Halting photons could lead to miniature particle accelerators, improved data transmission

December 22, 2014 by David L. Chandler
Plot of radiative quality factor as a function of wave vector for a photonic crystal slab. At five positions, this factor diverges to infinity, corresponding to special solutions of Maxwell equations called bound states in the continuum. These states have enough energy to escape to infinity but remain spatially localized.

Researchers at MIT who succeeded last year in creating a material that could trap light and stop it in its tracks have now developed a more fundamental understanding of the process. The new work—which could help explain some basic physical mechanisms—reveals that this behavior is connected to a wide range of other seemingly unrelated phenomena.

The findings are reported in a paper in the journal Physical Review Letters, co-authored by MIT physics professor Marin Soljačić; postdocs Bo Zhen, Chia Wei Hsu, and Ling Lu; and Douglas Stone, a professor of applied physics at Yale University.

Light can usually be confined only with mirrors, or with specialized materials such as photonic crystals. Both of these approaches block light beams; last year's finding demonstrated a new method in which the waves cancel out their own radiation fields. The new work shows that this light-trapping process, which involves twisting the polarization direction of the light, is based on a kind of vortex—the same phenomenon behind everything from tornadoes to water swirling down a drain.

In addition to revealing the mechanism responsible for trapping the light, the new analysis shows that this trapped state is much more stable than had been thought, making it easier to produce and harder to disturb.

"People think of this [trapped state] as very delicate," Zhen says, "and almost impossible to realize. But it turns out it can exist in a robust way."

In most natural light, the direction of polarization—which can be thought of as the direction in which the light waves vibrate—remains fixed. That's the principle that allows polarizing sunglasses to work: Light reflected from a surface is selectively polarized in one direction; that reflected light can then be blocked by polarizing filters oriented at right angles to it.

Vortices of bound states in the continuum. The left panel shows five bound states in the continuum in a photonic crystal slab as bright spots. The right panel shows the polarization vector field in the same region as the left panel, revealing five vortices at the locations of the bound states in the continuum. These vortices are characterized with topological charges +1 or -1.

But in the case of these light-trapping crystals, light that enters the material becomes polarized in a way that forms a vortex, Zhen says, with the direction of polarization changing depending on the beam's direction.

Because the is different at every point in this vortex, it produces a singularity—also called a topological defect, Zhen says—at its center, trapping the at that point.

Hsu says the phenomenon makes it possible to produce something called a vector beam, a special kind of laser beam that could potentially create small-scale particle accelerators. Such devices could use these vector beams to accelerate particles and smash them into each other—perhaps allowing future tabletop devices to carry out the kinds of high-energy experiments that today require miles-wide circular tunnels.

The finding, Soljačić says, could also enable easy implementation of super-resolution imaging (using a method called stimulated emission depletion microscopy) and could allow the sending of far more channels of data through a single optical fiber.

"This work is a great example of how supposedly well-studied physical systems can contain rich and undiscovered phenomena, which can be unearthed if you dig in the right spot," says Yidong Chong, an assistant professor of physics and at Nanyang Technological University in Singapore who was not involved in this research.

Chong says it is remarkable that such surprising findings have come from relatively well-studied materials. "It deals with photonic crystal slabs of the sort that have been extensively analyzed, both theoretically and experimentally, since the 1990s," he says. "The fact that the system is so unexotic, together with the robustness associated with topological phenomena, should give us confidence that these modes will not simply be theoretical curiosities, but can be exploited in technologies such as microlasers."

Explore further: New phenomenon could lead to novel types of lasers and sensors

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8 comments

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xstos
not rated yet Dec 22, 2014
What about phasers!? Make it so.
BONK__RS
5 / 5 (1) Dec 22, 2014
what about robust qubits?
Egleton
not rated yet Dec 22, 2014
what about robust qubits?

Beat me to it.
movementiseternal
Dec 22, 2014
This comment has been removed by a moderator.
movementiseternal
Dec 22, 2014
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katesisco
1 / 5 (2) Dec 23, 2014
Well, I did say that bh are actually mazes where light is trapped. The above seems to say the same thing. KateSisco papers are on Google Drive
swordsman
1 / 5 (3) Dec 23, 2014
"diverges to infinity"? Ridiculous!

Maxwell's equations are incomplete, as Planck discovered. They still do not understand electromagnetic theory.
Torbjorn_Larsson_OM
5 / 5 (4) Dec 23, 2014
The arxiv paper is here: http://arxiv.org/...37v1.pdf

The trapping in the center of the vortex is a topological defect (think defects in spin fields, or dislocations in crystals), but it looks to have connections with other optics and solid state physics as well. Among other things it connects with those vector beams of photonic crystal lasers, a new but exciting phenomena for me.

@swordsman: I don't understand your point.

All our theories have divergences of some kind or other, because they are thus far effective. They use classical electromagnetism to rapidly get a good enough understanding,

That classical electrodynamics breaks down is nothing new, say when applied to temperature radiation ("UV catastrophe") or particles (field sources and sinks), and that is why quantum field theory was discovered (and avoids some of those problems).

You seem to know of the progress made. Progress which we can hope for but never guarantee.

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