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Ultrawide Bandwidth Optomechanical Magnetometry Using Flux Concentration

Benjamin J. Carey, Nathaniel Bawden, Fernando Gottardo, James S. Bennett, Douglas Bulla, Scott Foster, Warwick P. Bowen

Abstract

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 below 20 nT Hz$^{-1/2}$ down to 3 Hz and less than 100 nT Hz$^{-1/2}$ 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.

Ultrawide Bandwidth Optomechanical Magnetometry Using Flux Concentration

Abstract

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 below 20 nT Hz down to 3 Hz and less than 100 nT Hz 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.
Paper Structure (14 sections, 8 equations, 4 figures)

This paper contains 14 sections, 8 equations, 4 figures.

Table of Contents

  1. Introduction
  2. Flux concentration
  3. Magnetometer Design
  4. Design of flux concentrators
  5. Mathematical description of flux concentration
  6. Numerical modeling of flux concentration
  7. Fabrication of flux concentrators
  8. Experimental setup
  9. Sensitivity enhancement
  10. Relative position
  11. Height dependence and field decay
  12. Operational bandwidth
  13. Nonlinear mixing based sensing
  14. Discussion and Conclusions

Figures (4)

  • Figure 1: a) Schematic of the experimental setup. A Helmholtz coil produces a magnetic field that is directed onto the sensor by the flux concentrator. Inset: Device architecture. b) 2D cross-section of COMSOL simulation, demonstrating the magnetic field lines and relative flux density. c) Measured power spectra of a 590 kHz 21 µT applied field with (conc.) and without (no conc.) the flux concentrator, with signal-to-noise ratio (SNR) and enhancement factor (EF) emphasized. Here the resolution bandwidth (RBW) is 1.97 kHz. The noise floor is indicated by the gray shaded region.
  • Figure 2: Heat-map of the magnetometer sensitivity as the position of the flux concentrator is varied. The size of the galfenol and flux concentrator are indicated by the dashed lines.
  • Figure 3: (a) Sensitivity spectra of the magnetometer for different flux concentrator heights above the device. (b) Measured and modeled enhancement of peak magnetic sensitivity as a function of flux concentrator height. Here, both the mathematical model, and FEM simulations results are determined using the geometry and permeability of the metglas flux concentrator.
  • Figure 4: a) Wide frequency spectrum of the magnetometer sensitivity from 1 MHz down to 1 Hz. The flux concentrator was approximately 10 µ m above the device and centered over the cantilever. The different colors represent different measurements. b) Low-frequency sensitivity using nonlinear mix-up. Inset: Power spectral density around the 590 kHz mix-up frequency for 2 µ T RMS and 21 µ T RMS signal fields at 300 Hz.