Indian physicists trap light in ‘nano’ soup

By K.S. Jayaraman, IANS

Bangalore : Indian scientists have demonstrated how to trap and retrieve light using a soup of micro and nano sized magnetic spheres – a major milestone in the path toward developing optical computers.

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The researchers claim that their unique mixture of tiny particles works at room temperature, holds photons – the particles of light – for far longer than other systems, and can also be tuned with a magnet to store any wavelength of visible light.

The discovery made by a team led by Rasbindu Mehta at Bhavnagar University in Gujarat was first announced in the November issue of the Current Science journal published by the Indian Academy of Science in Bangalore.

Now, a report published in the latest issue of the Royal Chemical Society of London’s journal Chemistry World says the discovery could pave the way for photonic ‘microchips’ that use photons for processing optical information in the same way electrons are manipulated in silicon chips in today’s electronic devices and computers.

For over a decade, scientists have been working towards light-based computing that could be many times faster than electronics as light travels at a speed of approximately 300,000 km per second.

Any microchip designed to process optical signals has to store photons, perhaps by slowing or trapping light in carefully designed crystals. Mehta’s team coated micron-size magnetite spheres with oleic acid and dispersed them through a ferrofluid, which is a suspension of much smaller magnetic nanoparticles.

When an external magnetic field was applied to the fluid, which was held in a glass cell, laser light passing through the medium was trapped inside. Photons escaped when the field was switched off.

“It is fantastic,” said Hema Ramachandran, who heads the photonics unit at the Raman Research Institute in Bangalore, and was one of several physicists who witnessed the demonstration.

“It was a chance discovery,” said Mehta’s colleague Rajesh Patel.

While investigating the optical properties of their transparent fluid, the researchers noticed that in a certain magnetic field range, light scattering – both forward and backward – became zero.

“We thought the light got trapped inside,” said Patel. “So, we switched off the laser (which was shining light through the system) and then the magnetic field, and there it was – a flash of colour lighting up our dark room.”

Mehta told IANS that his group is yet to develop a theory to explain this novel phenomenon. He believes that the spheres are aligned by the magnetic field and form micro cavities – filled by the ferrofluid – in which the photons get trapped, resonating back and forth.

Changing the external magnetic field alters the refractive index of the cavities that in turn decides which wavelength of light is trapped by the system.

And what is more, according to Mehta, photons can be stored for as long as the magnetic field is switched on.

“This is the first visual evidence of storage and retrieval of light for a long and controllable duration. In all other reports, storage time of photons is restricted to a few nanoseconds,” he said

Experiments by physicists in the US have involved stopping light altogether by using a gas of sodium or rubidium atoms chilled to near absolute zero, though that system is too complex and not practical for microchips.

Although Mehta’s team seems to have chanced on a method for taming light that is simple, inexpensive and unsophisticated, its practical application is still years away.