飞秒激光在光敏玻璃内制作微孔

用20倍显微物镜将波长为775 nm的飞秒激光聚焦在光敏玻璃（FOTURAN）内部,通过纵向写制模式由表面以下500 μm曝光至表面,并结合热处理和在浓度8%的氢氟酸超声溶液中腐蚀50 min,在FOTURAN内部制作了直径为几十μm的微孔.利用光学和扫描电子显微镜分析发现微孔具有圆形横截面和清晰边缘,目前得到的深宽比大约为7.通过在宽范围内改变入射激光能流（2.3~36.2 J/cm2）和写制速度（100~1 000 μm/s）,研究了这两项飞秒激光入射参量对制作微孔的影响.发现写制速度对制作微孔直径影响较小,而利用相对低的入射激光能流曝光可得到较大深宽比的微孔,并且在此情形下制作微孔的横截面更圆,璧面光滑度更高,并分析了原因.     

High-speed imaging of ultrasound driven cavitation bubbles in blind and through holes

The interest in application of ultrasonic cavitation for cleaning and surface treatment processes has increased greatly in the last decades. However, not much is known about the behavior of cavitation bubbles inside the microstructural features of the solid substrates. Here we report on an experimental study on dynamics of acoustically driven (38.5 kHz) cavitation bubbles inside the blind and through holes of PMMA plates by using high-speed imaging. Various diameters of blind (150, 200, 250 and 1000 µm) and through holes (200 and 1000 µm) were investigated. Gas bubbles are usually trapped in the holes during substrate immersion in the liquid thus preventing their complete wetting. We demonstrate that trapped gas can be successfully removed from the holes under ultrasound agitation. Besides the primary Bjerknes force and acoustic streaming, the shape oscillations of the trapped gas bubble seem to be a driving force for bubble removal out of the holes. We further discuss the bubble dynamics inside microholes for water and Cu²⁺ salt solution. It is found that the hole diameter and partly the type of liquid media influences the number, size and dynamics of the cavitation bubbles. The experiments also showed that a large amount of the liquid volume inside the holes can be displaced within one acoustic cycle by the expansion of the cavitation bubbles. This confirmed that ultrasound is a very effective tool to intensify liquid exchange processes, and it might significantly improve micro mixing in small structures. The investigation of the effect of ultrasound power on the bubble density distribution revealed the possibility to control the cavitation bubble distribution inside the microholes. At a high ultrasound power (31.5 W) we observed the highest bubble density at the hole entrances, while reducing the ultrasound power by a factor of ten shifted the bubble locations to the inner end of the blind holes or to the middle of the through holes.