolved pump-probe shadowgraphy, ultrafast continuous optical imaging, and four-dimensional ultrafast scanning electron microscopy, are comprehensively surveyed. Each technique is described in depth, beginning with its basic principle, followed by a description of its representative applications in laser-material interaction and its strengths and limitations. The consideration of temporal and spatial resolutions and panoramic measurement at different scales are two major challenges. Hence, the prospects for technical advancement in this field are discussed finally.