The evolution of 355 nm nanosecond laser-induced damage to ${{\rm CaF}_2}$ poly-crystals is investigated by using the time-resolved pump-probe shadowgraph technique. The damage morphologies of the front surface, rear surface, and interior of the ${{\rm CaF}_2}$ crystal are imaged by optical microscopy. When the rear surface is focused by one laser pulse throughout the front surface, three shock waves (SWs) and one SW are observed in the air beside the front and rear surfaces, respectively. When the laser energies are 40 mJ and 60 mJ, at delay time of 1000 ns the radii of SW fronts beside the front surface are 2569.8 µm and 2831.7 µm, while those beside the rear surface are 1012.9 µm and 1078.1 µm, respectively. The filamentary channels inside the ${{\rm CaF}_2}$ crystal are established before the end of a laser pulse at energies of 25, 40, or 60 mJ. The average propagation velocities of SWs along the filamentary channel are approximately 8.2 µm/ns. The maximum diameters of channels can reach approximately 53 µm and 128 µm for 25 mJ energy and 40 mJ energy, respectively. The experimental results help to explore the mechanism of laser-induced ${{\rm CaF}_2}$ poly-crystal damage in nanosecond regime.

中文翻译：

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355 nm 纳秒激光照射后 CaF2 多晶响应的时间分辨成像

通过使用时间分辨泵浦探针阴影图技术研究了355 nm 纳秒激光诱导对${{\rm CaF}_2}$多晶损伤的演变。${{\rm CaF}_2}$晶体的前表面、后表面和内部的损伤形态通过光学显微镜成像。当背面被一个激光脉冲聚焦在整个正面时，在正面和背面旁边的空气中分别观察到三个冲击波 (SW) 和一个 SW。当激光能量为 40 mJ 和 60 mJ 时，延迟时间为 1000 ns，前表面旁边的 SW 锋面半径分别为 2569.8 µm 和 2831.7 µm，而后表面旁边的半径分别为 1012.9 µm 和 1078.1 µm。${{\rm CaF}_2}$内的丝状通道在能量为 25、40 或 60 mJ 的激光脉冲结束之前建立晶体。SWs 沿丝状通道的平均传播速度约为 8.2 µm/ns。对于 25 mJ 能量和 40 mJ 能量，通道的最大直径可以分别达到大约 53 µm 和 128 µm。实验结果有助于探索纳秒范围内激光诱导${{\rm CaF}_2}$多晶损伤的机制。