  Surface phonon polaritons (SPhPs) - the surface-bound electromagnetic modes
of a polar material resulting from the coupling of light with optic phonons -
offer immense technological opportunities for nanophotonics in the infrared
(IR) spectral region. Here, we present a novel approach to overcome the major
limitation of SPhPs, namely the narrow, material-specific spectral range where
SPhPs can be supported, called the Reststrahlen band. We use an atomic-scale
superlattice (SL) of two polar semiconductors, GaN and AlN, to create a hybrid
material featuring layer thickness-tunable optic phonon modes. As the IR
dielectric function is governed by the optic phonon behavior, such control
provides a means to create a new dielectric function distinct from either
constituent material and to tune the range over which SPhPs can be supported.
This work offers the first glimpse of the guiding principles governing the
degree to which the dielectric function can be designed using this approach.

Materials Science