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MIT researchers control light with a shock

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CAMBRIDGE, Mass.--Light is faster than a speeding bullet, but MIT researchers found that light waves do some really weird things when they meet a speeding bullet head on.

Physicist Evan J. Reed, a postdoctoral associate in MIT's Research Laboratory of Electronics, and colleagues reported in the May 23 issue of Physical Review Letters that by sending a powerful shock wave through photonic crystals, they can change red light to green, for instance, slow light down in its tracks, and concentrate wide bandwidths of light such as those of the sun into narrower bandwidths that can be more efficiently harnessed by solar cells.

Some of the results may have applications in telecommunications and quantum optics.

A photonic crystal is a credit-card-thick stack of optical filters, the kind that might be used for coating lenses. When you launch a shock wave through these materials, they shatter in a fraction of a second. But right before that, they do some interesting things, Reed said.

Shock wave experiments allow researchers to see how materials react under severe stress. Because no one can go to extreme places like a planet's core, experiments like these help researchers see how materials' crystalline structure changes under high temperatures and pressure.

Reed, who probes material processes, is interested in what happens when shock waves propagate through periodic media. He had previously investigated silicon and metals. Although this is "basic shock wave physics," he said no one had ever tried it with photonic crystals.

It turns out that sending a shock wave through a photonic crystal allows researchers to gain ultimate control over light. By confining the light between the moving shock front and reflecting surface, incoming light can become trapped at the shock wave boundary, bouncing back and forth in a "hall of mirrors" effect. As the shock moves through the crystal, the light's wavelength is shifted slightly each time it bounces. If the shock wave travels in the opposite direction of the light, the light's frequency will get higher. If the wave travels in the same direction, the light's frequency drops.

By changing the way the crystal is constructed, researchers could control exactly which frequencies go into the crystal and which come out.

Three new phenomena emerged from the work:

The transfer of light frequency from the bottom of a bandgap to the top and vice versa.

The capture of light at near zero speed at the shock front for a controlled period of time.

  • The increase or decrease of the bandwidth of light by orders of magnitude with 100 percent energy conservation. This is significant because bandwidth broadening can be accomplished in other ways, but narrowing cannot be done easily or cheaply.

"No other system does this," Reed said. One of the problems with solar power, he said, is that the sun provides a broad spectrum of light that needs to be collapsed into a narrow bandwidth to harness it efficiently.

"The key is that this interface is moving," Reed said. "That's why these things happen. But you can observe the same effects in systems that do not involve shock waves. We can reproduce the physical effects in other kinds of systems."

In addition to Reed, authors are Marin Soljacic, an MIT Pappalardo postdoctoral fellow in physics, and John D. Joannopoulos, the Francis Wright Davis Professor of Physics at MIT.

This work is supported by the National Science Foundation's Materials Research Science and Engineering Center program.

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