Crystal Bash: Shocking changes to light’s properties

Sometimes a good, hard whack makes things work better. That’s what a new study suggests about light-manipulating microstructures known as photonic crystals.

GOING UP. Simulated light pulse speeding to the left through a photonic crystal (top to bottom) hits a shock wave (dotted vertical line) that’s heading right, gets a frequency boost (shown as higher position in bottom two panels), and reflects back (blue arrow). Reed et al./PRL

Those orderly arrangements of tiny films, rods, balls, or even holes exclude specific wavelengths of light–a trait of growing importance for many optical components, including lasers and optical fibers.

Given the potential of photonic crystals, researchers are racing to find better ways to make the structures (SN: 5/25/02, p. 334: Available to subscribers at Tiny tungsten beams lord over light). Now, a team at the Massachusetts Institute of Technology (MIT) suggests potential capabilities of photonic crystals that nobody had suspected.

Evan J. Reed and his colleagues have calculated the effects of smacking a photonic crystal to launch a wave of compression–a shock wave–through the structure. To simplify the calculations and computer simulations based on them, the team considered a so-called one-dimensional photonic crystal–a stack of thin alternating sheets of materials that differ in how quickly light traverses them.

Analyzing what would happen when a laser pulse enters a crystal and hits a shock wave, the scientists find that the light changes in unexpected and technologically interesting ways.

A Doppler shift of light underlies the new effects. Imagine a beam rebounding from a mirror that’s rushing toward the light source. The reflected light’s frequency is shifted slightly upward.

A related effect should occur in head-on collisions between light pulses and shock waves in photonic crystals, report Reed, Marin Soljacic, and John D. Joannopoulos in an upcoming Physical Review Letters. However, the frequency shift can be tens of thousands of times as large as normal Doppler shifts, Reed says.

In the MIT scenario, light becomes trapped and bounces around within the thin, moving zone where the shock wave is compressing the crystal. Every time the light beam bounces, “it’s picking up a little bit of Doppler shift,” Reed explains.

Because the surrounding crystal is designed to reject the light’s elevated frequency, the light remains in the compression zone until its ever-increasing Doppler shift has moved its frequency into a higher range that the crystal accepts.

The pulse can then reemerge into the rest of the crystal.

“This ability to move around the frequency of a pulse more or less to order . . . has many potential applications,” comments John B. Pendry of the Imperial College London in England. Yet the “elevator effect,” as Pendry calls it, is only one of several new tricks a shocked photonic crystal should perform.

Others include transforming a steady light beam into pulses, slowing light to the speed of the shock wave (SN: 4/19/03, p. 252: Available to subscribers at Light rambles through room-temperature ruby), and funneling a broad light pulse into a narrower frequency range, the MIT team finds.

“All of these effects could be useful in manipulating modern optical signals,” comments David J. Norris of the University of Minnesota, Twin Cities.

Implementing the effects may prove challenging, however. Scientists at Lawrence Livermore (Calif.) National Laboratory are preparing to use high-velocity projectiles to produce shock waves in crystals and test the MIT predictions. Even if those experiments succeed, the practicality of using strong shock waves remains limited. Not only are their effects short-lived, but they also would destroy the crystals.

Fortunately, Reed notes, there are gentler ways of creating shock waves. Moreover, using laser patterns projected onto so-called nonlinear crystals, researchers may be able to mimic a shock wave passing through a photonic crystal. That may achieve the newfound powers without applying a hard whack.


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