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.