Identifying NV center axes via spatially varying microwave fields for vector magnetometry

Vector magnetometry using an ensemble of nitrogen-vacancy (NV) centers holds great promise across a wide range of applications. Since NV centers are sensitive to external magnetic fields projected onto their crystallographic axes, distinguishing NV centers with different orientations is essential for fully extracting vectorial magnetic field information. While a calibrated magnetic bias field or optical polarization has been widely used for this purpose, the use of microwave fields remains relatively unexplored. Here, we demonstrate the identification of NV axes using the spatial variation of microwave magnetic fields generated by a microwave loop structure. Continuous-wave optically detected magnetic resonance scanned over the NV center ensemble shows distinct patterns that align well with a simplified model for each NV axis. After identifying the NV axes, we reconstruct the magnitude and orientation of the external magnetic field. This technique expands the potential for miniaturizing and enhancing NV-based vector magnetic sensors.