  Engineering of phonons, i.e., collective lattice vibrations in crystals, is
essential for manipulating physical properties of materials such as thermal
transport, electron-phonon interaction, confinement of lattice vibration, and
optical polarization. Most approaches to phonon-engineering have been largely
limited to the high-quality heterostructures of typical semiconductors. Yet,
artificial engineering of phonons in a variety of materials with functional
properties, such as complex oxides, will yield unprecedented applications of
coherent tunable phonons in future quantum acoustic devices. In this study, we
demonstrate artificial engineering of phonons in the atomic-scale SrRuO3/SrTiO3
superlattices, wherein tunable phonon modes were observed via confocal Raman
spectroscopy. In particular, the coherent superlattices led to the backfolding
of acoustic phonon dispersion, resulting in zone-folded acoustic phonons in the
THz frequency domain. We could further fine-tune the frequencies from 1 to 2
THz via atomic-scale precision thickness control. In addition, we observed a
polar optical phonon originating from the local inversion symmetry breaking in
the artificial oxide superlattices, exhibiting emergent functionality. Our
approach of atomic-scale heterostructuring of complex oxides will vastly expand
material systems for quantum acoustic devices, especially with the viability of
functionality integration.
