Tiny Sensor May Be Able To Screen For Explosives

A tiny sensor that can detect magnetic field changes as small as 70 femtoteslas -- equivalent to the brain waves of a person daydreaming -- has been demonstrated at the National Institute of Standards and Technology (NIST). The device may have applications such as homeland security screening for explosives.

To be described in the November issue of Nature Photonics,* the prototype device is almost 1000 times more sensitive than NIST’s original chip-scale magnetometer demonstrated in 2004 and is based on a different operating principle. Its performance puts it within reach of matching the current gold standard for magnetic sensors, so-called superconducting quantum interference devices or SQUIDs. These devices can sense changes in the 3- to 40-femtotesla range but must be cooled to very low (cryogenic) temperatures, making them much larger, power hungry, and more expensive.

The NIST prototype consists of a single low-power (milliwatt) infrared laser and a rice-grain-sized container with dimensions of 3 by 2 by 1 millimeters. The container holds about 100 billion rubidium atoms in gas form. As the laser beam passes through the atomic vapor, scientists measure the transmitted optical power while varying the strength of a magnetic field applied perpendicular to the beam. The amount of laser light absorbed by the atoms varies predictably with the magnetic field, providing a reference scale for measuring the field. The stronger the magnetic field, the more light is absorbed.

"The small size and high performance of this sensor will open doors to applications that we could previously only dream about," project leader John Kitching said.

To make a complete portable magnetometer, the laser and vapor cell would need to be packaged with miniature optics and a light detector. The vapor cell can be fabricated and assembled on semiconductor wafers using existing techniques for making microelectronics and microelectromechanical systems (MEMS). This design, adapted from a previously developed NIST chip-scale atomic clock, offers the potential for low-cost mass production.

As described in the new paper, NIST scientists demonstrated that the prototype mini-sensor produces a strong signal that changes rapidly with the strength of a magnetic field from the outside world. The device exhibits a consistent minimum level of electromagnetic static, or “white noise,” which indicates a stable limit on its overall sensitivity. The authors also estimated that a well-designed compact magnetometer with present sensitivity could operate continuously for weeks on a single AA battery. Magnetometers need to be designed with applications in mind; smaller vapor cells require less power but are also less sensitive. Thus, an application for which low power is critical would benefit from a very small magnetometer, whereas a larger magnetometer would be more suitable for a different application requiring high sensitivity. The NIST work evaluates the tradeoffs between size, power and performance in a quantifiable way.

“This result suggests that millimeter-scale, low-power, inexpensive, femtotesla magnetometers are feasible … Such an instrument would greatly expand the range of applications in which atomic magnetometers could be used,” the paper states.

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