## 3 Results and Discussions
### Analysis of Material Ablation Characteristics by Femtosecond Laser  
#### 3.2.1 Ablation Morphology of CNT Film Irradiated by Different Laser Pulse Energy and Number  
There are many factors influencing the interaction process between the femtosecond laser and material [13], such as the laser pulse energy and the number of pulses. The energy in the center of the spot is the highest, and the laser energy decreases away from the center of the spot due to the Gaussian distribution of laser intensity. Therefore, a morphology similar to a crater could be seen in the SEM image, as shown in Figure 3. Ablated CNT films with different pulse numbers (\(N\) = 5, 50, 200) were obtained under the pulse energy (\(E\)) of 45, 70, 135, and 165 nJ, respectively. When the pulse energy was 45 nJ, the tightness of the CNTs at the edge of the ablation hole was reduced, and some oligomers used for bonding were removed. The diameter of the hole was small, and the shape was irregular at low pulse energy. By increasing the number of pulses, the ablation region gradually formed as the crater but did not change. Obviously, when the pulse energy increased to 70 nJ, the ablation area of the micro holes gradually increased with the increase of the laser energy, and the oligomer ablation area at the edge of the ablation hole gradually disappeared. Especially at the pulse energies of 135 and 165 nJ, a large amount of CNT filaments were present around the ablation holes, and distinct bright spots at the boundary of ablation holes could be observed by SEM. The above phenomenon was mainly caused by the Gaussian distribution of the incident laser energy, and the cumulative effect of femtosecond laser on the CNT film was weak. When the number of pulses was five and the pulse energy was 45 nJ, the energy of the incident laser pulse was close to the ablation threshold of the material, the hydrocarbon oligomer in the CNT film was preferentially removed. The spot area where the incident region of the beam reached the ablation threshold of CNTs was small, resulting in a poor quality of the ablation hole. As the pulse energy gradually increased to 165 nJ, ablation holes became regular and the depth increased. Meanwhile, the pulse energy density at the edge of the spot reached the ablation threshold of the CNT film, and the diameter of the hole began to increase. Moreover, the laser reached an in-deep layer after the surface was removed, so that the vaporization of the in-depth material was reversely sprayed off the body under the actions of high temperature and high pressure. The vaporized materials  
Figure 3: Ablation morphology at different pulse energies with the number of pulses \(N\) = 5, 50, 200.  
re-solidified on the edge of the ablation hole to form a bright spot structure with the partially fractured CNTs thrown around the hole under the force generated by a thermo elastic shock wave. Different pulse energies and different numbers of pulses were used in the experiments to analyze the influence of pulse number on the diameter of the ablation holes. The diameter of the ablation hole changed with the increasing number of pulses at different pulse energies, as shown in Figure 4. When the pulse energy was 25 nJ, the minimum diameter of the hole was 7.8 \(\upmu\)m, and it increased with the increasing of the number of pulses. However, the ablation diameter increase was not significant. When the pulse energy was 165 nJ, the diameter of the hole was 12.9 \(\upmu\)m, and the energy was far exceeded the ablation threshold of the material. Thus, increasing the number of pulses did not have a substantial effect on the diameter of the ablation hole. This result was related to the interaction between the laser and the material--the area of the region where the material ablation threshold reached was relatively stable. When the energy was low, the laser energy irradiated at the edge of the spot could not reach the ablation threshold of the material, and the partial material at the center of the spot was removed by ablation.