Color centers in diamond, such as nitrogen-vacancy (NV) and silicon-vacancy (SiV) centers, are attractive for quantum communication, quantum photonics, and bioscience applications. While NV centers possess a long coherence time at room temperature, their use as single-photon sources is limited by low zero-phonon line (ZPL) emission (around 4%) and strong inhomogeneous broadening due to their electric dipole moment. SiV centers, conversely, exhibit superior properties for single-photon emission, including a high ZPL (70%), a narrow ZPL, a high photon emission rate, and near-infrared emission (ZPL at 738 nm). These features make them ideal for photonic devices. Current SiV center fabrication methods, including ion implantation and chemical vapor deposition (CVD), have limitations: ion implantation causes structural damage requiring high-temperature annealing with low efficiency, while CVD struggles with precise high-temperature control. Neither method allows for in situ observation of color center introduction and vacancy migration. This research aims to provide fundamental thermodynamic and kinetic insights into SiV center formation by employing temporally controlled annealing, offering a novel annealing scheme to improve conversion efficiency and a guiding framework for synthesizing other color centers.

Existing literature extensively covers the synthesis of SiV centers in diamond using ion implantation and CVD methods. Ion implantation, while widely available, is inefficient and introduces substantial damage. CVD, although capable of producing high-quality diamonds, faces challenges in controlling the high-temperature growth environment and precise doping. High-pressure high-temperature (HPHT) methods using laser-heated diamond anvil cells (DACs) have been explored for synthesizing nanodiamonds with SiV centers using various precursors. Previous work has demonstrated the use of adamantane as a superior precursor for laser-induced HPHT diamond synthesis and nitrogen-functionalized adamantane's conversion to fluorescent diamonds at high pressure and moderate temperature. However, a detailed mechanistic study on the formation of SiV centers in diamond using temporally controlled annealing was lacking, motivating this current work.

This study directly synthesized SiV centers in diamond using a silicon-containing adamantane precursor (adamantylethyltrichlorosilane, AdaSi) and a HPHT approach in a laser-heated DAC. The DAC system allowed for mapping temperature and pressure conditions for SiV formation from AdaSi and exploration of various growth and annealing conditions. The system's transparency enabled in situ measurements using continuous wave (CW) and pulsed lasers for precise annealing control. Pressure was calibrated using ruby fluorescence and diamond Raman shifts. A 100-nm tungsten layer served as a laser absorber, deposited on mica, sandwiched with AdaSi to ensure homogeneous laser heating. The laser heating was temporally controlled using an acousto-optic modulator (AOM), achieving ultrafast heating and cooling rates. A novel ultrafast thermometry system with 1 µs temporal resolution was developed, utilizing the integrated intensity ratio of two radiation windows (900–1450 nm and 1550–2200 nm) to determine temperatures above 1000 K. The system's accuracy was estimated at ±50 K. A pressure-temperature synthesis diagram was constructed, and the formed diamond's quality was characterized *ex situ* using SEM, Raman spectroscopy, and EDS. To investigate the effects of annealing, both continuous wave (CW) and pulsed laser annealing treatments were performed. The pulse width and repetition frequency of the pulsed laser were systematically varied to find the optimal parameters. Finally, nudged elastic band (NEB) calculations using density functional theory (DFT) were conducted to understand SiV and vacancy diffusion dynamics, determining energy barriers and diffusivities.

The minimum temperature required to form SiV centers at 20 GPa was determined to be 1200 K. The formation energy was estimated to be above 0.18 kJ/cm². CW annealing resulted in a rapid decrease in SiV PL intensity, suggesting SiV center migration to the surface at elevated temperatures. In contrast, sub-µs pulsed annealing significantly enhanced SiV PL intensity. Pulsed annealing with a 200-ns pulse width at 50 kHz resulted in a 250% increase in SiV concentration, which remained stable for hours. NEB calculations revealed a significantly higher energy barrier for SiV diffusion (12.3 eV) compared to vacancy diffusion (2.7 eV). This difference in diffusion rates explains the observed stabilization of SiV centers during pulsed annealing. The vastly different diffusion rates indicate that vacancies, uncoupled to silicon impurities, readily diffuse to the diamond surface while SiV centers remain stable in the lattice. The improved SiV PL intensity during pulsed annealing correlated with a narrowing of the ZPL peak, signifying enhanced diamond quality.

The findings directly address the research question of improving SiV center creation efficiency. The dramatic increase in SiV PL intensity with sub-µs pulsed annealing, compared to the rapid decay observed with CW annealing, highlights the importance of precise temporal control over annealing. The theoretical calculations corroborate the experimental observations, explaining the observed stability of SiV centers under pulsed annealing conditions. The vastly different diffusion coefficients between SiV centers and vacancies support the hypothesis that short-pulsed annealing minimizes the migration of SiV centers out of the lattice while allowing the formation of new centers. The results significantly advance SiV center fabrication, potentially impacting quantum technologies. The approach of using ultrafast thermometry and AOM-modulated laser heating, combined with NEB calculations, provides a robust methodology applicable to other color centers and other materials systems.

This study successfully demonstrated a novel sub-µs pulsed annealing treatment for significantly enhancing the creation of SiV centers in diamond. The 2.5-fold increase in SiV concentration achieved by this method represents a significant advancement in SiV center fabrication. The combination of experimental techniques and theoretical calculations provides a comprehensive understanding of the underlying dynamics. Future research could explore the optimization of pulse parameters for various SiV concentrations and explore the application of this pulsed annealing strategy to other impurity-vacancy centers in diamond and to other materials, such as perovskite solar cells and transition metal dichalcogenide-based transistors.

The study's primary limitation stems from the *ex situ* characterization of diamond quality. While this allowed detailed analysis using advanced techniques, it prevented direct real-time observation of defect migration and formation during the annealing process. The accuracy of the ultrafast thermometer (±50 K) introduces uncertainty in the precise temperature during the annealing. Furthermore, while the NEB calculations provided valuable insights into diffusion dynamics, they were performed at ambient pressure and might not fully capture the behavior at high pressure.