Dual Doppler Effect in Wedge-Type Photonic Crystals.

The dual Doppler effect, in the simultaneous occurrence of both normal and inverse Doppler effect in one moving two-dimensional wedge-type photonic crystal, is proposed. An improved finite-different time-domain algorithm is used to verify this phenomenon. The spatial Fourier Transformation has been applied to complex electrical field data to reveal the mechanism. The harmonics with negative spatial frequencies show a lagging phase evolution, while those with positive spatial frequencies show a front phase evolution. Different wedge-type photonic crystals are designed to filter out the required harmonics based on the systematic study of spatial Fourier Transformation and wave vector diagram. Our work paves a new way for Doppler cooling of atomic gases, radar deception, invisibility cloaks, microstructure dual frequency interferometer and so on.