Creation and Engineering of Nitrogen-Related Defects in Diamond with Nanosecond Ultraviolet Laser Pulses

Color centers in diamond have been extensively investigated for their potential applications in quantum computing, quantum communication networks, and quantum sensing. Multiple methods have been proved to be effective in generating the diamond color centers, including ion implantation, energetic particle irradiation, and ultrafast laser writing. Ultrafast laser writing is promising due to its efficient defect creation, three-dimensional position control, and minimal damage in the surrounding host lattice. This method typically uses intense laser pulses with femtosecond pulse widths causing nonlinear interactions between the pulses and the diamond lattice. Most research has been focused on the femtosecond lasers. Using nanosecond lasers to generate color centers has not been sufficiently explored. Compared to femtosecond lasers, nanosecond lasers have prolonged irradiation and may provide extended periods of both free carrier generation and diffusion as well as expanded local annealing. These effects could enhance the ability to control the creation and activation of color centers. Presently, there is not a color center fabrication approach that works for all quantum applications. As work on diamond color centers moves from fundamental research to manufacturing, nanosecond lasers are a promising method to generate color centers.
In this work, we explored the nanosecond laser's capability and effectiveness to controllably generate color centers in diamond. The nanosecond laser irradiation was performed using the fourth harmonic of a Nd:YAG laser system (center wavelength of 266 nm, pulse width of 6 ns) and a 3×3×0.25 mm single crystal CVD diamond (nitrogen concentration [N] < 1 ppm, Element Six) in a vacuum chamber (1 × 10^-6 Torr). The laser pulses were focused through the objective lens resulting in a laser spot with a diameter of 50 µm on the sample surface. The depth of focus of the optical system was approximately 1 mm, thus the shape and size of the laser spot is constant through the sample depth. The creation and engineering of various nitrogen-related color centers was monitored by Raman and photoluminescence spectroscopy under both confocal and non-confocal conditions. Color centers such as NV⁰, H3 ([N-V-N]⁰), and self-interstitials associated with nitrogen atoms and vacancies were generated by the laser irradiation without additional annealing. We observed that concentration distribution patterns of these color centers were formed around the laser irradiated spots on the diamond sample. These patterns were constant through the depth of the sample. The concentrations and patterns of the engineered color centers can be tuned by both the fluence of a single laser pulse and the number of laser pulses. Generally, when laser irradiation varied from low pulse fluence and a low pulse number (e.g., ~1 J/cm² × 1 pulse) to high pulse fluence and a high pulse number (e.g., ~8 J/cm² × 40 pulses), the size and shape of the concentration patterns changed accordingly. We are currently evaluating the impact of the laser irradiation on the diamond lattice quality by assessing the diamond peak in Raman spectroscopy. A qualitative model involving exciton creation, diffusion, and relaxation was proposed to explain the above phenomena.
Our results demonstrate the viability of using nanosecond pulsed laser to create and modify nitrogen-related defects in diamond. The model we propose contributes to elucidating the mechanisms of the interaction between diamond defects and laser pulses with a nanosecond pulse width. Our work provides insight into a potential new method to fabricate future color center-based qubits such as NV^-, SiV^-, and NiV^- centers.