Understanding the Magnetic Resonance Spectrum of Nitrogen Vacancy Centers in an Ensemble of Randomly Oriented Nanodiamonds

Nanodiamonds containing nitrogen vacancy (NV–) centers show promise for a number of emerging applications including targeted in vivo imaging and generating nuclear spin hyperpolarization for enhanced NMR spectroscopy and imaging. Here we develop a detailed understanding of the magnetic resonance behavior of NV– centers in an ensemble of nanodiamonds with random crystal orientations. Two-dimensional optically detected magnetic resonance spectroscopy reveals the distribution of energy levels, spin populations, and transition probabilities that give rise to a complex spectrum. We identify overtone transitions that are inherently insensitive to crystal orientation and give well-defined transition frequencies that access the entire nanodiamond ensemble. These transitions may be harnessed for high-resolution imaging and generation of nuclear spin hyperpolarization. The data are well described by numerical simulations from the zero- to high-field regimes, including the intermediate regime of maximum complexity. ...