Small and rugged optical magnetometer can measure femtotesla-level magnetic fields

Based on a magnetite-containing nanocomposite and an in-line fiber-optic Sagnac interferometer, the device could be used for mapping the brain’s magnetic fields.

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Researchers from the University of Arizona and TIPD (both in Tucson, AZ), Urbix Resources (Mesa, AZ), and the University of Leuven (Leuven, Belgium) have developed a magneto-optical (MO) sensor for measuring very weak magnetic fields, such as those produced when neurons fire in the brain. The inexpensive and compact sensor, which takes advantage of a newly developed polymer-nanoparticle MO composite that is sensitive to magnetic fields, can detect the brain’s magnetic field, which is 100 million times weaker than the magnetic field of earth—and thus could offer an alternative to the magnetic resonance imaging (MRI) systems currently used to map brain activity without the expensive cooling or electromagnetic shielding required by MRI machines. The device has a millisecond-scale response time and a sensing area as small as 100 µm2.

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The new sensing material consists of dysprosium-doped magnetite and cobalt ferrite nanoparticles dispersed in a polymer matrix. The composite imparts a detectible polarization rotation (Faraday rotation) in light when very weak magnetic fields are present. Based on output from the MO composite, a compact fiber-optic in-line Sagnac interferometer determines the magnetic field with a sensitivity of 20 fT∕Hz2 (magnetic fields in the human brain are on the order of 50 to 500 fT). The researchers used the sensor to measure the magnetic field created by electrical impulses produced during the human heartbeat. They were able to detect a clear magnetic signal exhibiting high contrast, demonstrating the technology’s potential as a simple replacement for electrocardiography (ECG) tests commonly performed to detect heart problems. Next, the researchers plan to study the long-term stability of the sensors and how well they withstand environmental changes. They next want to fabricate several hundred sensors to make a 256-channel system for evaluating and imaging the entire magnetic field of a human brain. Because the fibers and optical elements of the system are commercially available, the sensors could potentially be made inexpensively. Reference: B. Amirsolaimani et al., Opt. Lett. (2018); https://doi.org/10.1364/ol.43.004615.

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