doi:10.1038/nindia.2013.114 Published online 27 August 2013
Six years ago, physicists from a little known Indian university demonstrated a simple technique to stop light and store it in nano-sized magnetic spheres . They have now gone a step further by successfully transporting the "packet" of trapped light from one place to other along a magnetic field as if it were a piece of cargo .
Researchers Rasbindu Mehta and Rajesh Patel of Maharaja Krishnkumarsinh University in Bhavnagar have reported a technique of trapping light and transporting it. The technique will be useful in optical communication.
The search for an efficient storage technique for optical energy is of prime importance for photonic applications like bio-optics, solar windows, optical communication and other photonic devices. Scientists world-wide have been trying to produce systems that will control photons — particles of light — in the same way one controls electrons in microchips. Such systems would use light, rather than electricity.
Trapping light in any medium was considered an impossibility until 1987 when Sajeev John, now at University of Toronto, Canada, and Eli Yablonovitch of Bell Communications Research, USA independently showed the possibility to develop 'photonic band gap' (PBG), akin to electronic semiconducting material like silicon and germanium. Since then several PBG materials have been developed to trap and release light. But, according to the Bhavnagar scientists, the conventional PBG materials are difficult to fabricate and are not able to store photons long enough for useful applications.
The technique reported by Mehta and Patel in 2007 overcame these limitations. Their material — micron-size magnetic spheres dispersed in a ferro-fluid (FF) — is easy to prepare and their technique for storing the photons (through application of a varying magnetic field) is simple enough. At a critical magnetic field, light from a laser passing through this medium gets trapped and gets out when the field is switched off. But they soon realised their success in storing light at a fixed place was of little use.
"We thought that if this light can be transported and released at a distant spot, it will be useful for optical communication," Mehta, who continues research in the university's physics department after retirement, told Nature India.
Their latest report details a remarkably simple method of accomplishing exactly this. While there have been earlier attempts to transport stored light, the Bhavnagar scientists say their technique of loading and off-loading light is "simple, inexpensive and operates at room temperature."
According to the researchers their ferro-colloid material is transparent to light, has normal refractive index and is able to store photons for few hundreds of seconds, much longer than achieved by others. Further they are able to store and retrieve light "in the entire region of visible spectrum" while techniques developed by other groups have restrictions on the working frequency band.
The most intriguing effect, according to their report, is the trapping and release of light with the help of an externally applied varying magnetic field. In other words, their technique exploits the difference in the refractive index between the micrometer-sized magnetizable spheres and the surrounding ferro-fluids to induce resonance in the system by "tuning" the applied magnetic field.
"Tuning is sustained as long as the field is present and the light remains trapped within the magnetic spheres," said Mehta. "This light can be transported some distance by moving the spheres along with the magnetic field and released by switching off the field." According to the researchers, it is possible to increase the transport distance by using higher power lasers.