An ultra-sensitive magnetic-field detector could usher in a new generation of sensors for medical and security applications.
Developed by researchers at MIT in the US the new device is claimed to be 1,000 times more energy-efficient than its predecessors and could, it is claimed, lead to miniaturised, battery-powered devices for medical and materials imaging, contraband detection, and even geological exploration.
Magnetic-field detectors – or magnetometers – are already widely used but existing technologies have drawbacks: some rely on gas-filled chambers; others work only in narrow frequency bands, limiting their utility.
The MIT system owes its advanced performance to the innovative use of synthetic diamonds with nitrogen vacancies (NVs) – a defect resulting from a missing atom in the lattice, adjacent to a nitrogen atom.
Electrons in this vacancy interact with magnetic fields, making them useful for sensing applications. A diamond chip about one-twentieth the size of a thumbnail could contain trillions of nitrogen vacancies, each capable of performing its own magnetic-field measurement. However, aggregating all of these measurements has been a problem.
Probing a nitrogen vacancy requires striking it with laser light, which it absorbs and re-emits. The intensity of the emitted light carries information about the vacancy’s magnetic state.
When a light particle – a photon – strikes an electron in a nitrogen vacancy, it kicks it into a higher energy state. When the electron falls back down into its original energy state, it may release its excess energy as another photon. A magnetic field, however, can flip the electron’s magnetic orientation, or spin, increasing the difference between its two energy states. The stronger the field, the more spins it will flip, changing the brightness of the light emitted by the vacancies.
Making accurate measurements with this type of chip requires collecting as many of those photons as possible.
In order to achieve this, 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.
Prof Dirk Englund, one of the designers of the device, said that this confers considerable advantages. “You can get close to a meter in path length,” he said in a statement. ”It’s as if you had a meter-long diamond sensor wrapped into a few millimeters.” As a consequence, the chip uses the pump laser’s energy 1,000 times as efficiently as its predecessors did.
Because of the geometry of the nitrogen vacancies, the re-emitted photons emerge at four distinct angles. A lens at one end of the crystal can collect 20 per cent of them and focus them onto a light detector, which is enough to yield a reliable measurement.