Shocking Color Effects

A photonic crystal is a lattice of structures (sometimes an arrangement of rods or a solid filled with a pattern of holes) with a periodic alteration in the index of refraction. In such a material waves with only a select band of frequencies may propagate successfully. Other frequencies are forbidden. What happens, though, when a shock wave moves through the lattice, momentarily compressing or expanding the characteristic spacings? A new "computational experiment" (detailed computer simulation) provides an intriguing answer. Evan J. Reed, Marin Soljacic, and John Joannopoulos at MIT determine that a light beam moving in a shock-modified photonic crystal will undergo two unexpected changes: a Doppler shifting hundreds or even 10,000 times bigger than usual and a bandwidth narrowing. There are plenty of phenomena that can broaden a signal's bandwidth but none yet known that would narrow the bandwidth of an arbitrary signal in this way (and by factors of 4 or more). As for the Doppler shift (a change in the frequency of the light owing to its reflection from a moving target), the light reflecting from the shock wave can be "up converted" (e.g., turned from red light into green light) with an efficiency that should match or exceed the up conversions achieved with nonlinear optical materials. Furthermore, the shock conversion process is tunable and independent of light intensity.

According to Evan Reed (617-253-5482) the MIT research should generate great surprise and interest among those who work with photonic crystals. The next step will be to implement the computational results in the laboratory with samples and actual shock waves, although for the sake of eventual commercial applications (frequency conversion and signal modulation) future modifications in photonic crystals will not have to be initiated with guns or laser pulses but with less destructive acousto-optic effects. The photonic-crystal modulations might even be actuated with some kind of MEMS (microelectromechanical systems) device. (Reed et al., Physical Review Letters, 23 May 2003; website http://ab-initio.mit.edu )