The Nitrogen-Vacancy-Nitrogen Color Center: A Ubiquitous Visible and Near-Infrared-II Quantum Emitter in Nitrogen-Doped Diamond

Photoluminescent defects in diamond, such as the nitrogen-vacancy (NV) color center, are at the forefront of emerging optical quantum technologies. Most emit in the visible and near-infrared spectral region below 1000 nm (NIR-I), limiting their applications in photonics, fiber communications, and biology. Here, we show that the nitrogen-vacancy-nitrogen (N₂V) center, which emits in the visible and near-infrared-II (NIR-II, 1000-1700 nm), is ubiquitous in as-synthesized and processed nitrogen-doped diamond, ranging from bulk samples to nanoparticles. We demonstrate that N₂V is also present in commercially available state-of-the-art NV diamond sensing chips made via chemical vapor deposition (CVD). In high-pressure high-temperature (HPHT) diamonds, the photoluminescence (PL) intensity of both N₂V charge states, N₂V⁰ in the visible and N₂V^- in the NIR-II, increases with increasing substitutional nitrogen concentration. We determine the PL lifetime of N₂V^- to be 0.3 ns and compare a quantum optical and density functional theory model of the N₂V^- with experimental PL spectra. Finally, we show that detonation nanodiamonds (DND) exhibit stable PL in the NIR-II, which we attribute to the N₂V color center, and use this NIR-II PL to image DNDs inside skin cells. Our results contribute to the scientific and technological exploration and development of the N₂V color center and help elucidate interactions with other color centers in diamond.