A new super-efficient magnetic-field detector could pave the way for miniaturised devices for medical imaging and contraband detection.
The sensor developed by researchers at the Massachusetts Institute of Technology relies on synthetic diamonds with nitrogen vacancies (NVs) – defects that are extremely sensitive to magnetic fields.
Finding out a vacancy’s magnetic state requires it to be zapped with laser light, which it absorbs and re-emits, with the intensity indicative of its state. But the efficiency of this process has been a major limiting factor in the technology's adoption to date.
"In the past, only a small fraction of the pump light was used to excite a small fraction of the NVs," said Dirk Englund, one of the designers of the new device detailed in the latest issue of Nature Physics. "We make use of almost all the pump light to measure almost all of the NVs."
In previous experiments, lead author and graduate student Hannah Clevenson says, researchers often excited the nitrogen vacancies by directing laser light at the surface of the diamond chip, but this results in most of the light going straight through the diamond.
The innovation of the MIT researchers was to calculate the angle at which the laser beam should enter the crystal so that it remains confined, bouncing off the sides in a pattern that spans the length and breadth of the crystal before all of its energy is absorbed increasing the efficiency of the chip 1,000 times compared to its predecessors.
"You can get close to a meter in path length," Englund added. "It's as if you had a meter-long diamond sensor wrapped into a few millimetres."
A nitrogen vacancy is a missing atom in diamond’s carbon atom lattice, adjacent to a nitrogen atom. Electrons in one of these vacancies can be boosted into a higher energy state when struck by a photon and when they return to their original energy state, they sometimes release their excess energy as another photon.
A magnetic field, however, can flip the magnetic orientation of these electrons increasing the difference between their two energy states. The stronger the field the more electrons are affected and the brighter the light emitted by the vacancies becomes.
Because of the geometry of the nitrogen vacancies, the re-emitted photons emerge at four distinct angles and a lens at one end of the crystal is used to collect 20 per cent of them and focus them onto a light detector.
“We gain an enormous advantage by adding this prism facet to the corner of the diamond and coupling the laser into the side,” said Clevenson. “All of the light that we put into the diamond can be absorbed and is useful."
Magnetic-field detectors are already used for everything from medical and materials imaging, to contraband detection to geological exploration and the researchers hope increasing the efficiency of detectors will allow the creation of handheld, battery-powered devices in these applications.