Date Published: June 14, 2018
Publisher: AIP Publishing LLC
Author(s): Eugene Freeman, Cheng-Yu Wang, Vedant Sumaria, Steven J. Schiff, Zhiwen Liu, Srinivas Tadigadapa.
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.
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
The chip-scale glassblowing process was first pioneered by Eklund and Shkel15 for mechanical resonance applications. The process was modified and adapted for optical resonance as published in our earlier work,14 which provides more detail on the fabrication process and WGM resonance achievable using these chip-scale microbubbles.
In conclusion, a 1.1 × 107 Q-factor chip-scale whispering gallery mode magnetometer was experimentally demonstrated with a sensitivity of 1.43 GHz/mT. The phenomenon is determined to be mostly due to mechanical deformation of the radius of the borosilicate microbubble with a small contribution from the photoelastic effect. An experimental limit of detection of 60 nT Hz−1/2 was measured for our current setup and determined to be dominated by laser noise. A thermorefractive analysis for a fused silica microbubble predicts an ultimate limit of detection of 52 pT Hz−1/2 at 100 Hz is possible at room temperature.