Understanding phonon polaritons and epsilon-near-zero modes in sapphire nanocones across the broad Reststrahlen band (385-1050 cm-1).

Tailoring light-matter interactions is crucial for advancing nanophotonics. Surface phonon polaritons are promising candidates for enhanced light-matter interactions due to their efficient, low-loss light confinement. In this work, we experimentally investigate the Reststrahlen bands in sapphire across the infrared spectrum, spanning ω = 385-1050 cm-1. We extended this investigation to nanocone-patterned sapphire resonators, with specific attention to its in-plane and out-of-plane permittivity components. Infrared spectroscopy and full-wave simulations revealed a range of optical excitations, including three surface phonon polaritons with quality factors as high as 40 ± 1, two hyperbolic volume phonon polaritons with quality factors as high as 83 ± 4, and one epsilon-near-zero mode with a quality factor of 122 ± 8 across the various Reststrahlen bands. Furthermore, confocal Raman scattering measurements showed enhanced Raman signals with maximum enhancement factors of 7.0 ± 0.6 on the nanostructured surface, indicating coupling between phonons and phonon-polaritons. Finally, finite element modeling of polarizability demonstrates good quantitative agreement with the measured results. This study is the first exploration of sapphire nanostructures and gives an in-depth understanding of phonon polaritons and epsilon-near-zero modes from nanocone-structured sapphire. The reported Raman enhancement attributed to coupling of phonon and phonon polariton modes holds promise for sensing through surface-enhanced Raman spectroscopy.