Researchers have developed a light-based technique for measuring very weak magnetic fields, such as those produced when neurons fire in the brain.  The inexpensive and compact sensors could offer an alternative to the magnetic resonance imaging (MRI) systems currently used to map brain activity.

"A portable, low-cost brain imaging system that can operate at room temperature in unshielded environments would allow real-time brain activity mapping after potential concussions on the sports field and in conflict zones where the effect of explosives on the brain can be catastrophic," said researcher member Babak Amirsolaimani of the University of Arizona, Tucson.

As detailed in The Optical Society (OSA) journal Optics Letters, the researchers fabricated the magnetic sensors using optical fibers and a newly developed polymer-nanoparticle composite that is sensitive to magnetic fields. The sensors can detect the brain's magnetic field, which is 100 million times weaker than the magnetic field of the earth.

The researchers also showed that the new sensor could detect the weak magnetic pattern of a human heartbeat and can detect magnetic fluctuations that change every microsecond from an area as small as 100 square microns.

"The all-optical design of the sensor means it could be fabricated inexpensively on a silicon photonics chip, making it possible to produce a system that is almost as small as the sensor's 10-micron-diameter optical fiber," said Amirsolaimani. "Multiple sensors could then be used together to provide high spatial resolution brain mapping."

The new sensors could help scientists better understand the activity of the brain and diseases of the brain such as dementia and Alzheimer's. They might also be useful for measuring the magnetic fields used to predict volcanic eruptions and earthquakes, identify oil and minerals for excavation and detect military submarines.

Optical detection of magnetic fields

The optical method for detecting weak magnetic fields takes advantage of the fact that a magnetic field causes the polarization of light to rotate, with the degree of rotation dependent on the material through which the light passes.

The researchers developed a new composite material made of nanoparticles dispersed in a polymer that imparts a detectable polarization rotation in light when very weak magnetic fields are present.

They selected nanoparticles based on magnetite and cobalt because these materials exhibit very high magnetic sensitivity. They then optimized the size, spacing, and coating of the nanoparticles to create a composite material that is extremely sensitive to magnetic fields.

The researchers detected the polarization rotation using an optical interferometer. This works by splitting laser light into two paths, one of which passes through the highly-sensitive material while the other does not. The polarization of each light path is detected and compared to measure fluctuations in very small magnetic fields.