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IRG-I Nugget

The color of shock waves in photonic crystals

Light bandwidth narrows when light bounces between an advancing shock front and a mirror at the right-hand boundary of this image. Over the course of the simulation, the shock front moves about 10 pre-shock lattice periods a, and the frequency band is narrowed by about a factor of four. Much higher narrowing factors can be achieved in other scenarios.

The ability to significantly change the frequency of light by an arbitrary amount with high efficiency has been a long and challenging problem. Photonic crystals, materials with periodic modulations of index of refraction, provide a powerful new mechanism for controlling the properties of light and hence, are an attractive candidate system for frequency manipulation in ways that are not possible with conventional methods.

In a recent article (Reed, et al., Phys. Rev. Lett. 90, 203904, 2003), members of IRG-I report on the discovery of two unexpected and stunning physical phenomena when light reflects from a shock wave that propagates through a photonic crystal (a material that has a periodic modulation of the index of refraction). The first of these new phenomena is the observation of anomalous large "Doppler shifts" in light reflected from the shock wave. These frequency shifts are orders of magnitude larger than normal Doppler shifts and result in color shifts observable with the naked eye. The physical mechanism that gives rise to these frequency shifts is fundamentally new and completely different than existing, known mechanisms like nonlinear frequency conversion. The second new observable effect is the narrowing of the bandwidth of light that reflects from the shock wave with no losses (see Figure). There are many physical systems that increase the bandwidth of light, but to our knowledge, no existing classical systems are capable of narrowing the bandwidth of an arbitrary signal. While these two effects are presented within the context of a shock wave propagating in a photonic crystal, their generality make them amenable to observation in a variety of non-destructive and reusable time-dependent photonic crystal systems. These effects could impact technologies ranging from telecommunications to solar power to quantum information processing.

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