Hyperbolic phonon polaritons in calcite for nanoscale infrared confinement

Phonon polaritons are collective oscillations resulting from the coupling of photons with optical phonons in polar materials and are supported within a material-specific spectral region called the reststrahlen band. 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. 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 as they behave optically like a dielectric and a metal along different crystal axes. Here, we report on the first observation of hyperbolic phonon polaritons (HPs) in calcite nanopillar arrays, demonstrate the aspect ratio dependence of the HP resonance frequencies, discuss fabrication challenges, and compare our results to numerical simulations and analytical models. The results of this work are an important first step towards creating a library of materials with the appropriate phonon properties to extend phonon polariton applications throughout the infrared.