Stefan Forstner

Cavity optomechanical magnetometry

A cavity optomechanical magnetometer is demonstrated. The magnetic-field-induced expansion of a magnetostrictive material is resonantly transduced onto the physical structure of a highly compliant optical microresonator and read out optically with ultrahigh sensitivity. A peak magnetic field sensitivity of 400  nT  Hz(-1/2) is achieved, with theoretical modeling predicting the possibility of sensitivities below 1  pT  Hz(-1/2). This chip-based magnetometer combines high sensitivity and large dynamic range with small size and room temperature operation.

Ultrasensitive Optomechanical Magnetometry

A cavity optomechanical magneto-meter operating in the 100 pT range is reported. The device operates at earth field, achieves tens of megahertz bandwidth with 60 μm spatial resolution and microwatt optical-power requirements. These unique capabilities may have a broad range of applications including cryogen-free and microfluidic magnetic resonance imaging (MRI), and investigation of spin-physics in condensed matter systems.

Model of a microtoroidal magnetometer

We present a model of a cavity optomechanical magnetic field sensor based on a microtoroidal resonator. The magnetic field induced expansion of a magnetostrictive material is transduced onto the physical structure of a highly compliant optical microresonator. The resulting motion is read out optically with ultra-high sensitivity. According to our theoretical model sensitivities of up to 750 fT/√ Hz may be possible. The simultaneous presence of high-quality mechanical and optical resonances in microtoroids greatly enhances both the response to the magnetic field and the measurement sensitivity.

Sensitivity of cavity optomechanical field sensors

This article presents a technique for modeling cavity optomechanical field sensors. A magnetic or electric field induces a spatially varying strain across the sensor. The effect of this strain is accounted for by separating the mechanical motion of the sensor into eigenmodes, each modeled by a simple harmonic oscillator. The force induced on each oscillator can then be determined from an overlap integral between strain and the corresponding eigenmode, with the optomechanical coupling strength determining the ultimate resolution with which this force can be detected.

Optomechanical magnetometer with nano-Tesla sensitivity

We demonstrate an optomechanical magnetometer based on microtoroidal resonators that combines the giant magnetostriction of Terfenol-D with the ultrahigh optical transduction sensitivity of microtoroids and achieves detection sensitivities in the range of nT Hz−1/2.

Modelling of Cavity Optomechanical Magnetometers

Cavity optomechanical magnetic field sensors, constructed by coupling a magnetostrictive material to a micro-toroidal optical cavity, act as ultra-sensitive room temperature magnetometers with tens of micrometre size and broad bandwidth, combined with a simple operating scheme. Here, we develop a general recipe for predicting the field sensitivity of these devices. Several geometries are analysed, with a highest predicted sensitivity of 180 pT/Hz at 28 μm resolution limited by thermal noise in good agreement with previous experimental observations. Furthermore, by adjusting the composition of the magnetostrictive material and its annealing process, a sensitivity as good as 20 pT/Hz may be possible at the same resolution. This method paves a way for future design of magnetostrictive material based optomechanical magnetometers, possibly allowing both scalar and vectorial magnetometers.

Quantum enhanced optomechanical magnetometry

Quantum-enhanced measurements of magnetic fields are experimentally demonstrated using a microcavity optomechanical magnetometer and squeezed states of light. We attain an improvement of the magnetic field sensitivity of 20% using 2.2dB phase-squeezed states.

Towards a scalable ultrasensitive optomechanical magnetometer

Optomechanical magnetometers have been previously demonstrated to achieve sensitivities comparable to the state-of-the-art magnetometers of comparable size. They combine the ultra-precise cavity optomechanical measurement and magnetostrictive response of terfenol-d. We have achieved reproducible sensitivities of 600 pT/√Hz using a combination of standard photolithography and sputter deposition techniques which provides a pathway for scalable magnetometers for diverse applications.

Cavity optomechanical magnetometry on a chip

A microscale, picotesla range, silicon-chip optical magnetometer. Earth-field, fiber coupled operation, with 60 μm resolution and 40 MHz bandwidth lead to potential applications in microfluidic-MRI, spin physics in condensed matter systems and ultracold atom clouds.

Ultrasensitive cavity optomechanical magnetometry

Ultra-low field magnetic field sensors are essential for many applications including geology, mineral exploration, archaeology and medicine [1]. However, such magnetometers typically either require cryogenic systems to operate, or suffer poor dynamic range, limiting their ability to operate in ambient conditions. Here, we demonstrate a microscale room temperature cavity optomechanical magnetometer operating in the picoTelsa sensitivity range (see Fig. 1a). The peak sensitivity of 200 pT Hz-1/2 exceeds that of any previous room temperature magnetometer of its size, and is a factor of 2,000 better than the only previously reported cavity optomechanical magnetometer [2]. Furthermore, we demonstrate that an inherent paramagnetic nonlinearity allows mix-up of low frequency to the RF frequency band, enabling magnetic field sensing in the Hz-kHz frequency window crucial for applications such as medical imaging, geosurvey, and magnetic anomaly detection (see Fig. 1b). This presents an enabling step towards real applications of cavity optomechanical magnetometers in high-performance microscale magnetometry.