# Throughput Improvement in Femtosecond Laser Ablation of Nickel by Double Pulses

^{1}

^{2}

^{3}

^{*}

## Abstract

**:**

## 1. Introduction

^{14}W/cm

^{2}) and extremely short pulse width (approximately 10

^{−15}s); thus, these lasers can process most materials. Materials processed using femtosecond lasers have an extremely small heat-affected zone (HAZ), which can safely be ignored [8,9]. Therefore, femtosecond lasers are increasingly used in micro/nano manufacturing.

## 2. Materials and Methods

^{3}, and each nickel sample’s upper surface was mechanically polished such that their upper surface roughness was 10 nm. Finally, the nickel sample was in an air atmosphere during the experiment. The optical microscope (OM) was provided by the Olympus company, and the atomic force microscope (AFM) was provided by the Bruker company.

## 3. Results

^{2}) and double pulses (F

_{1}+ F

_{2}= 1.0 J/cm

^{2}) with three energy ratios. The diameter and depth of the ablated craters were characterized using the OM and AFM. When measuring the diameter of the craters, the magnification of the OM was 100×. When measuring the depth of the craters, the area scanned by the AFM was 50 × 50 μm

^{2}. Each group of experiments with different parameters (delay time or laser fluence ratio) was repeated 6 times, and the diameters and depths were measured using OM and AFM, respectively, and rounded to obtain the average value. The results of the OM and AFM are displayed in Figure 3 and Figure 4, respectively. In addition, when the sum of the double-pulse or single-pulse laser fluence was 0.5 J/cm

^{2}, the circle (as shown in Figure 3a) was not seen in the nickel sample.

_{1}+ F

_{2}), the ablation result obtained with a double-pulse delay time of 0 ps is close to that obtained for a single pulse. However, some unusual phenomena were observed in this study’s double-pulse ablation experiments. As displayed in Figure 5, 28 craters were ablated when the delay time of the double pulses with three energy ratios was 0 ps. The diameters of these craters were measured using the OM, and the corresponding results are presented in Figure 6 and Table 1. The diameters of the craters ablated by double pulses with an energy ratio of 1:9 were concentrated in the middle area (10–35 μm). At the aforementioned energy ratio, craters were formed in all 28 experiments, and eight of the craters had a diameter of more than 35 μm. When the energy ratio of the double pulses was 5:5, 20 craters were formed over the 28 experiments. At total of 14 of these craters had a diameter of more than 35 μm. Finally, when the energy ratio of the double pulses was 2:8, 22 craters were formed over the 28 experiments. At total of 10 of these craters had diameters exceeding 35 μm. The formation of the craters was influenced by the interference of double pulses when the delay time was 0 ps. The double pulses’ interference was strongest when their energy ratio was 5:5; therefore, the crater diameter had the widest distribution with the lowest proportion of values in the middle range (10–35 μm) when the energy ratio of the double pulses was 5:5. It was proved that the processing effect of double pulses at zero delay is obviously different from that of a single pulse, and this result can be used to develop multi-scale interference processing. In order to avoid the interference phenomenon of double pulses, vertical polarization optical pulse processing, achieved by changing the polarization of one of the double pulses, can also be used.

## 4. Discussion

## 5. Conclusions

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

## References

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**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 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.

**Table 1.**The distribution of the ablation craters’ diameters under a double-pulse delay time of 0 ps.

Diameter | 1:9 | 2:8 | 5:5 |
---|---|---|---|

0~10 | 0 | 25.0% | 28.6% |

10~15 | 14.2% | 0 | 0 |

15~20 | 10.7% | 10.7% | 0 |

20~25 | 17.9% | 0 | 0 |

25~30 | 10.7% | 7.2% | 10.7% |

30~35 | 17.9% | 21.4% | 10.7% |

35~40 | 28.6% | 35.7% | 50.0% |

Symbol | Value |
---|---|

${C}_{e}$ | $1065\text{}\mathrm{J}\xb7{\mathrm{m}}^{-3}\xb7{\mathrm{K}}^{-1}$ [31] |

${C}_{l}$ | $4.1\text{}\times \text{}{10}^{6}\text{}\mathrm{J}\xb7{\mathrm{m}}^{-3}\xb7{\mathrm{K}}^{-1}$ [30] |

${G}_{0}$ | $3.6\text{}\mathrm{W}\xb7{\mathrm{m}}^{-1}\xb7{\mathrm{K}}^{-1}$ [32] |

${K}_{0}$ | $90\text{}\mathrm{W}\xb7{\mathrm{m}}^{-1}\xb7{\mathrm{K}}^{-1}$ [31] |

${\delta}_{b}$ | $13.5\text{}\mathrm{nm}$ [33] |

${t}_{pi}$ | $35\text{}\mathrm{fs}$ |

$c$ | $3\text{}\times \text{}{10}^{8}\mathrm{m}\xb7{\mathrm{s}}^{-1}$ |

$\omega $ | $c$$\text{}/800\text{}\mathrm{nm}$ |

$e$ | $-1.6\text{}\times \text{}10{}^{-19}\text{}\mathrm{C}$ |

${\epsilon}_{0}$ | $8.85418717\text{}\times \text{}10{}^{-12}\text{}\mathrm{F}\xb7{\mathrm{m}}^{-1}$ |

${m}_{e}$ | $9.1\text{}\times \text{}10{}^{-31}\text{}\mathrm{Kg}$ |

$N$ | 2 |

${N}_{A}$ | 6.022 × 10^{−23} |

$\rho $ | $8.88\text{}{\mathrm{g}*\mathrm{cm}}^{-3}$ |

$Y$ | 59 |

${A}_{e}$ | $0.59\text{}\times \text{}10{}^{-7}\text{}{\mathrm{s}}^{-1}\xb7{\mathrm{K}}^{-2}$ [31,34] |

${B}_{l}$ | $1.4\text{}\times \text{}{10}^{11}{\mathrm{s}}^{-1}\xb7{\mathrm{K}}^{-1}$ [31,34] |

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**MDPI and ACS Style**

Chu, K.; Guo, B.; Jiang, L.; Hua, Y.; Gao, S.; Jia, J.; Zhan, N.
Throughput Improvement in Femtosecond Laser Ablation of Nickel by Double Pulses. *Materials* **2021**, *14*, 6355.
https://doi.org/10.3390/ma14216355

**AMA Style**

Chu K, Guo B, Jiang L, Hua Y, Gao S, Jia J, Zhan N.
Throughput Improvement in Femtosecond Laser Ablation of Nickel by Double Pulses. *Materials*. 2021; 14(21):6355.
https://doi.org/10.3390/ma14216355

**Chicago/Turabian Style**

Chu, Kunpeng, Baoshan Guo, Lan Jiang, Yanhong Hua, Shuai Gao, Jingang Jia, and Ningwei Zhan.
2021. "Throughput Improvement in Femtosecond Laser Ablation of Nickel by Double Pulses" *Materials* 14, no. 21: 6355.
https://doi.org/10.3390/ma14216355