Phonon polaritons are collective nuclear charge oscillations resulting from the coupling of photons with optical phonons in polar materials. They are supported within a material-specific spectral region called the reststrahlen band, which is bounded by the transverse and longitudinal optical phonons. In this region, the material behaves optically like a metal; it is highly reflective and has a negative real part of the permittivity. When polar materials are nanostructured, phonon polaritons can enable a variety of near-field optical effects such as sub-diffraction light confinement. Phonon polaritons are similar to plasmon polaritons but have much lower optical losses because of the slower scattering rates of phonons, resulting in narrower resonance bands. Interestingly, a polar material which supports phonon polaritons can also have anisotropic optical properties, such that different components of its permittivity tensor have opposite signs. These materials are referred to as hyperbolic since, within the reststrahlen band, they behave optically like a dielectric and a metal along different crystal axes. Here, we compare our results from nanopillar and nanohole arrays fabricated in calcite, which support hyperbolic phonon polaritons (HPs). The HP modes observed in the nanohole arrays appear at higher frequencies than those in the nanopillar arrays. We have developed a qualitative understanding of the nature of the HP modes in the nanohole arrays, in which the modes are most likely confined to the constrictions between holes. In such a model, changing the pitch and/or the diameter of the holes will change the dimensions of the constrictions and result in changing the resonance frequency of the HP modes in a predictable way. Being able to theoretically describe HPs in calcite is a first step towards understanding the nature of these modes in a variety of materials in order to expand the range of phonon polariton materials available for mid-IR nanophotonic applications.

Phonon polaritons for mid-infrared photonics

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