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, which is bounded by the transverse and longitudinal optical phonons. In this region, the material behaves optically like a metal due to the negative real part of the permittivity, resulting in the incident light being strongly reflected. When polar materials are nanostructured, phonon-polaritons can enable a variety of near-field optical effects such as sub-diffraction light confinement. 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 as they behave optically like a dielectric in one direction and like a metal in the other. Here, we report on the observation of hyperbolic phonon-polaritons (HPhPs) in calcite nanopillar arrays, demonstrate the aspect ratio dependence of the HPhP resonance frequencies, and verify our results through comparison to an analytical model. Calcite is an ideal low-loss material for studying HPhPs that could find applications in mid-IR nanophotonic devices.