Zero phase delay induced by wavefront modulation in photonic crystals
  A new mechanism for generation of efficient zero phase delay of electromagnetic wave propagation based on wavefront modulation is investigated in this paper. Both numerical simulations and experiment have demonstrated the zero phase delay behaviors of wave propagation in a two-dimensional triangular photonic crystal. The zero phase delay propagation, independent of the propagating distance, is attributed to an invariable wavefront modulation of photonic crystal in the direction of wave propagation, where the phase velocity is perpendicular to the group velocity with parallel wavefronts (or phasefronts) extending along the direction of energy flow. This effect can be extended to the three-dimensional cases or other artificially engineered materials, and may open a new route to obtain perfect zero-phase-delay propagation for electromagnetic wave instead of using zero-index or zero-averaged-index materials and have significant potential in many applications. 
Dong Guoyan, Zhou Ji and Cai Luzhong
physics.optics cond-mat.mtrl-sci