Microwave Magnetic Field Coupling with Nitrogen-Vacancy Center Ensembles in Diamond with High Homogeneity

Electron spin resonance measurements with nitrogen-vacancy (NV) center ensembles in diamond are strongly dependent on a uniform microwave magnetic field. Three different types of microwave resonators are used and are well coupled with the spin ensembles at 2.87 GHz for zero-field splitting of the NV centers. The magnitude and the uniformity of both the horizontal and vertical magnetic fields are extracted and analyzed within a 1 × 1 × 0.5 mm³ volume on a diamond sample surface, and the results indicate that the field homogeneity is up to 200 times better than that of the traditional copper wire microwave delivery model. The horizontal magnetic field magnitude homogeneity is better than 5 % over an area of 1 mm² on the thin film diamond sample with NV ensembles. The average Rabi oscillation frequency is estimated to be 2.3 MHz per 1 W of microwave input power upon strong coherent coupling between the resonators and the spin ensembles. The effect of the nonuniform microwave magnetic field on the spin signal is also discussed. The approach used here will find widespread application in microwave coupling with spin ensembles in thin films.