## 3 Results and Discussions
### Femtosecond Laser Cutting CNT Film  
#### 3.3.1 Raman Analysis of Femtosecond Laser Cutting CNT Film  
Raman spectroscopy is a crucial technique for the characterization of materials such as graphene and CNTs. It can be used to probe the interactions between the polymer chains and the nanotubes in carbon-based composites, because Raman scattering is resonantly enhanced and induces strong bands in a polymer matrix. Therefore, SEM images and Raman spectroscopy are effective methods for  
Figure 5: (**a**) Surface morphology of hole under \(E=25\) nJ ablation; (**b**) schematic illustration on the ablation mechanism with different pulse energies; and (**c**) surface morphology of hole under \(E=165\) nJ ablation. (The direction of double arrows in the figure is the polarization direction of the laser). analyzing the structural changes of CNTs after femtosecond laser processing. First, we performed Raman spectroscopy on different areas of the unprocessed material and obtained their Raman spectrum, as shown in Figure 6. Sometimes under certain irradiation conditions, the bands in Raman spectra are affected by carbon deposition in the atmosphere. But in this paper, the Raman characteristic band is the area ratio after peak fitting, so we ignored the carbon deposition in the laser-induced air [31]. CNT film is a typical carbon material formed by stacking CNTs, and the CNTs are formed by curling different layers of graphene. In order to determine the uniformity of the material, the Raman spectra of unprocessed CNT film of composites irradiated are displayed in Figure 6. There were two typical Raman characteristic bands: The D band was around 1350 cm\({}^{-1}\), and a high frequency shoulder of the D\({}^{\prime}\) band was located around 1623 cm\({}^{-1}\)[32]. Though the two characteristic bands were all double resonance processes, the two characteristic bands were formed in different ways. The degree of defects in CNT films was reflected in their Raman D and D\({}^{\prime}\) bands. The D\({}^{\prime}\) band appeared on the right side of the G band, and the intensity was slightly lower than D band [33]. The characteristic G band formed as a single resonance process was located near 1582 cm\({}^{-1}\). The intensity of the G band was correlated with the thickness of the detected CNT films, and the ratio of the D band to the G band (\(I_{\rm D}/I_{\rm G}\)) could characterize the defect density. At different positions of the sample, the D band of 1350 cm\({}^{-1}\) appeared to reflect the defects and disorder of CNTs. Compared with the highly oriented pyrolytic graphite (HOPG), the G band had no Raman shift. Therefore, the information of Raman spectral characteristic showed a certain amount of CNTs. The typical characteristic bands D and G were fitted by the least-squares method using the Lorentz line, and the relative area ratio \(I_{\rm D}/I_{\rm G}\) was between 0.5 and 0.52. This indicates that, although the CNT film had certain defects, the uniformity of the CNT film was good. It was observed by SEM that there were many CNTs coated with binders on the surface of the CNT film, which is consistent with the results obtained by Raman spectroscopy. In the cutting process, the photon energy of the short wavelength was higher than that of the long wavelength under the same pulse energy and spot size, so the laser with shorter wavelength had more energy to remove the material. Additionally, the CNT film absorbed more laser light with a shorter wavelength [34]. Therefore, a femtosecond laser with \(\lambda\) = 515 nm and \(\lambda\) =1030 nm was selected to analyze the effect of wavelength on cutting quality. The other parameters were: A pulse duration of 300 fs, a repetition frequency of 40 kHz, a pulse energy of 100 nJ, and a scanning speed of 0.1 mm/s (The polarization direction of the laser was perpendicular to the laser scanning direction).