Laser threshold magnetometry senses weak magnetic fields

10 Oct 2018

Fraunhofer project employs NV-doped diamond as a laser medium.

Measurements of the weak magnetic fields arising from electrical activity in nerve cells gives a valuable insight into the health of living tissues and organs, as well as facilitating imaging procedures such as magnetic resonance tomography.

The equipment needed to make those measurements, however, has tended to be large and complex, often requiring extreme cooling of the magnetic field sensors themselves.

A new approach under development by the DiLaMg project led by Fraunhofer IAF could offer a valuable alternative. It aims to develop and optimize the use of nitrogen vacancy (NV) centers in diamond as highly sensitive magnetic field detectors.

"This is a new approach to sensing, which I have developed under the name laser threshold magnetometry, or LTM," said project leader Jan Jeske of Fraunhofer IAF.

NV centers in diamond are units of the diamond structure containing both one nitrogen atom and a carbon vacancy. They are known to absorb green light and emit red light, with a luminosity affected by the intensity of an exterior magnetic field. Since the centers are so small, this effect can in theory detect magnetic fields with high spatial resolution and sensitivity.

DiLaMag intends to use the NV-doped diamond as a laser medium, and use the change in fluorescence that arises under the influence of a magnetic field to shift the laser from below threshold to above threshold. Effectively, the project envisages constructing a laser from the NV centers themselves.

"NV centers are being widely explored for magnetometry, but the signal has always been spontaneous emission rather than stimulated emission or laser output," Jeske commented to Optics.org. "This new scheme promises a strong improvement in precision and sensitivity."

In a 2016 New Journal of Physics paper announcing the principle, Jeske predicted that LTM may lead to magnetometers with sensitivities two to three orders of magnitude better than existing NV−demonstrations, and comparable to state-of-the-art SQUID (superconducting quantum interference device) magnetometers.

Sensors: the missing element

"Sensing is possible even while remaining above threshold the whole time," said Jeske. "Just above threshold a small change in the brightness is strongly amplified by the laser cavity system, due to competition between spontaneous and stimulated emission. A possible sensing configuration could use the brightness of the laser as a direct signal of magnetic field intensity."

DiLaMag is intended to run until 2023, funded by the German Federal Ministry of Education and Research. The first half of the project will tackle material improvements and the development of the sensor, including ways to enrich diamond with as many NV centers as possible while minimizing losses through absorption, scattering and double scattering,

The second half of the project will address applications for the technique, alongside partner groups at University clinics in Freiberg and Heidelberg. Possible uses may include measurement of brain and heart activities of unborn babies, helping with prenatal treatment of disease.

"In the long term a highly sensitive magnetometer has great potential in low-field MRI and magneto-encephalography (MEG), replacing the expensive SQUID technology," said Jeske.

"Magnetic fields travel through the body undisturbed, so functional neuroimaging techniques like MEG have the potential to be more precise than their electric counterparts. Vapor cell magnetometers are another fast-developing technology. The missing element so far has been the right sensors."