The ability to slow down and trap light has become a hot topic in physics since it was first observed in the 1990s. The ability to trap electromagnetic waves has important applications in areas such as information storage, sensing, and quantum optics.
But the field has not progressed quite as quickly as many had hoped. That’s largely because of the complexity of the experimental setup and the difficulty of releasing the waves with their original properties after they have been trapped.
Today, Toshihiro Nakanishi and pals at Kyoto University in Japan reveal a new approach to this problem that has the potential to bring the routine storage and release of electromagnetic waves closer to reality.
Conventional light trapping relies on atoms such as caesium and rubidium that have special combinations of ground and excited states. These atoms absorb at one specific frequency. However, if they are excited by zapping them with a laser at another frequency, called a probe, light can then pass through.
This phenomenon is called electromagnetically induced transparency. The trapping comes about by switching off the probe laser while light is passing through a cloud of these atoms. This light then becomes trapped.
But there is another way to achieve this kind of trapping, say Nakanishi and co. Instead of a cloud of atoms, these guys have created a metamaterial but does the same job.
Metamaterials are periodic arrays of subwavelength–sized components that influence the passage of electromagnetic waves. Their beauty is that they can be engineered with properties no natural material possesses.
In this case, Nakanishi and co have created a metamaterial in which each repeating unit contains two variable capacitors. One of the capacitors is designed to absorb and radiate waves at a particular frequency while the other is designed to trap them.
If the capacitors are tuned to the same frequency, any light at that frequency is absorbed and trapped. Detuning the capacitors then releases the electromagnetic waves, allowing them to continue on their way.
That’s the theory, at least. Impressively, Nakanishi and co have built a proof-of-principle device that does exactly this with microwaves.
That’s the first time anybody has demonstrated the storage and release of electromagnetic waves using a metamaterial.
What’s more impressive, however, is that the released waves have the same phase distribution as the originals. “The electromagnetic waves were stored and released, while maintaining the phase distribution in the propagating direction,” they say.
And things could get significantly better. The proof of principle experiments were done with a metamaterial made of only three layers. But these results suggest that a more extensive metamaterial structure could store and release waves of arbitrary shape and polarisation.
For the moment, the technique is limited to microwaves although Nakanishi and co give a brief description of how this might be translated to optical frequencies.
But even microwave metamaterials have application in areas such as radar and telecoms. Nakanishi and co give little detail about the kind of application their approach would make possible so suggestions please in the comments section below.
Ref: arxiv.org/abs/1304.2443: Storage of Electromagnetic Waves in a Metamaterial that Mimics Electromagnetically Induced Transparency