Abstract
A whispering gallery mode resonator based magnetometer using chip-scale glass microspherical shells is described. A neodynium micro-magnet is elastically coupled and integrated on top of the microspherical shell structure that enables transduction of the magnetic force experienced by the magnet in external magnetic fields into an optical resonance frequency shift. High quality factor optical microspherical shell resonators with ultra-smooth surfaces have been successfully fabricated and integrated with magnets to achieve Q-factors of greater than 1.1 × 107 and have shown a resonance shift of 1.43 GHz/mT (or 4.0 pm/mT) at 760 nm wavelength. The main mode of action is mechanical deformation of the microbubble with a minor contribution from the photoelastic effect. An experimental limit of detection of 60 nT Hz−1/2 at 100 Hz is demonstrated. A theoretical thermorefractive limited detection limit of 52 pT Hz−1/2 at 100 Hz is calculated from the experimentally derived sensitivity. The paper describes the mode of action, sensitivity and limit of detection is evaluated for the chip-scale whispering gallery mode magnetometer.
Highlights
INTRODUCTIONLow cost magnetic sensing for bio-magnetic applications are being pursued over a wide range of technologies including atomic magnetometers,[1] nitrogen-vacancy in diamond magnetometers,[2,3] and magnetoelectric magnetometers.[4,5]
Room temperature, low cost magnetic sensing for bio-magnetic applications are being pursued over a wide range of technologies including atomic magnetometers,[1] nitrogen-vacancy in diamond magnetometers,[2,3] and magnetoelectric magnetometers.[4,5]Recently some efforts have been made to take advantage of optical resonance methods of sensing to probe magnetic fields.[6,7,8,9,10] One such strategy is to immerse a fiber based resonator into a magnetic fluid, and quantify the spectral response while applying an external magnetic field
Whispering gallery mode (WGM) optical resonators have demonstrated exceptional sensitivity to mechanical deformation, with shot noise limited displacement sensitivities of 10−19 m Hz−1/2 experimentally measured at room temperature.[12]
Summary
Low cost magnetic sensing for bio-magnetic applications are being pursued over a wide range of technologies including atomic magnetometers,[1] nitrogen-vacancy in diamond magnetometers,[2,3] and magnetoelectric magnetometers.[4,5]. Whispering gallery mode (WGM) optical resonators have demonstrated exceptional sensitivity to mechanical deformation, with shot noise limited displacement sensitivities of 10−19 m Hz−1/2 experimentally measured at room temperature.[12] Recently, Forstner et al.[10] used a toroidal WGM resonator with a Terfenol-D bead to experimentally demonstrate an optomechanical method of magnetic sensing with 400 nT Hz−1/2 limit of detection at mechanical resonance of approximately 10 MHz, which is too fast to detect biomagnetic signals. We use recently developed chip-scale glass microspherical shells as the high quality factor WGM resonator platform[14] and integrate with a micro-magnet to couple magnetic force into mechanical deformation of the microbubble. We estimate the thermorefractive limited magnetic limit of detection for a fused silica microbubble
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