Strong coupling effects between ir-inactive zone folded LO phonon and localized surface phonon polariton modes in SiC nanopillars

Abstract

While plasmonics have a broad range of technological applications including infrared photovoltaics and photodetectors, plasmonic metals are subject to high optical losses in the long-wave infrared spectral regime. In order to reduce optical losses in the infrared, alternatives to plasmonic metals are being explored. One promising alternative employs polar dielectric materials, which exhibit a highly-reflective, optically-metallic spectral band (Reststrahlen band), bounded by the LO and TO optical phonons, and are capable of supporting plasmonic-like resonance in the infrared. In polar dielectrics, plasmonic-like resonances, known as surface phonon polariton (SPhP) resonances, arise from a coupling between incident light and collective oscillations of bound lattice charges, which are mediated by the optical phonons. In this study, we have examined the SPhP resonances of SiC nanopillars with constant height of 950 nm and width in the range of 200–400 nm, as a function of their aspect ratio (AR=Length/Width=0.5–16). As the nanopillar width is decreased, we have found that localized SPhP resonances redshift towards the zone folded LO (ZFLO) phonon that is normally not infrared active. However, as localized SPhP resonances are spectrally tuned through the ZFLO mode, we have found that the latter mode becomes infrared active. Furthermore, reflectance measurements have revealed strong coupling between the ZFLO and both the monopolar and dipolar localized SPhP resonances.