Precise machining of ceramics by using a femtosecond terawatt laser

在第一年，一套用于在五个轴上控制目标位置的精确平台被安装在大型真空室中，该真空室是飞秒太瓦激光系统的一部分。为了找到硅晶体介质击穿的临界条件，进行了一些初步的加工试验，在脉宽为300 ps的情况下，烧蚀的阈值激光能量密度为0.4 J/cm ^2，与已发表的数据外推吻合得很好。在大气条件下，阈值注量降低。通过改变脉冲宽度为50 fs的飞秒激光的能量，实验研究了几种陶瓷材料，如熔融石英，光学玻璃（BK-7），氧化铝晶体，氟化钙晶体的烧蚀阈值的值。采用三种方法分析了氧化铝晶体的刻蚀阈值：（1）监测样品表面的散射光;（2）用激光能量空间分布对样品凹坑形状进行高斯拟合;（3）用五光子吸收机制公式拟合刻蚀速率。它们之间的一致性很好，分别为：（1）0.35、（2）0.37和（3）0.37 J/cm ^2。这些值依赖于材料，但没有揭示出与其不良间隙的大小的简单线性关系。由于氟化钙晶体的机械强度较弱，在激光辐照后产生的热应力和/或冲击波的作用下，晶体会破碎。对字符、设计标记等掩模图形进行了试加工。在深度方向上获得尖锐的边缘，但是激光的干涉使图案变形。这就要求对没有抛物面反射镜的光学系统进行改造。总之，这种飞秒激光加工技术是有希望的，具有一定强度的陶瓷可以精确地加工，而没有热损伤。

In the first year, a set of precise stages for controlling a target position at five-axes were installed in the large vacuum chamber which was a part of the femtosecond, terawatt laser system. Some preliminary tests of machining were carried out in order to find a critical condition for dielectric breakdown in silicon crystal.The threshold laser fluence was 0.4 J/cm^2 for ablation in the case of a pulse width of 300 ps and was in good agreement with the extrapolation of the published data. The threshold fluence decreased under atmospheric condition. By changing energy of femtosecond laser at a pulse width of 50 fs, values of ablation threshold were experimentally examined for several ceramic materials such as fused quartz, optical glass(BK-7), alumina crystal, and calcium fluoride crystal. Threshold values for alumina crystal were analyzed by three methods as follows : (1) monitoring the scattered light from the sample surface, (2) gaussian fitting of laser energy spatial distribution to the crater shape of sample, and (3) fitting of the etching rate to the formula for five-photons absorption mechanism. They were in good agreement each other as follows : (1) 0.35, (2) 0.37, and (3) 0.37 J/cm^2, respectively. Such values were dependent on materials but did not reveal a simple linear relationship with magnitude of their bad gap. Calcium fluoride crystal was broken by the effect of thermalstress and/or shock waves that were generated after the irradiation of laser, because the crystal had a weak mechanical strength. A trial machining were carried out for patterning of some mask patterns such as character and designed mark. Sharp edges were obtained in the direction of the depth, but interference of laser light deformed the pattern. It requires modifying optical system without parabolic mirror. In conclusion, this femtosecond laser machining technology is promising that ceramics having a certain strength can be machined precisely without thermal damage.