Substrate-Mediated Hyperbolic Phonon Polaritons in MoO₃

Hyperbolic phonon polaritons (HPhPs) are hybrid excitations of light and coherent charge oscillations that exist in strongly optically anisotropic 2D materials (e.g., MoO₃). These polaritons propagate through the material’s volume with long lifetimes, enabling novel mid-infrared nanophotonic applications by compressing light to sub-diffraction dimensions. Here, the dispersion relations and HPhP lifetimes (up to ≈ 2 ps) in single-crystal α-MoO₃ are determined by Fourier analysis of real-space, nanoscale-resolution polariton images obtained with the photothermal induced resonance (PTIR) technique. Measurements of MoO₃ crystals deposited on periodic gratings showed longer HPhPs propagation lengths (≈ 2 ×) and lower optical compressions in suspended regions compared to regions in direct contact with the substrate. Additionally, PTIR data reveal polymeric contaminants, resulting from sample preparation, localized under parts of the MoO₃ crystals. This work enhances the ability to engineer nanophotonic devices by leveraging substrate morphology to control polariton propagation.