Hyperbolic Phonon Polaritons in Calcite Nanopillar and Nanohole Arrays for Nanoscale Infrared Confinement

Abstract

Abstract Body: 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; 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. Polar materials that support phonon polaritons can also have anisotropic optical properties, such that the optical constants depend on the propagation direction and polarization of the incoming light. When the principal components of the permittivity tensor have opposite signs, the material is referred to as hyperbolic. Hyperbolic materials are somewhat unusual as they behave optically like a dielectric and metal along different crystal axes, and this anisotropy of the metal-like response has important implications for the physics of its supported modes. Here, we report on the first experimental observation of hyperbolic phonon polaritons (HPs) in calcite nanopillar arrays, demonstrate the aspect ratio dependence of the HP resonance frequencies, discuss fabrication challenges, observe a new, possibly higher order mode as the pitch is reduced, and compare our results to both numerical simulations and an analytical model. We also found that the simulated electric field distributions for the HP modes are not localized to the pillars but extend appreciably into the calcite substrate. In addition, we have fabricated nanohole arrays in calcite, which also support HPs. However, the HP resonances appear to exhibit a slight aspect ratio dependence but opposite in trend to what was observed for the nanopillars. Additionally, the nanohole arrays have fewer HP modes and these modes are blueshifted in comparison to the nanopillars. We are currently using finite difference time domain simulations to characterize the HP modes supported in the nanohole arrays and to understand the differences between these modes and the HP modes confined within the nanopillars. Calcite is an ideal low-loss material for studying HPs that could find applications in mid-IR nanophotonic devices, and so these results are an important step toward creating a library of materials with the appropriate phonon properties to span the infrared.