Cumulative Mechanism and Nonlinear Propagation Effects of Nitrogen-Vacancy Center Formation Induced by Femtosecond Laser Writing.

Nitrogen-vacancy (NV) centers in diamond are a promising building block for quantum information and sensing technologies due to their excellent quantum properties and room-temperature operation. A key requirement to obtain quantum-grade NV centers lies in a controlled NV fabrication process. While ion implantation is the dominant fabrication process, laser irradiation has shown great potential, but it is still underexplored. In this work, we investigate the fabrication of NV centers using a 515 nm femtosecond laser with a repetition rate of 500 kHz in a single-step process with no separate seed and vacancy diffusion pulses and no post-annealing step. By varying the energy per pulse, laser exposure time (number of pulses) and focus depth, we identify how these parameters influence NV creation. We analyze the impact of laser-writing-associated processes such as graphitization, ablation, and self-focusing on NV creation, density, and photoluminescence. High energy density near the surface promotes graphitization and ablation, minimizing NV creation and altering NV creation depth. Conversely, self-focusing effects shift the NV creation toward the surface as pulse energy increases. By increasing the exposure time to more than 30 s, NVs can be created at even lower pulse energies (2.26 nJ) suggesting that cumulative processes play an important role in NV formation. Overall, these results provide insights into single-step NV laser writing and propose a framework for guiding optimized NV fabrication, in which the accumulation of structural and electronic precursors is proposed as a key mechanism contributing to NV formation.