Fig. 1. Schematic of picosecond dual-pulse laser ablation test platform (HWP: half wave-plate; QWP: quarter wave-plate; HR: high reflectance mirror; PBS: polarization beam splitter. The insert shows focal spot of dual-pulse recorded by beam analyzer)

Fig. 2. Variation of ablation morphology with increasing laser fluence under different shapes of dual-pulse radiation. (a0)-(d0) Dual-pulse train with various shape factors; (a1)-(a4) ablation morphology of glass under different laser fluences when η=0; (b1)-(b4) ablation morphology of glass under different laser fluence when η=0.33; (c1)-(c4) ablation morphology of glass under different laser fluences when η=0.50; (d1)-(d4) ablation morphology of glass under different laser fluences when η=0.67

Fig. 3. Optical microscopy images and SEM images of four types of representative ablation morphologies. (a1)-(d1) Optical microscopy images; (a2)-(d2) SEM images; (a3)-(d3) detail enlargement of SEM images

Fig. 4. Distribution of five kinds of dual-pulse ablation morphologies in different pump and probe fluences

Fig. 5. Size definition of typical dual-pulse ablation morphologies. (a) Ablation morphology for phase Ⅳ; (b) ablation morphology for phase Ⅴ

Fig. 6. Under dual-pulse irradiation with different shapes, Dcore2 and Douter2 as a function of laser fluence. (a)(d) Dcore2 and Douter2 as a function of pump fluence; (b)(e) Dcore2 and Douter2 as a function of probe fluence; (c)(f) Dcore2 and Douter2 as a function of train-pulse fluence

Fig. 7. Ablation size of core and outer rings as a function of train-pulse fluence with η=0.5

Fig. 8. Relation between ablation threshold and shape factor for dual-pulse train

Fig. 9. Relationship between ablation morphology and laser fluence for different pulse intervals of dual-pulse with η=0.50. (a) Pulse interval of 333 ps; (b) pulse interval of 667 ps

Fig. 10. Under dual-pulse with η=0.50, relationship between ablation size and laser fluence for different pulse intervals of dual-pulse. (a) Dcore2 as a function of pump fluence; (b) Douter2 as a function of train-pulse fluence

Fig. 11. Schematics of ablation process of K9 glass by picosecond dual-pulse. (a) Ablation morphologies for phases Ⅰ and Ⅱ; (b) ablation morphology for phase Ⅲ; (c) ablation morphology for phase Ⅳ; (d) ablation morphology for phase Ⅴ