*4.2.2 Femtosecond laser irradiation method*

The femtosecond laser irradiation used to generate NV defects can cause strong damage to diamond, even more leading to graphitization. In 2017, Chen et al. used a single-pulse laser writing to controllably generate NV centers on the diamond surface without the occurrence of macroscopic crystal graphitization, which promoted the development of femtosecond laser irradiation for NV defect generation [50].

The principle of the femtosecond laser method shown in **Figure 10a** is that electron beam, generated by ionizing the air due to the strong electric field formed by the propagation of the femtosecond laser, can lead to the generation of vacancies in diamond. After annealing process, the produced vacancies can combine with nitrogen atoms to form NV defects [53]. Compared with ion implantation, the NV defect generated by femtosecond laser irradiation method possesses a narrower optical linewidth and a longer decoherence time. The NV color centers in diamond exhibit a narrow optical linewidth of 13.05 ± 0.2 MHz and a long decoherence time of 445 ± 27.6 μs [51].
