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Applied physics

Hybrid sensors ring the changes

An improved design for a class of magnetometer greatly increases the sensitivity of these devices — and might be the vanguard of a new generation of hybrid sensors that combine different types of signal to increase sensitivity.

When seventeenth-century sailors undertook their dangerous journeys around the Cape of Good Hope or to the Spice Islands, they had an invaluable navigational tool on board: the compass. Only with the aid of this, the most precise measurement device of those times, were they able to accomplish their daring feats. Since then, precision measurements of magnetic fields have been key drivers of basic science and of a whole wealth of technologies with applications ranging from navigation to the medical sciences. Writing in Advanced Materials, Forstner et al.1 report a vastly improved design for a certain class of magnetometer — a hybrid sensor that measures magnetic fields using an optical signal.

Various technologies have been developed to measure magnetic fields. The record holders for sensitivity are devices known as atom-vapour magnetometers2. Others can measure the dipolar magnetic interaction between two electrons3, and still others, most notably diamond spin sensors, have a compact design that might allow magnetometers to be made as small as one nanometre across. The central element of Forstner and colleagues' sensor is a 'whispering gallery mode' micro-resonator: a doughnut-shaped device that traps light waves and is known for its strong optical confinement and ultra-high quality factor (a measure of the energy lost from the device; high quality factors correspond to low rates of energy loss). The frequency at which light resonates in the doughnut depends on the doughnut's geometry.

To sense magnetic fields, Forstner et al. combined the resonator with a magnetostrictive material. This material expands in a magnetic field and thereby exerts a force on the cavity system, changing the micro-resonator's shape and thus its resonance frequencies. So, by measuring the change in resonance frequencies, the strength of the magnetic field can be determined.

The authors placed the magnetostrictive material in the middle of the doughnut, where it does not interfere with optical measurements and is most effective in changing the shape of the resonator (Fig. 1). Because of the high quality factor of the device, tiny distortions of its shape — and therefore tiny changes of magnetic field — can be detected. In a proof-of-principle experiment, Forstner and co-workers demonstrated that their device has a magnetic-field sensitivity of 100 picotesla (1 picotesla is 10−12 tesla) for measurements of 1 second duration. Other kinds of magnetometer, such as atom-vapour types, are sensitive to much lower fields4,5 (less than 1 femtotesla; 1 femtotesla is 10−15 tesla), but Forstner and co-workers' approach is three orders of magnitude more sensitive than previous hybrid optomechanical sensors6, mostly because of the integration of the magnetostrictive material into the micro-resonator. Moreover, the researchers extended the frequency range of magnetic fields that can be sensitively detected by the sensor by using the nonlinear properties of the magnetostrictive material.

Figure 1: An optomechanical magnetometer.
figure1

Forstner et al.1 have constructed a magnetic-field sensor in which a doughnut-shaped 'whispering gallery mode' resonator device surrounds a piece of magnetostrictive material. Light is guided through a tapered optical fibre into the sensor, where it is trapped by the resonator. In the presence of a magnetic field, the magnetostrictive material expands, distorting the resonator and changing the frequency at which light resonates in the device. The change of resonance frequency is measured by a spectrum analyser and used to determine the strength of the magnetic field.

Compared with other, more-sensitive sensor types, the main advantage of Forstner and colleagues' device is its small size, which is limited to the size of the doughnut (about a few tens of micrometres). Furthermore, the device works at room temperature and tolerates background magnetic fields. This opens up various potential applications, such as navigation without the need for a global positioning system, and the detection of weak biomagnetic fields from neurons.

Forstner and co-workers' achievements follow a recent trend in sensor design: the use of optomechanical sensors, in which optical and mechanical signals are combined to enhance sensitivity. By incorporating advances in nanophotonics and the fabrication of nanoscale structures, optomechanical sensors have been made that perform better than their electromechanical counterparts, mostly because the noise in optical readouts is much lower than in electronic ones. Excellent examples include state-of-the-art force sensors7 and accelerometers8, and a detector of microwave photons9.

Hybrid designs based on principles other than optomechanics might also be adopted to improve the sensitivity of sensors. Consider diamond spin sensors. Rather than using optical resonances, diamond spin sensing relies on the precision measurement of transitions between resonance states that are associated with a quantum property of electrons: spin. Such sensors can be responsive to various quantities, including magnetic10 and electric fields11, temperature12, pressure13 and strain14. But the sensitivity of these devices — particularly for temperature and strain — is limited by the nature of the ground-state spin wave function of the electrons used in the sensing mechanism13.

It has been proposed15 that this problem could be overcome by constructing a hybrid sensor using the same magnetostrictive material as that used by Forstner and colleagues. However, the material would function in exactly the opposite way to that reported by these authors: a strain (or an electric field) would generate a magnetic field, which is sensitively detected by the electron spins, which in turn are read out optically. The resulting diamond hybrid sensor is predicted to be about 1,000 times more sensitive for pressure, force or electric field than diamond spins alone, and would retain excellent spatial resolution.

Hybrid sensors thus seem to be an upcoming theme in sensor technology. An important further step will be to use advances in quantum technology to achieve the limits of accuracy. The resulting quantum hybrid sensors could potentially revolutionize sensor technology in various disciplines, enabling unprecedented opportunities in technology and basic science.

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Correspondence to Jörg Wrachtrup.

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Wrachtrup, J., Finkler, A. Hybrid sensors ring the changes. Nature 512, 380–381 (2014). https://doi.org/10.1038/512380a

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