Researchers from the Indian Institutes of Technology (IIT) at Mumbai and Kharagpur have built a microscope that can image magnetic fields within microscopic two-dimensional samples that change over milliseconds. This has a huge potential for scientific applications, such as in measuring biological activity of neurons and dynamics of vortices in superconductors. The work, led by IIT Bombay professor Kasturi Saha, from the Department of Electrical Engineering, has been published in Scientific Reports. This is the first time that such a tool has been built to image magnetic fields that change within milliseconds.
Capturing change
Prof. Saha explains that the ideal frame rate to capture a changing magnetic field is one that captures data at twice the frequency of the changing field. Signals in nature exhibit a range of frequencies — magnetism in geological rock samples and rare earth magnets can be constant over months; magnetic nanoparticle aggregation inside living cells takes place in minutes; action potentials in neurons are fast, taking milliseconds, whereas precession of atomic spins in complex molecules takes only microseconds. The instrument that this team has built works in the millisecond range.
The key aspect of this sensor is a “nitrogen vacancy (NV) defect centre” in a diamond crystal. Such NV centres act as pseudo atoms with electronic states that are sensitive to the fields and gradients around them (magnetic fields, temperature, electric field and strain).
“Notably, the fluorescence emitted from these NV centres encodes the magnetic field information,” says Prof. Saha. “During the measurement of ultra-small magnetic fields, the change in the fluorescence levels is extremely small and therefore, limits the imaging frame rate and degrades the signal-to-noise ratio of the measurement.”
In order to overcome this limitation, the researchers employed a “lock-in detection scheme” which selects light fluctuations of a small frequency range, rejecting others, and thereby improving the sensitivity to small changes in fluorescence.
Improved frame rates
Earlier reported magnetic field imaging frame rates were close to 1-10 minutes per frame. This would increase to about half an hour per frame for challenging samples like biological cells. The instrument built by this group exhibits an imaging frame rate of about 50-200 frames per second, which would translate into a frame acquisition time of about 2-5 milliseconds. “It enables imaging of millisecond scale magnetisation changes in micro-magnets, dynamic micro scale thermometry in cells and with further improvements, might enable probing action potentials in mammalian neurons,” says Prof. Saha.
A special diamond crystal, one micrometre thick, embedded with a high density of such NV centres is created. This acts as a sensor when a thin two-dimensional sample is brought close to it — less than 10 micrometre. Using this technique, the researchers can image a 150 micrometre by 150 micrometre field of view, which is quite an achievement.
“The NV centre imaging technique is a unique tool in the context of imaging microscale magnetic field variations in any sample,” says Prof. Saha.
The team had started a collaboration with IIT Kharagpur in 2017 with the ambitious target of building a novel system to image the brain. They collaborated with Sharba Bandopadhyay, who brought in an expertise in neurobiology and bioengineering to complement the knowledge of quantum optics, quantum computing and quantum sensing that was Prof. Saha’s forte.
“We have, along with PhD student Madhur Parashar, developed an algorithm to image neurons in 3D using NV quantum sensors,” says Prof. Saha.
This work was published in Communications Physics in 2020. We have jointly filed a patent for the present work, she adds.
