Pure Transverse Phonon-Polaritons in Laterally Bounded Piezoelectric Superlattices

Existing studies on transverse phonon-polaritons in one-dimensional piezoelectric superlattices, based on the assumption of infinite lateral dimensions (perpendicular to the periodic direction of ferroelectric domains), have shown that only transverse superlattice vibrations with a strain component along the periodic direction can couple with electromagnetic waves to generate transverse phonon-polaritons. Real samples, however, inevitably have finite lateral dimensions, indicating that the infinite-lateral-size model requires modification. In this study, we find that in laterally finite systems, pure transverse superlattice vibrations (those without any strain component along the periodic direction) can also couple with electromagnetic waves, giving rise to a new class of pure transverse phonon-polaritons. Theoretical analysis reveals that the energy of this mode is primarily confined to the crystal surface and propagates as surface waves. Experimental verification confirms the existence of this polariton, and this result provides a new degree of freedom for the design of microwave devices based on piezoelectric superlattices.