November 13, 2015 feature
Device can theoretically trap a light 'bit' for an infinite amount of time
Sylvain Lannebère and Mário G. Silveirinha at the University of Coimbra in Portugal have published a paper on the new light-trapping device in a recent issue of Nature Communications.
As the researchers explain in their paper, previous research has confined light for finite periods of time using mirrors and specially engineered materials, but confining light indefinitely—even in theory—remains elusive.
"Light is an object difficult to tame," the researchers wrote in their paper. "No matter how elaborate and intricate are the material constructions that may be used to screen it from the exterior environment, there is always some residual coupling with the radiation continuum, and hence light—if not absorbed by the material walls—always finds its way out."
But now the scientists may have found a way to keep light in.
"We have unveiled a way to trap light in an open nanostructure with a lifetime which, in ideal circumstances, can be infinitely large," Silveirinha told Phys.org.
To overcome light's penchant for escaping, Lannebère and Silveirinha utilized an idea proposed by John von Neumann and Eugene Wigner in 1929, and later extended by others, which has led to the discovery that transparent structures with tailored geometries can perfectly confine light by scattering it in a very specific way.
Lannebère and Silveirinha showed that this strategy for confining light can be achieved by shining light on a spherical "meta-atom," so-named because it allows light to have only a specific quantized energy value (creating a light "bit"), similar to how an atom allows electrons to occupy only certain quantized energy levels.
In order for the scattered light to stay confined, the wavelength of light in the meta-atom's core must have a very specific value. As the scientists explain, the trick to letting the incoming light enter the meta-atom but not escape afterwards relies on a nonlinear effect that enables squeezing the light wavelength in the core until it reaches a critical value at which the light is perfectly confined by the meta-atom walls. This critical light wavelength in the core is determined by the amount of stored energy, so that the light trapping occurs only for a very precise (quantized) energy value.
Based on this strategy, the optical meta-atom can trap a bit of light for a time limited only by imperfections in the materials. To compensate for this inevitable loss, the researchers suggest using an optical gain element, like a weaker version of the gain used in lasers. This addition would also enable the meta-atom to serve as a basic one-bit optical memory.
While storing light for long times is highly desirable, there are also times when the light needs to be released. In order to quickly free the trapped light, the researchers have proposed illuminating the meta-atom with a second light pulse, which would collide and mix with the trapped light in a way that would cause a quick release.
In general, the ability to confine light in a small region of space has a variety of applications, including light-emitting sources, chemical and biological sensors, and other nanoscale photonics devices.
Currently, the scientists are exploring planar-type designs for the meta-atom that can be more easily integrated on a chip. They also hope that their theoretical work may trigger the attention of other groups who have the means to realize the meta-atom experimentally.
And what would a trapped light bit look like?
"In theory, the stored light cannot be seen from the outside," Silveirinha said. "However, any realistic solution requires a gain mechanism to keep the light confined in the meta-atom. If the gain element is switched off, then the light within the meta-atom will be released, and then it may be seen with the naked eye!"
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