利用相干光照明的纹影成像装置研究飞秒激光脉冲烧蚀铝靶喷射物相位的超快时间演化

本文提出了一种基于纹影成像装置的新型相位检测方法, 并使用该方法对飞秒激光烧蚀铝靶产生的喷射物的超快相位演化过程进行了实验研究. 与传统的纹影法不同, 本文的相位检测方法使用相干光作为成像照明光, 利用未透过样品的背景光作为参考光, 借助透过样品后在纹影装置刀口处衍射的照明光与背景照明光的干涉, 检测样品的相位; 其最显著的优点是能够清晰反映被测样品mπ或2mπ (m为整数) 的相位改变. 利用该方法, 结合抽运-探测技术, 研究了激光流量为5.4 J/cm²的50 fs脉冲激光烧蚀铝靶产生的喷射物的超快相位演化. 实验发现, 烧蚀过程中形成的喷射物可分为三个相位不同的区域, 分别对应等离子体态的喷射物、后续的垂直靶面喷射的物质和冲击波. 其中, 等离子体态的喷射物在0–9.0 ns的时间延迟内, 由于膨胀和电子复合作用, 相位变化超过π; 而后续的垂直靶面的喷射物在此时间内的相位变化没有超过π. 关键词： 相位检测方法 纹影成像技术 相干光照明 抽运-探测技术

Investigation of the ultrafast phase evolution of the ejected material generated during femtosecond laser ablation of aluminum by the coherent light illuminated schlieren apparatus

A novel phase measurement method based on the schlieren apparatus is proposed, and the ultrafast phase evolution of the ejected material generated during the femtosecond laser ablation of aluminum is experimentally studied by this method. Different from the conventional schlieren technique, the phase measurement method presented in this work uses coherent light as the illuminating light. The specimen's phase under-test is derived with the help of the interference between the light which irradiates the surroundings of the specimen and the light which transmits through the specimen and diffracts on the razor edge of the schlieren apparatus. One remarkable merit of this method is that it can clearly exhibit the specimen's phase variation of mπ or 2mπ (m is an integer). The ultrafast process of the ejected material generated during the 5.4 J/cm², 50 fs laser pulses ablation of the aluminum target is investigated by this novel phase measurement method and the pump-probe technique. Results show that the ejected material is composed of three sequentially appearing regions with different phase evolving processes, which are respectively corresponding to the ejected plasma-state material, the successively ejected material normal to the target surface and the shock wave. It is also found that during the time interval of 0–9.0 ns after the femtosecond pulse strikes the target, the phase of the ejected plasma-state material varies beyond π due to the expansion and recombination, but the phase variation of the successively ejected material does not exceed π.
Keywords： phase measurement method schlieren technique coherent light illumination pump-probe technique