Buckled Fabry-Perot cavity magnetometer reaches 2 pT/Hz^{1/2} sensitivity via GMM substrate actuation

The sensor exploits substrate bending: external fields expand the GMM layer, flexing the chip and modulating cavity length at mechanical resonances. This decouples GMM volume from optical mode volume, allowing a compact, low-order cavity with Q around 10^3 and high optomechanical coupling for simple slope readout. Fabrication used thin-film buckling on fused silica followed by direct bonding, yielding devices operable at room temperature without cryogenic requirements typical of SQUID systems.

Peak performance of 2 pT/Hz^{1/2} was measured in ordinary lab conditions, with thermomechanical noise projected to support 0.1 pT/Hz^{1/2} once laser and electronics noise are reduced. The design avoids the size-sensitivity trade-offs seen in fiber-based or MEMS magnetometers by keeping the optical cavity small while scaling the magnetostrictive actuator independently. Related work on cavity optomechanics for force sensing shows similar resonance-enhanced coupling but rarely integrates macroscopic GMM actuators.

Key limitation is the current readout noise floor, not fundamental thermomechanical limits. Strengthening evidence requires shielded tests quantifying actual thermal noise and direct comparisons against fluxgate or NV-center sensors at identical frequencies. Scalability to arrays for biomedical or geophysical mapping follows if bonding yield and laser stabilization improve.

Future integration with photonic chips could enable portable, low-power magnetometry for unshielded environments where SQUIDs remain impractical.