| Summary: | The negatively charged nitrogen-vacancy(NV<sup>−</sup>) defect in diamond has many applications such as sensing and nanoscale imaging. It is recently becoming a promising
candidate in quantum technologies such as quantum computing and quantum communication. Ultrafast laser fabrication of NV<sup>−</sup> in diamond has proven its ability to generate NV<sup>−</sup>with demanding position accuracy with minimal lattice damage. However, there is a possibility that the formed NV<sup>−</sup> would be destroyed and is assumed to be the consequence of the presence of the nearby interstitial carbon. The GR1 centre in diamond is a common optical centre attributed to the neutral monovacancy of the carbon atom (<em>V</em><sup>0</sup>). In this thesis, <em>ab initio</em> simulations are conducted to investigate the ground state electronic interaction between interstitial carbon and neutral monovacancy, interstitial carbon and negatively charged nitrogen-vacancy in diamond. The results suggest that their optical properties might change due to hybridized orbitals within them. Additionally, a carbon-nitrogen machine-learning potential is developed for investigating the diffusion mechanism of interstitial carbon with the presence of NV<sup>−</sup> centre. According to the molecular dynamics simulations, it could either diffuse away or recombine with the position of vacancy, depending on its initial position and diffusion time. Furthermore, linear-scaling time-dependent density functional theory (TDDFT) is performed, trying to interpret the changes in the optical spectrum during laser annealing where various levels of hybridized orbitals are observed, potentially mapping the difference of photoluminescence signals.</p>
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