Phonon Polaritons and Epsilon Near Zero Modes in Sapphire Nanostructures

TL;DR

This study explores the unique light-matter interactions in nanostructured sapphire within its Reststrahlen bands, identifying multiple phonon polariton modes and demonstrating enhanced Raman signals, advancing nanophotonic applications.

Contribution

It provides the first detailed investigation of LWIR phonon polaritons and ENZ modes in sapphire nanostructures using spectroscopy and simulations, revealing mode coupling and potential applications.

Findings

Identification of three SPhPs, two HVPhPs, and one ENZ mode in sapphire nanostructures.

Enhanced Raman signals indicating strong mode coupling.

Confirmation of mode dynamics through finite element modeling.

Abstract

Surface phonon polaritons (SPhPs) are promising candidates for enhanced light--matter interactions due to their efficient and low-loss light confinement features. In this work, we present unique light-matter interactions in saphhire within its Reststrahlen bands (RBs) across the long-wave infrared (LWIR) spectrum ($\omega = 385$-$1050~\mathrm{cm}^{-1}$). Particularly, we investigated the nanocone-patterned sapphire resonator array, with specific attention to its in-plane and out-of-plane permittivity components. Through Fourier transform infrared spectroscopy measurement and full-wave photonic simulations, we identified a range of optical excitations in the RBs, including three SPhPs, two hyperbolic volume phonon polaritons (HVPhPs), and one epsilon-near-zero (ENZ) mode. The depth-resolved confocal Raman spectroscopy revealed strongly enhanced Raman signals on the nanostructured surface, suggesting the mode coupling between phonons and phonon-polaritons, which was further confirmed by the finite element modeling of polarizability. This exploratory study provides in-depth insights into the dynamics of LWIR phonon polaritons and ENZ modes in the nanostructured sapphire, indicating its great potential for innovative nanophotonic applications.