Broadband group-velocity anomaly in transmission through a terahertz photonic crystal slab

We have measured the complex transmission coefficient of a two-dimensional photonic crystal slab for propagation perpendicular to the slab, using terahertz time-domain spectroscopy. At high frequencies, where the lattice parameter exceeds the wavelength, we find that the group delay is equal to that of empty space despite the fact that the volume-weighted average dielectric of the slab is epsilon(ave)similar to 3.8. In contrast to most other examples of anomalous group delay effects, this unusual phase transparency persists over a very broad spectral bandwidth, demonstrating that such anomalies do not require the presence of a nearby isolated resonance. Our results are consistent with finite-difference time-domain simulations.