Fig. 1. Schematic illustrations of (a) experimental setup for the GHz burst LIPAA process, (b) a pulse form of GHz burst mode containing P intra-pulses, and the measured waveforms of (c) a GHz burst pulse with 10 intra-pulse numbers and (d) a pulse at the single-pulse mode operation as comparison.

Fig. 2. (a) Dependence of the ablated depth by GHz burst LIPAA process on the intra-pulse number in GHz burst pulse for the various intra-pulse fluence (0.18, 0.22, 0.27, 0.36, 0.44 and 0.53 J/cm²) and (b) pulse forms of GHz burst mode with various number of P intra-pulses.

Fig. 3. Dependence of ablation depth on laser fluence for (a) GHz burst LIPAA process (circle, P=10, 1 shot) and conventional single-pulse mode direct ablation (triangle, 10 shots) and (b) GHz burst LIPAA process (circle, P=10, 1 shot) and single-pulse mode LIPAA process (square, 10 shots). Schematic schemes with pulse forms of each process are illustrated below graphs.

Fig. 4. SEM images of ablated spots on the sapphire surfaces with different configurations: GHz burst LIPAA (P= 10) at intra-pulse fluence of (a) 0.22 J/cm², (b) 0.27 J/cm², and (c) 0.53 J/cm², single-pulse mode LIPAA (10 shots of fs laser pulses at a repetition rate of 10 Hz) at same intra-pulse fluence of (d) 0.22 J/cm², (e) 0.27 J/cm², and (f) 0.53 J/cm² with (g) schematic scheme of two-step process, and single-pulse mode direct fs laser ablation at laser fluences of (h) 2.33 J/cm², (i) 3.13 J/cm², and (j) 8.05 J/cm². Schematic illustrations of each scheme are shown at left side of SEM images.