Understanding the magnetic resonance spectrum of nitrogen vacancy centers in an ensemble of randomly-oriented nanodiamonds

TL;DR

This study provides a comprehensive analysis of the magnetic resonance properties of nitrogen vacancy centers in nanodiamonds with random orientations, identifying orientation-insensitive transitions for potential imaging and hyperpolarization applications.

Contribution

It introduces a detailed spectral analysis of NV- centers in randomly oriented nanodiamonds and highlights overtone transitions suitable for high-resolution imaging and hyperpolarization.

Findings

Identification of orientation-insensitive overtone transitions

Numerical simulations accurately describe the spectrum across regimes

Potential for efficient nuclear hyperpolarization using frequency-swept DNP

Abstract

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. We evaluate the prospects of nanodiamond ensembles specifically for nuclear hyperpolarization and show that frequency-swept dynamic nuclear polarization may transfer a large amount of the NV- center's hyperpolarization to nuclear spins by sweeping over a small region of its spectrum.