A cross-section of the miniature optical magnetometer (left), and the fabricated sensor prototype (right). [Image: Babak Amirsolaimani / University of Arizona in Tucson]
U.S. researchers from the University of Arizona’s College of Optical Sciences have developed a new optical material—a composite made of nanoparticles in a polymer—that they used to create a miniaturized optical magnetometer that can detect faint magnetic fields, like those produced by heart and brain activity (Opt. Lett., doi: 10.1364/OL.43.004615).
The polymer, which contains nanoparticles made of magnetically sensitive magnetite and cobalt, is combined with an interferometer to make a sensor that can optically detect a magnetic field by measuring how the field rotates polarized light. And unlike magnetic resonance imaging (MRI) and electrocardiography (ECG), the all-optical sensor does not require expensive and bulky cooling systems or electromagnetic shielding.
During a prototype demonstration of the sensor, the researchers were able to detect a human heartbeat. They believe their miniature magnetic-field optical sensor shows promise as a smaller, more affordable and portable alternative to MRIs and ECGs for measuring and monitoring heart and brain activity in the field or in low-resource settings.
Nanocomposite polymer
The electrical impulses created by heartbeats and brain activity create weak magnetic fields that can be measured outside of the body. However, most existing magnetometers fall short of the high spatio-temporal resolution needed to detect these fields at room temperature and without electromagnetic shielding.
Nasser Peyghambarian and his team say that they have created a new nanocomposite-based magnetometer system that is capable of real-time high-resolution magnetic-field measurements without needing to stabilize the surrounding thermal and electromagnetic environment. The key to their sensor is the nanoparticle-containing polymer that they fabricated specifically for this use.
The polymer contains ferrite-based magnetite and cobalt nanoparticles that have been optimized (for size, spacing and coating) to maximize their sensitivity to magnetic fields and minimize optical scattering. The magneto-optical nanoparticles are sensitive enough to create a detectible polarized-light rotation in the presence of magnetic fields 100 million times weaker than the magnetic field of the Earth.
Measuring a human heartbeat
The miniature optical magnetometer consists of a 500-µm-thick film of the nanoparticle polymer sandwiched between a quarter-wave plate (QWP) and a silver coating, all connected to a compact fiber-optic interferometer. The QWP creates two orthogonal polarizations in the right- and left-handed circular polarized light originating from a 1,310-nm laser beam. The silver coating acts as a mirror, and the interferometer measures the rotation of polarized light, which is specific to the strength of the surrounding magnetic field.
Using a prototype of the sensor, the researchers were able to detect the relatively weak magnetic signal from a human heartbeat, showing that the sensor can detect “magnetic fluctuations that change every microsecond from an area as small as 100 µm2.”
Although it hasn’t been tested yet, the team says that the sensor could also detect the magnetic field created by a neuron firing in the brain. Multiple sensors could potentially be strung together in a network, according to the team, to measure more complex brain activity with high spatio-temporal resolution.