Electron Paramagnetic Resonance spectroscopy of a scheelite crystal using microwave photon counting

Counting the microwave photons emitted by an ensemble of electron spins when they relax radiatively has recently been introduced as a sensitive new method for electron paramagnetic resonance spectroscopy at millikelvin temperatures. Here, we apply this spin fluorescence method to a scheelite crystal of CaWO4, finding some known ($\mathrm{Er}^{3+}$, $\mathrm{Yb}^{3+}$, $\mathrm{Nd}^{3+}$ and $\mathrm{Fe}^{3+}$) and other unknown paramagnetic impurities. Investigating the zero nuclear spin isotope ($I=0$) transition of $\mathrm{Er}^{3+}:\mathrm{CaWO}_4$ as a model system, we provide a quantitative analysis of the time-dependent photon counting rate following an excitation pulse, as a function of its power. The achieved signal-to-noise ratio is found to be an order of magnitude higher than the one obtained by inductively-detected Hahn echo under identical conditions. Finally, we use spin fluorescence spectroscopy at low excitation power to probe the properties of rare-earth-ions close to a metallic wire deposited on the surface; our data reveal line distortion caused by the mechanical strain imparted by the thermal contractions of the metal relative to the underlying crystal. Coherent oscillations are also observed for the most highly strained ions.