Ultrahigh concentration of NV⁻ centers embedded in the CVD epi-diamond layer near the interface with an HPHT diamond substrate

The negatively charged nitrogen vacancy (NV⁻) center ensemble in as-grown chemical vapor deposition (CVD) diamond is a promising candidate for quantum sensing due to its long coherence time and excellent optical properties. However, achieving a high concentration of NV⁻ centers in as-grown CVD diamond remains a critical challenge, which constrains the performance of NV⁻ based sensors. In this study, we observe that NV⁻ center formation efficiency is significantly enhanced during the initial growth phase, with a coherence time $T_2^{*}$ of 1.1 μs. These findings demonstrate that high-concentration NV⁻ centers can be achieved in as-grown diamonds, greatly enhancing their utility in high-performance magnetometers and quantum sensing.