Group-Velocity Anomaly Modes in Hybrid Bands of Photonic Crystals Made of Ferroelectrics

In photonic crystals composed of ferroelectrics, the hybrid bands with corresponding to new additional band gaps are expected to appear around the Brillouin zone’s center and boundaries. In this hybrid bands, the group-velocity anomaly modes related to the phonon-polariton branches are expected to be discovered. Propagation characteristics of the group-velocity anomaly modes in the hybrid bands of one-dimensional photonic crystals fabricated by ferroelectric Li2Ge7O15 single crystals are discussed on the basis of finite element method and finite-difference time-domain numerical analyses and experimental results obtained by terahertz time-domain spectroscopy. It is founded that the electric-field intensity of the standing-wave mode at the end point of the dielectric band branch is found to be localized around all of the ferroelectrics plates in the photonic crystal. In contrast, group-velocity anomaly mode in the vicinity of the standing-wave mode is strongly localized around the first ferroelectrics plate on the incident side and decays as it propagates through the following ferroelectrics plates.