Ultrawide Bandwidth Optomechanical Magnetometry Using Flux Concentration

Low‐frequency magnetic fields carry vital information for neuroscience, navigation, and Earth science. However, they are generally weak, making it challenging to measure them with compact, room‐temperature magnetometers. To overcome this challenge, we combine an on‐chip optomechanical magnetometer with a high‐permeability flux concentrator. Beyond boosting sensitivity and bandwidth, exploiting the concentrator's nonlinear response converts low‐frequency magnetic fluctuations into higher‐frequency signals where the sensor is intrinsically most responsive. This sidesteps the technical noise that has long constrained the application of optomechanical magnetometry at low‐frequencies. Our measurements show order‐of‐magnitude improvements in sensitivity and extend performance into the sub‐hertz regime, achieving 20 nT/ down to 3 Hz and 100 nT/ at 0.1 Hz. Because this approach requires no redesign of the underlying architecture, it can be readily applied across magnetometer technologies, opening the way to practical low‐frequency sensing for applications from brain activity mapping to undersea navigation and biomedical diagnostics.