Dual resonances induced by amplitude-modulated light in a spin-exchange relaxation-free magnetometer

Spin-exchange relaxation-free (SERF) magnetometers enable femtotesla-level biomagnetic imaging, yet their scalability into high-density arrays is impeded by inter-sensor crosstalk arising from conventional magnetic-field modulation. While optical amplitude modulation offers a crosstalk-free alternative, its resonance mechanisms within the SERF regime remain unexplored. Here, we report a dual-resonance mechanism induced by amplitude-modulated light in a SERF magnetometer. Our theoretical model characterizes this behavior as the coexistence of a central zero-field resonance and symmetric parametric resonances at high atomic densities, in excellent agreement with experimental measurements. Systematic temperature and pump-power sweeps reveal the evolution of the dual-resonance profile, demonstrating that high alkali density sharpens and amplifies the zero-field resonance, whereas strong optical pumping restores the dominance of parametric resonances. Exploiting these distinct resonances, we utilize the parametric component to extract the slowing-down factor and spin polarization, which allows in situ coil calibration in both probe and pump channels with errors below 0.5%. In contrast, operation on the zero-field resonance significantly suppresses long-term drifts compared with DC detection and yields a sensitivity of 4.96 fT/Hz^1/2. Ultimately, these findings elucidate the modulation physics within the SERF regime and establish a crosstalk-free framework that paves the way for high-precision bio-magnetic arrays.