Throughput Improvement in Femtosecond Laser Ablation of Nickel by Double Pulses

Abstract

In this study, femtosecond laser double pulses were tested to improve their nickel ablation efficiency. The experimental results indicated that compared with single pulses, double pulses with different delay times generated craters with larger diameters and depths. The results obtained for three sets of double pulses with different energy ratios indicated that double pulses with an energy ratio of 1:9 had the highest ablation efficiency, followed by those with energy ratios of 2:8 and 5:5. The double pulses with the aforementioned three energy ratios achieved the maximum ablation efficiency when the delay time was 3-4 ps. Compared with single pulses, double pulses with an energy ratio of 1:9 generated craters with an up to 34% greater depth and up to 14% larger diameter. In addition, an interference effect was observed with a double pulse delay time of 0 ps, which has seldom been reported in the literature. The double pulses were simulated using the two-temperature model. The simulation results indicated that double pulses with an energy ratio of 1:9 with a delay time of 4 ps can perform the strongest ablation. These simulation results are in line with the experimental results.

Keywords: double pulses; femtosecond laser; nickel; two-temperature model.

Figure 1

Schematic of the light path in the experiment (1—fs laser; 2—neutral density filter; 3—diaphragm; 4—mirror; 5—beam splitter; 6—one-dimensional linear translation stage; 7—shutter; 8—CCD camera; 9—dichroic mirror; 10—plano-convex lens; 11—sample; 12—six-degrees-of-freedom translation stage).

Figure 2

Schematic of the pulses used for ablation, with a single pulse and with double pulses with varying energy ratios: (a)—single pulse; (b)—a double pulse with an energy ratio of 5:5; (c)—double pulses with a different energy ratio (1:9 or 2:8). The maximum repetition rate of the laser was 1 kHz.

Figure 3

A comparison of the diameters of the craters ablated by the double pulses (with three energy ratios) and single pulse. The red, black, and pink lines represent the results obtained for double pulses with energy ratios of 1:9, 2:8, and 5:5, respectively. The downward green triangle represents the results obtained for the single pulse. Inset (a)—image of a crater ablated by the single pulse; inset (b)—image of a crater ablated by double pulses with an energy ratio of 1:9 and a delay time of 4 ps. Insets (a,b) have the same scale bar.

Figure 4

A comparison of the depths of the craters ablated by the double pulses (with three energy ratios) and single pulse. The red, black, and pink lines represent the results obtained for double pulses with energy ratios of 1:9, 2:8, and 5:5, respectively. The downward green triangle represents the results obtained for the single pulse.

Figure 5

The results of the ablation experiments when the double-pulse delay time was 0 ps: (a)—energy ratio of 1:9; (b)—energy ratio of 2:8; (c)—energy ratio of 5:5. Panels (a–c) have the same scale. The scale bar is 200 μm.

Figure 6

Diameter range histograms of the craters ablated by double pulses with three laser fluence ratios when the delay time was 0 ps.

Figure 7

The process of calculating the depth of nickel ablation.

Figure 8

Changes in the lattice and electron temperatures over time under irradiation by double pulses with three energy ratios: (a)—electron temperatures obtained by irradiation with double pulses with a delay time of 4.0 ps and energy ratios of 1:9 (solid red line), 2:8 (dotted blue line), and 5:5 (dotted pink line); (b)—lattice temperatures obtained by irradiation with double pulses with a delay time of 4.0 ps and energy ratios of 1:9 (solid red line), 2:8 (dotted blue line), and 5:5 (dotted pink line).

Figure 9

Changes in the maximum lattice temperatures with respect to the delay time for irradiation by double pulses with three energy ratios. The solid red, blue, and black lines represent the results obtained using double pulses with energy ratios of 1:9, 2:8, and 5:5, respectively.