Shock waves may enable wavelength modulation
Based on a computer simulation, researchers at the Massachusetts Institute of Technology (MIT; Cambridge, MA) believe that it should be possible to linearly transform the wavelength of a light beam in a Doppler-like fashion by applying a mechanical shock to a photonic crystal.
Based on a computer simulation, researchers at the Massachusetts Institute of Technology (MIT; Cambridge, MA) believe that it should be possible to linearly transform the wavelength of a light beam in a Doppler-like fashion by applying a mechanical shock to a photonic crystal. Experimental teams at Los Alamos National Labs (LANL; Los Alamos, NM) and at Lawrence Livermore National Lab (LLNL; Livermore, CA) are attempting to actually produce the simulated results.
Evan Reed and colleagues at MIT simulated the effects of light scattering from a shock wave in a photonic crystal by using finite-difference time-domain simulations of Maxwell's equations in one dimension of a single-polarization light pulse entering a multilayer dielectric material at normal incidence. They observed three unexpected phenomena: a tunable frequency shift, capture of light at the shockwave front for a controllable time period, and bandwidth narrowing.1
"The effect of the shock wave is to compress the lattice constant of the photonic crystal," Reed said. The observed effects occur because the bandgap frequencies behind the shock are different from those in front and because the frequencies of electromagnetic modes change at the shock front, he explained. So when light enters the system at certain frequencies it can couple into the modes and change its frequency (see figure).
"This particular type of frequency conversion is a fundamentally new way of doing frequency conversion, because the photonic-crystal system is a completely linear system and the materials have a completely linear optical response," Reed said in comparing it to nonlinear optical methods of wavelength conversion. "This means that you can get these shifts for arbitrarily low intensities of light. In fact, you can even do it for single photons."
To verify whether these simulated results take place in real systems, researchers at LANL have set up a high-intensity laser to launch a shock into a multilayer film; and an LLNL group plans to launch a shock wave using a projectile from a large gun. If these tests prove successful, it should eventually be possible to reproduce the effect in more technologically relevant systems such as acousto-optic modulators or MEMS (microelectromechanical systems) devices containing photonic crystals, said Reed.
- E. J. Reed et al., Phys. Rev. Lett. 90(20) 203904-1 (May 23, 2003).