Fig. 1. Sketch of the pump–probe imaging system. BS, beam splitter; M, ultrafast mirror.

Fig. 2. Morphology of three types of shockwaves induced by fs laser single pulse irradiation on PMMA surface with laser fluence of 11.9J/cm2 at a probe delay of 16 ns. HSW1, HSW2, and CSW are the three types of shockwaves, which represent the HSW in air, the HSW in the sample, and the CSW in the sample, respectively. The solid arrows depict the propagation direction of each shockwave. The dashed line indicates the contour of HSW2.

Fig. 3. (a) Propagation distance of HSW1, HSW2, and CSW as a function of probe delay after single pulse irradiation with laser fluence of 11.9J/cm2. (b) Propagation distance of HSW1, HSW2, and CSW as a function of the pulse number with laser fluence of 11.9J/cm2; the probe delay of HSW1 and HSW2/CSW is 1 and 16 ns, respectively.

Fig. 4. CSW evolution with pulse number N. All shadowgraphs were captured at the same position with laser fluence of 11.9J/cm2 at a probe delay of 16 ns. The dashed line represents the surface of the sample and laser propagated from air in the Z direction.

Fig. 5. Schematic diagram of the focal range and FL generation when (a) the first pulse is focused on the flat surface and (b) the subsequent pulse is irradiated on the pre-ablated structure.