Photons stored in switchable magnetic fluids

Physicists at the Bhavnagar University in Gujarat, India, have demonstrated how to trap and retrieve light using a soup of micro- and nano-sized magnetic spheres.1 The unusual fluid, they say, works at room temperature, holds photons for far longer than other systems, and can also be tuned with a magnet to store any wavelength of visible light. ’The discovery could pave the way for lab-on-a-chip devices for processing optical information,’ Rasbindu Mehta, who led the team, told Chemistry World.

For over a decade scientists have been working towards light-based computing: where circuits control photons - particles of light - in the same way that they currently manipulate electrons. Photon-based computing should be faster than electronics. It would also cut out the ungainly components used to convert between the optical signals that transmit data and the electrical signals that manipulate and store it. 

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 (in this case held in kerosene). 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. 


Red light passes through the fluid with no external magnetic field applied (left-hand diffraction pattern); is stopped by a particular magnetic field strength (middle); photons appear again when the magnet is switched off (right).

’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. She said she was planning some experiments with Mehta’s group.

’It was a chance discovery,’ explained 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.’ 

A rigorous theoretical explanation is yet to come, but the researchers believe that the spheres are aligned by the magnetic field and form microcavities - 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, by changing the dispersion of both the ferrofluid and the larger spheres. This in turn decides which wavelength of light is trapped by the system.   And what is more, said 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. 

Physicist Ortwin Hess, at the Advanced Technology Institute in the University of Surrey, said the results were ’very interesting’. ’As I understand it, the spheres act as resonators (every resonator stores light) for a particular wavelength,’ he told Chemistry World.   ’The interesting fact is that via the ferroelectric properties of the materials, a particular wavelength can then be controlled and released by application of a field.’   Hess recently published a scheme to slow down and eventually store light using artificially designed solid metamaterials, but his idea is still theoretical.2Other experiments have involved stopping light altogether by using a gas of sodium or rubidium atoms chilled to near absolute zero, though that system is not practical for microchips.   

Many photonic crystals also control light by being patterned on the nano and micro-scale, and having regions of variable refractive index. Pumping a second laser light at photonic crystals can even alter their photon-trapping properties. But Mehta’s team seem to have chanced on a simpler fluid system with the easy dynamic control for which photonic crystal designers yearn.

Killugudi Jayaraman 

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