Precise machining of ceramics by using a femtosecond terawatt laser

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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.