Silicon-photonic optomechanical magnetometer

TL;DR

The paper tackles the challenge of realizing chip-scale, room-temperature optomechanical magnetometers that are compatible with photonics and electronics. It introduces silicon-on-insulator magnetometers enabled by a post-release lithography workflow that preserves functional galfenol films on released structures, enabling co‑integration of magnetostrictive actuation with photonic crystal cavities. The photonic-crystal slot cavities provide strong transduction, achieving a coupling rate of $G_{OM} \approx 99.8~\\mathrm{GHz}/\\mathrm{nm}$ and a slot-cavity Q around $10^3$, with a transduction gain of about $27\times$ over prior designs. A peak magnetic-field sensitivity of $800~\\mathrm{pT}/\\sqrt{\\mathrm{Hz}}$ is demonstrated at $243~\\mathrm{kHz}$, illustrating a viable path to scalable, room-temperature, on-chip magnetometers for applications in biomedical imaging, navigation, and geophysical surveying.

Abstract

Optomechanical sensors enable exquisitely sensitive force measurements, with emerging applications across quantum technologies, standards, fundamental science, and engineering. Magnetometry is among the most promising applications, where chip-scale optomechanical sensors offer high sensitivity without the cryogenics or magnetic shielding required by competing technologies. However, lack of compatibility with integrated photonics and electronics has posed a major barrier. Here we introduce silicon-on-insulator optomechanical magnetometers to address this barrier. A new post-release lithography process enables high-quality metallisation of released mechanical structures, overcoming the incompatibility between silicon-on-insulator fabrication and functional magnetic films. This allows us to employ photonic-crystal cavities that enhance motion-to-optical signal transduction by over an order of magnitude. The resulting devices achieve magnetic field sensitivity of 800 pT Hz^-1/2, three orders of magnitude beyond previous waveguide-integrated designs. The advances we report provide a path towards high-performance, room temperature and chip-integrated magnetometers for applications ranging from biomedical imaging and navigation to resource exploration.

Figures (4)

• Figure 1: Overview of the post-release fabrication process. A freestanding optomechanical device is flooded with diluted resist solution (AZ + EBR) prior to photolithography and galfenol deposition. The excess galfenol and structure supporting resist are removed via a magnetically-assisted lift-off process, followed by critical point drying, resulting in the final device.
• Figure 2: a) Schematic diagram of the device design and working principle. A magnetic field ($\vec{B}$) induces deformation of a galfenol (yellow) platform, which pulls on a silicon (blue) slot cavity. The resulting in-plane deformation of the cavity ($\delta w$) induces a change in its optical resonance wavelength ($\delta \lambda$). This can be detected by probing the reflected light. The red arrows indicate coupling of the optical mode to an on-chip waveguide. b) False color SEM image of a magnetometer with close-up images of the slot cavity. Silicon, blue; galfenol, yellow.
• Figure 3: Finite element method modeling with COMSOL. a) Modeshape of the fundamental out-of-plane vibrational mode. b) Intensity distribution of the 1D photonic slot cavity's optical mode (normalised by the peak intensity).
• Figure 4: Magnetometer characterization. a) Experimental setup for sensitivity measurements. b) Optical spectrum reflected from the slot cavity. The Lorentzian fit (dashed) gives an optical quality factor of 930. c) Power spectrum showing the mechanical mode and the signal from an applied field measured with a 30 Hz RBW. d) Sensitivity spectrum around the mechanical resonance. The black dashed line shows the sensitivity calculated from the thermomechanical noise model using the relevant parameter derived both experimentally and from the FEM model