arXiv subjects

Benjamin J. Carey

Publications and source records attributed to Benjamin J. Carey.

9 recordsLinked to original sources

arXiv2026

Ultrawide Bandwidth Optomechanical Magnetometry Using Flux Concentration

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.

physics.optics↗

arXiv2025

Silicon-photonic optomechanical magnetometer

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.

physics.optics↗

arXiv2025

Precision optomechanical accelerometer via hybrid test mass integration

Accelerometers offer motion sensing capabilities across a wide range of areas, enabling navigational awareness in consumer goods and defense applications, and playing a key role in monitoring and control systems. To date, on-chip accelerometers have largely utilized a single device layer or substrate as a test mass. This constrains the test mass to the dimensions and density of the device layer or substrate, ultimately limiting the sensitivity of the device. We demonstrate a new approach which utilizes a pick-and-place bonding technique to increase the test mass of an on-chip accelerometer. By bonding a high-density platinum sphere to a nanomechanical silicon nitride trampoline membrane, we achieve a quality factor of 1900 in air with 95 mg test mass, corresponding to a thermomechanical noise limited acceleration sensitivity of $0.8\,\mathrm{n}g/\sqrt{\mathrm{Hz}}$. We optically probe the device's response to applied accelerations with increasing level of acoustic and vibration isolation, measuring a peak sensitivity of $5.5\,\mathrm{n}g/\sqrt{\mathrm{Hz}}$ at 117 Hz in air, limited by environmental vibrations. This represents the best peak sensitivity reported using a chip-integrated test mass.

physics.app-ph↗

arXiv2025

Silicon Double-Disk Optomechanical Resonators from Wafer-Scale Double-Layered Silicon-on-Insulator

Whispering gallery mode (WGM) optomechanical resonators are a promising technology for the simultaneous control and measurement of optical and mechanical degrees of freedom at the nanoscale. They offer potential for use across a wide range of applications such as sensors and quantum transducers. Double-disk WGM resonators, which host strongly interacting mechanical and optical modes co-localized around their circumference, are particularly attractive due to their high optomechanical coupling. Large-scale integrated fabrication of silicon double-disk WGM resonators has not previously been demonstrated. In this work we present a process for the fabrication of double-layer silicon-on-insulator wafers, which we then use to fabricate functional optomechanical double silicon disk resonators with on-chip optical coupling. The integrated devices present an experimentally observed optical quality factors of the order of 10^5 and a single-photon optomechanical coupling of approximately 15 kHz.

physics.optics↗

arXiv2025

Fibre-coupled photonic crystal hydrophone

Many applications, including medical diagnostics, sonar and navigation rely on the detection of acoustic waves. Photonic hydrophones demonstrate comparable sensitivity to piezoelectric-based hydrophones, but with significantly reduced size, weight and power requirements. In this paper we demonstrate a micron-sized free-standing silicon photonic hydrophone. We demonstrate sensitivity on the order of $\sim$mPa/$\sqrt{\textrm{Hz}}$ from 10-200 kHz, with a minimum detectable pressure of 145 $μ$Pa/$\sqrt{\textrm{Hz}}$ at 22 kHz. We also deployed our hydrophone in a wave flume to evaluate its suitability for underwater measurement and communication. Our hydrophone matches the sensitivity of commercial hydrophones, but is many orders of magnitude smaller in volume, which could enable high spatial resolution imaging of micron-sized acoustic features (i.e., living cell vibrations). Our hydrophone could also be used in underwater communication and imaging applications.

physics.optics↗

arXiv2023

Quantitative Profilometric Measurement of Magnetostriction in Thin-Films

A DC non-contact method for measuring the magnetostrictive strain in thin-films is demonstrated, achieving a state-of-the-art sensitivity of 0.1 ppm. In this method, an optical profilometer is used to measure the curvature induced in a magnetostrictively coated coverslip under a DC field through phase-sensitive interferometry. From this the magnetostrictive stress and strain are calculated using Stoney's formula. This addresses limitations of conventional techniques that measure magnetostriction based on the deflection of a cantilever under an AC field, which require complex dedicated set-ups and are sensitive to vibrational noise. Further, it reveals information about the anisotropy of the film and allows for the possibility of measuring multiple samples simultaneously. The theoretical sensitivity limits are derived, predicting a shot-noise-limit of 0.01 ppm. The method is implemented to measure the magnetostrictive hysteresis and piezomagnetic coupling of thin-film galfenol. Degradation in film performance is observed above a thickness of 206 nm, alongside a change in coercivity. This prompts investigation into the growth and optimization of galfenol films for use in devices.

physics.app-ph↗

arXiv2023

Waveguide-integrated and portable optomechanical magnetometer

Optomechanical magnetometers enable highly sensitive magnetic field sensing. However, all such magnetometers to date have been optically excited and read-out either via free space or a tapered optical fiber. This limits their scalability and integrability, and ultimately their range of applications. Here, we present an optomechanical magnetometer that is excited and read out via a suspended optical waveguide fabricated on the same silicon chip as the magnetometer. Moreover, we demonstrate that thermomechanical noise limited sensitivity is possible using portable electronics and laser. The magnetometer employs a silica microdisk resonator selectively sputtered with a magnetostrictive film of galfenol (FeGa) which induces a resonant frequency shift in response to an external magnetic field. Experimental results reveal the retention of high quality-factor optical whispering gallery mode resonances whilst also demonstrating high sensitivity and dynamic range in ambient conditions. The use of off-the-shelf portable electronics without compromising sensor performance demonstrates promise for applications.

physics.optics↗

arXiv2022

Excitonic Absorption Signatures of Twisted Bilayer WSe$_{2}$ by Electron Energy-Loss Spectroscopy

Moiré twist angle underpins the interlayer interaction of excitons in twisted van der Waals hetero- and homo-structures. The influence of twist angle on the excitonic absorption of twisted bilayer tungsten diselenide (WSe$_{2}$) has been investigated using electron energy-loss spectroscopy. Atomic-resolution imaging by scanning transmission electron microscopy was used to determine key structural parameters, including the nanoscale measurement of the relative twist angle and stacking order. Detailed spectral analysis revealed a pronounced blueshift in the high-energy excitonic peak C with increasing twist angle, up to 200 meV when compared to the AA$^{\prime}$ stacking. The experimental findings have been discussed relative to first-principle calculations of the dielectric response of the AA$^{\prime}$ stacked bilayer WSe$_{2}$ as compared to monolayer WSe$_{2}$ by employing the \textit{GW} plus Bethe-Salpeter equation (BSE) approaches, resolving the origin of higher energy spectral features from ensembles of excitonic transitions, and thus any discrepancies between previous calculations. Furthermore, the electronic structure of moiré supercells spanning twist angles of $\sim$9.5-46.5$^{\circ}$ calculated by density functional theory (DFT) were unfolded, showing an uplifting of the conduction band minimum near the $Q$ point and minimal change in the upper valence band concurrently. The combined experiment/theory investigation provides valuable insight into the physical origins of high-energy absorption resonances in twisted bilayers, which enables to track the evolution of interlayer coupling from tuning of the exciton C transitions by absorption spectroscopy.

cond-mat.mes-hall↗

arXiv2022

Substrate influence on transition metal dichalcogenide monolayer exciton absorption linewidth broadening

The excitonic states of transition metal dichacolgenide (TMD) monolayers are heavily influenced by their external dielectric environment based on the substrate used. In this work, various wide bandgap dielectric materials, namely hexagonal boron nitride (\textit{h}-BN) and amorphous silicon nitride (Si$_3$N$_4$), under different configurations as support or encapsulation material for WS$_2$ monolayers are investigated to disentangle the factors contributing to inhomogeneous broadening of exciton absorption lines in TMDs using electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM). In addition, monolayer roughness in each configuration was determined from tilt series of electron diffraction patterns by assessing the broadening of diffraction spots by comparison with simulations. From our experiments, the main factors that play a role in linewidth broadening can be classified in increasing order of importance by: monolayer roughness, surface cleanliness, and substrate-induced charge trapping. Furthermore, because high-energy electrons are used as a probe, electron beam-induced damage on bare TMD monolayer is also revealed to be responsible for irreversible linewidth increases. \textit{h}-BN not only provides clean surfaces of TMD monolayer, and minimal charge disorder, but can also protect the TMD from irradiation damage. This work provides a better understanding of the mechanisms by which \textit{h}-BN remains, to date, the most compatible material for 2D material encapsulation, facilitating the realization of intrinsic material properties to their full potential.

cond-mat.mes-hall↗