Picotesla-sensitivity microcavity optomechanical magnetometry

Cavity optomechanical systems have enabled precision sensing of magnetic fields,by leveraging the optical resonance-enhanced readout and mechanical resonance-enhanced response.Previous studies have successfully achieved mass-produced and reproducible microcavity optomechanical magnetometry(MCOM)by incorporating Terfenol-D thin films into high-quality(Q)factor whispering gallery mode(WGM)microcavities.However,the sensitivity was limited to 585 pT Hz^(−1/2),over 20 times inferior to those using Terfenol-D particles.In this work,we propose and demonstrate a high-sensitivity and mass-produced MCOM approach by sputtering a FeGaB thin film onto a high-Q SiO2 WGM microdisk.Theoretical studies are conducted to explore the magnetic actuation constant and noise-limited sensitivity by varying the parameters of the FeGaB film and SiO2 microdisk.Multiple magnetometers with different radii are fabricated and characterized.By utilizing a microdisk with a radius of 355μm and a thickness of 1μm,along with a FeGaB film with a radius of 330μm and a thickness of 1.3μm,we have achieved a remarkable peak sensitivity of 1.68 pT Hz^(−1/2)at 9.52 MHz.This represents a significant improvement of over two orders of magnitude compared with previous studies employing sputtered Terfenol-D film.Notably,the magnetometer operates without a bias magnetic field,thanks to the remarkable soft magnetic properties of the FeGaB film.Furthermore,as a proof of concept,we have demonstrated the real-time measurement of a pulsed magnetic field simulating the corona current in a high-voltage transmission line using our developed magnetometer.These high-sensitivity magnetometers hold great potential for various applications,such as magnetic induction tomography and corona current monitoring.

Coupling ideality of standing-wave supermode microresonators

Standing-wave supermode microresonators that are created through the strong coupling between counter-propagating modes have emerged as versatile platforms for sensing and nonlinear optics.For example,these microresonators have shown potential in nanoparticle sizing and counting,as well as enhancing the single-photon optomechanical coupling rate of stimulated Brillouin scattering.However,it has been observed that the relation between the mode linewidth and on-resonance transmission of the split supermodes differs obviously from that of the non-split modes.This behavior is typically quantified by the coupling ideality(I),which remains inadequately explored for the standing-wave supermodes.In this study,we theoretically and experimentally investigate the coupling ideality of standing-wave supermodes in a commonly employed configuration involving a Si O2microresonator side-coupled to a tapered fiber.Our findings demonstrate that,even with a single-mode tapered fiber,the coupling ideality of the standing-wave supermodes is limited to 0.5,due to the strong backscattering-induced energy loss into the counter-propagating direction,resulting in an additional equivalent parasitic loss.While achieving a coupling ideality of 0.5 presents challenges for reaching over-coupled regimes,it offers a convenient approach for adjusting the total linewidth of the modes while maintaining critically-coupled conditions.

Micropascal-sensitivity ultrasound sensors based on optical microcavities

Whispering gallery mode(WGM)microcavities have been widely used for high-sensitivity ultrasound detection,owing to their optical and mechanical dual-resonance enhanced sensitivity.The ultrasound sensitivity of the cavity optomechanical system is fundamentally limited by thermal noise.In this work,we theoretically and experimentally investigate the thermal-noise-limited sensitivity of a WGM microdisk ultrasound sensor and optimize the sensitivity by varying the radius and a thickness of the microdisk,as well as using a trench structure around the disk.Utilizing a microdisk with a radius of 300μm and thickness of 2μm,we achieve a peak sensitivity of 1.18μPa Hz^(-1/2)at 82.6 k Hz.To the best of our knowledge,this represents the record sensitivity among cavity optomechanical ultrasound sensors.Such high sensitivity has the potential to improve the detection range of air-coupled ultrasound sensing technology.

Free spectral range magnetic tuning of an integrated microcavity

Tunable whispering-gallery-mode(WGM)microcavities are promising devices for reconfigurable photonic applications such as widely tunable integrated lasers and reconfigurable optical filters for optical communication and information processing.Scaling up these devices demands the ability to tune the optical resonances in an integrated manner over a full free spectral range(FSR).Here we propose a high-speed full FSR magnetic tuning scheme of an integrated silicon nitride(Si_(3)N_(4))double-disk microcavity.By coating a magnetostrictive film on the spokes and the central pad of the Si_(3)N_(4) cavity,magnetic tuning can be realized using a microcoil integrated on the same chip.An FSR tuning can be achieved by combining magnetostrictive strain with strong optomechanical interactions provided by the double-disk microcavity.We calculate the required magnetic flux density to tune an FSR(B_(FSR))as a function of several key geometric parameters,including the air gap,radius,width of the spokes and ring of the double-disk cavities,as well as the thickness of the magnetostrictive film.The proposed structure enables a full FSR tuning with a required magnetic flux density of milli-Tesla(mT)level.We also study the dynamic response of the integrated device with an alternating current(AC)magnetic field driving,and find that the tuning speed can reach hundreds of kHz in the air.