激光等离子体效应对薄膜损伤特性的影响

高能激光的发展对光学元件的抗损伤能力要求越来越高,其中光学薄膜是最薄弱的环节之一。实验研究了激光的聚焦位置对石英基片上HfO2/SiO2减反射薄膜损伤形貌的影响,研究发现:激光等离子体的高压冲击波对薄膜产生强烈的冲击剥离效应,其压强随膨胀半径的增加而迅速减小。激光等离子体光谱的辐射波长小于入射激光波长,这会增强薄膜对辐射光能量的吸收;位于深紫外波段、能量大于HfO2薄膜带隙的光子能量,将被薄膜直接吸收,从而加剧薄膜的电离破坏。激光等离子体的辐射效应和冲击波效应的共同作用决定了薄膜的损伤形貌。当激光聚焦到薄膜表面时,冲击波压强极大会使薄膜发生大面积的电离去除,同时基底发生击穿;当两者距离大到一定距离时,冲击波只会使得中心处小面积薄膜发生剥离,基底未出现断裂。

The influence of laser plasma effects on the characteristics of thin film damage

Optical components with higher surface quality and higher damage threshold requirement are necessary in high-energy/power laser system, which strongly depends on the performance of optical thin films. The damage morphologies on the surface of the HfO2/SiO2 anti-reflection film, caused by focused laser pulses, were investigated in the present paper. The studies revealed that the shock wave formed with the expansion of laser plasma, and its velocity and pressure decease rapidly with the radius. The spectrum of laser plasma, recorded by EEP2000 spectrometer, shows that the wavelength of laser plasma radiation is shorter than incident laser, which will increase the probability of multi-photon absorption; the photon energy in deep ultraviolet region, higher than the band gap of HfO2, can be absorbed directly. The ionization effect of laser plasma can easily induce film damage. The combination of shock wave and ionization effect determines the damage morphology of films. In the case of laser pulse focused on the film surface, the radiation and shock wave effects are the highest, not only the film is removed, but also the quartz substrate is broken-down. When the focus point is away from the film surface to a certain distance, the radiation of laser plasma and shock wave decrease rapidly, as a result, no damage can be found except that the thin-film can be peeled away from the substrate.