Enhanced light-matter interaction in Sapphire nanostructures

Phonon polaritons (PhPs) have garnered much attention due to efficient light confinement with minimal optical loss within the Infrared (IR) spectral range. This effect has been extensively studied in nanostructured polar dielectric systems, including surface phonon polaritons (SPhP) within the metallic Reststrahlen band (RB) of silicon carbide and hyperbolic volume phonon polaritons (HvPhP) from the hyperbolic RBs of hexagonal boron nitride. Sapphire (Al₂O₃) is a polar dielectric material that exhibits multiple RBs in the IR spectrum, with both metallic and hyperbolic regions of interest. While the optical properties of Al₂O₃ have been measured and studied from the visible to IR, further investigation is necessary to explore the properties of PhPs in Al₂O₃ nanostructures.

In this study, the IR (385 to 1050 cm^-1) reflectivity and Raman scattering (RS) of a Al₂O₃ nanocone array have been investigated, with specific attention to its in-plane and out-of-plane permittivity components. Across the various RBs of Al₂O₃, we have observed the excitation of a diverse range of optical resonances spanning three SPhPs, two HvPhP, and one epsilon-near-zero (ENZ) mode. Three SPhP modes were observed within one of the metallic RBs, where the permittivity is negative in all directions include two s-polarized modes at 734 cm^-1 and 790 cm^-1 , and one p-polarized mode at 772 cm^-1. Besides, two HvPhP excitations were excited under p-polarized reflectivity at 497 cm^-1 and 502 cm^-1 within one of the Type I hyperbolic RBs. Furthermore, an ENZ mode was observed in one of the dielectric regions for both polarizations. This ENZ resonance arises due to the strong dispersion of the in-plane permittivity component close to the material’s hyperbolic transition point at 565 cm^-1. On the other hand, the confocal RS measurement further revealed enhanced RS signals on the nanostructured surface, demonstrating the presence of phonon–PhP coupling. Lastly, finite element modeling of the electromagnetic fields has been performed, showing an excellent theoretical agreement with the measured reflectivity and RS results. Overall, this study indicates that Al₂O₃ nanostructures can serve as a promising platform for advanced IR nanophotonic applications.