Shocked Photonic Crystals Tune Light
Daniel S. Burgess
In a series of "numerical experiments," researchers at Massachusetts Institute of Technology in Cambridge have discovered novel frequency-shifting and bandwidth-narrowing phenomena that occur when light interacts with a shock wave in a photonic crystal. They suggest that the effects may have applications in telecommunications and photovoltaics.
Figure 1. Computer simulations have revealed novel phenomena that occur when light interacts with a shock wave in a photonic crystal. In this series, the light propagating to the left is momentarily trapped at the front of the shock wave, is up-converted to a higher frequency and leaves to the right at a higher frequency. Images courtesy of MIT.
Evan J. Reed, a postdoctoral researcher working on the project with Marin Solja(breve)ci´c and John D. Joannopoulos, explained that the computer simulations revealed a classical process similar to Doppler shifting. For models of light incident with the front of a shock wave that compressed the lattice by a factor of two, they found that the light was momentarily confined at the front, where it experienced an up-conversion process that moved it to the top of the bandgap of the crystal, enabling it to escape in the opposite direction (Figure 1). When they introduced a reflective surface to one end of the photonic crystal, the shock wave narrowed the bandwidth of the light trapped between the moving front and the surface by a factor of four (Figure 2).
Figure 2. In this case, the light is confined between the front of the shock wave and a reflective surface at the right of the plot (top). As the front advances through the photonic crystal, the bandwidth narrows by a factor of four (bottom).
Because the simulations were based on direct solutions of Maxwell's equations, the researchers are confident that it will be possible to observe the phenomena in the lab. To that end, they are collaborating with a team at Lawrence Livermore National Laboratory in Livermore, Calif., to produce the shock waves in photonic crystals using a gas gun. Alternative means might include high-intensity lasers.
Each approach would destroy the photonic crystal, albeit after the phenomena occur, and the practical application of the effects likely would require less-extreme methods. "These effects can be observed with acoustic waves or MEMS [microelectromechanical systems] devices, rather than shock waves," Reed said. "Approaches of this sort are likely to have the most technological relevance."
Potential uses, he said, might be converting the frequency of telecom signals to prevent bottlenecks in the network, selectively reducing the bandwidth of the sun's spectrum to improve the efficiency of solar cells, and trapping light for single-photon quantum-information processing. "There already are applications of this new technology, but it is likely that nondestructive and repeatable methods of producing these effects will be required for them to be practical and cost-effective. Like any new technology, the utility is likely to be a function of cost and ease of use, etc."
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