## 3 Results and Discussions
### Analysis of Material Ablation Characteristics by Femtosecond Laser
#### 3.2.2 Mechanisms of Femtosecond Laser Ablation of CNT Film  
The large thermal stress caused by the laser irradiated on the CNT film will induce damage to the material, which may lead to complex structural modifications or even a conversion to other polymers [25]. Based on the analysis of the effect of laser parameters on the material, the surface morphology of the hole and the shape of the inner wall were mainly caused by the involved interaction mechanisms between the ultra-short pulse laser and the material. Figure 5 shows the morphology (the number of pulses was 200) and ablation schematics of CNT film by different laser energies.  
Figure 4: Ablation diameter versus number of pulses under different pulse energy conditions indicated in the plot legend.  
When the laser pulse energy was low, the vaporization of the oligomer on the surface of the CNT film occurred during the pulse ablating process. The CNTs and the oligomer at the center of the spot were simultaneously removed due to the Gaussian distribution of laser intensity. However, the lower energy on the outer edge of the spot only could cause the oligomer to be removed, and the surface material on the spot became loose and porous, which was favorable for the escape of the vapor. Carbon-carbon bonds between different carbon atoms in CNTs broke by absorbing photon energy. The pulse energies changed their interatomic distance and broke their bonds with the surrounding atoms [26]. The CNTs jet produced by laser ablation and unbroken CNTs formed complex hole structures that affected the energy density acting on the material and could not exceed the damage threshold of the material, thus resulting in the small influence of the number of pulses on ablation depth. When the laser pulse energy was high, the increasing number of pulses made more energy deposited on the inner wall, thus accelerating the ablation of the material and eventually forming a through hole. At the same total fluence of a laser, multi-pulse ablation can reduce the compressive stress and tensile stress at the ablation position of film [27]. A laser can easily cause air to breakdown near the processed micro holes to form atmospheric plasma, and the laser intensity gradually decreases with the direction of the beam propagation. The laser energy irradiated on the inner wall of the processed hole cannot remove all the material, so the diameter of the bottom will be smaller than the entrance of the ablated hole [28].  
During the laser ablation of the CNT film, laser-induced air breakdown and material vaporization in the holes created an enormous pressure, which will caused the ejection of the vaporized material from the ablated hole. The ejection material was rapidly condensed on the edge of hole under the effect of cooling air. Since the CNT film was composed of a large number of CNTs and oligomers, the fractured CNTs caused by laser ablation were thrown outside the hole under the pressure to form a projection of the outer edge of the CNT ablation hole.  
At the same time, after forming a through hole, the plasma formed by the laser-induced air breakdown suppressed the corrosion flowing back to the inlet of the ablation hole. As such, most of the vapor escaped from the bottom of the ablation hole. This was related to the carbon-carbon bond in the low-energy destruction CNT layer described in the literature [29]. At a higher laser pulse energy and number of pulses, the nanotubes were transformed into different nanocarbon forms, e.g., carbon nanoparticles and small carbon clusters [30].