A team of scientists at the Massachusetts Institute of Technology has developed an ultrasensitive magnetic-field detector that could lead to smaller devices for medical and materials imaging.
Laser light enters a synthetic diamond from a facet at its corner and bounces around inside the diamond until its energy is exhausted; this excites the NVs that can be used to measure magnetic fields. Image credit: H. Clevenson / MIT Lincoln Laboratory.
Synthetic diamonds with nitrogen vacancies (NVs) have long held promise as the basis for efficient, portable magnetometers (magnetic-field detectors).
A diamond chip about 1/20 the size of a thumbnail could contain trillions of the NVs, each capable of performing its own magnetic-field measurement. The problem has been aggregating all those measurements.
Probing a NV requires zapping it with laser light, which it absorbs and re-emits. The intensity of the emitted light carries information about the vacancy’s magnetic state.
“In the past, only a small fraction of the pump light was used to excite a small fraction of the NVs. We make use of almost all the pump light to measure almost all of the NVs,” said Prof Dirk Englund of MIT, the senior author on the paper published in the journal Nature.
In previous experiments, physicists often excited the NVs by directing laser light at the surface of the chip.
“Only a small fraction of the light is absorbed. Most of it just goes straight through the diamond. We gain an enormous advantage by adding the prism facet to the corner of the diamond and coupling the laser into the side. All of the light that we put into the diamond can be absorbed and is useful,” said study lead author Hannah Clevenson, a graduate student at MIT.
The MIT team calculated the angle at which the laser beam should enter the crystal so that it will remain confined, bouncing off the sides in a pattern that spans the length and breadth of the crystal before all of its energy is absorbed.
“You can get close to a meter in path length. It’s as if you had a meter-long diamond sensor wrapped into a few millimeters,” Prof Englund said.
As a consequence, the team’s device uses the pump laser’s energy 1,000 times as efficiently as its predecessors did.
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Hannah Clevenson et al. Broadband magnetometry and temperature sensing with a light-trapping diamond waveguide. Nature Physics, published online April 06, 2015; doi: 10.1038/nphys3291
