Phonon-polariton dispersion and the polariton-based photonic band gap in piezoelectric superlattices

The phonon-polariton dispersion relation is calculated in the piezoelectric superlattice (PSL). In this specific case, the coupling could take place not only between the electromagnetic (EM) waves (photons) and the transverse superlattice vibrations (transverse phonons), but also between the EM waves and the longitudinal superlattice vibrations (longitudinal phonons), which gives rise to two types of phonon polaritons. It differs with the case in an ionic crystal, where the phonon polariton could only originate from the coupling between transverse lattice vibrations (transverse optical phonons) and EM waves. The two types of phonon polaritons are experimentally observed in the one-dimensional PSL with a periodicity of micrometer scale, corresponding to the frequency region of microwave. The PSL is based on a congruent lithium tantalite (LiTaO3) crystal and fabricated by the method of electric field poling at room temperature. Both microwave absorption and dielectric abnormality are observed and the sizes of band gaps are measured for the two types of phonon polaritons.