Suppressing laser-power noise with a multifunctional liquid crystal polarization grating in miniaturized optically pumped magnetometers

Optically pumped magnetometers (OPMs) enable high-resolution biomagnetic imaging, yet conventional single-beam designs are constrained by bulky, alignment-intensive polarization optics and susceptibility to laser-power noise. Here, we propose and demonstrate a compact, noise-suppressed OPM that uses a single planar liquid-crystal polarization grating (LCPG) functioning simultaneously as a high-efficiency polarization converter and a beam splitter. At the component level, the LCPG replaces multiple bulk elements, converts 795 nm light with 95% first-order diffraction efficiency and an ellipticity of ~44.6°, and exhibits robust performance against variations in incident-light polarization direction, ambient temperature, and angle of incidence. At the sensor level, a power-differential configuration effectively suppresses noise originating from pump-power fluctuations. We fabricated a probe with a total volume of 4 cm³ and benchmarked it against a conventional OPM: the LCPG-enabled differential mode achieves a sensitivity of 8.6 fT/Hz^1/2, representing an ~28% improvement over the conventional configuration. Importantly, the approach is compatible with mature, high-throughput, and cost-effective liquid-crystal manufacturing. These results demonstrate a scalable design strategy for OPMs that unites component-level efficiency, robustness, and cost-effectiveness with sensor-level compactness and noise suppression, paving the way for next-generation chip-scale quantum sensors.