  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.
