Origin of the sharp cwODMR contrast dip at y-axis field alignment

Determine the physical mechanism responsible for the reproducible sharp reduction in continuous-wave optically detected magnetic resonance (cwODMR) contrast of the f_A and f_B transitions of the S=1 carbon-related spin defect in hexagonal boron nitride when the external magnetic field is applied directly along the defect’s y axis, as observed during rotation of a 51 mT bias field in the yz plane.

Background

The paper investigates a low-symmetry S=1 carbon-related spin defect in hexagonal boron nitride (hBN) for vectorial quantum magnetometry. The defect exhibits three ground-state spin transitions (f_A, f_B, f_C) with cwODMR contrast that persists under arbitrarily oriented magnetic fields, a key advantage over NV centers in diamond.

In angle-resolved measurements with a 51 mT bias field rotated in the yz and xy planes, the authors observe systematic changes in cwODMR contrast as a function of field orientation. Specifically, when the bias field aligns directly along the defect y axis, the cwODMR contrast of f_A and f_B shows a sharp dip that is reproducible across measurements.

The authors' photophysical model captures many features of the field-dependent ODMR behavior; however, the specific cause of the sharp contrast dip at exact y-axis alignment remains unidentified, indicating an unresolved mechanism in the defect’s spin-photodynamics under this configuration.