Fast, broad-band magnetic resonance spectroscopy with diamond widefield relaxometry

We present an alternative to conventional Electron Paramagnetic Resonance (EPR) spectroscopy equipment. Avoiding the use of bulky magnets and magnetron equipment, we use the photoluminescence of an ensemble of Nitrogen-Vacancy centers at the surface of a diamond. Monitoring their relaxation time (or T1), we detected their cross-relaxation with a compound of interest. In addition, the EPR spectra are encoded through a localized magnetic field gradient. While recording previous data took 12 minutes per data point with individual NV centers, we were able to reconstruct a full spectrum at once in $3\; \textrm{seconds}$, over a range from $3$ to $11\; \textrm{Gauss}$. In terms of sensitivity, only $0.5\; {\mu}\textrm{L}$ of a $1\; {\mu}\textrm{M}$ hexaaquacopper (II) ion solution was necessary.