Observation of Localized Resonant Phonon Polaritons in Biaxial α-MoO₃ Nanoparticles

Anisotropic subwavelength particles uniquely combine the strong, tunable response of nanostructures with the exotic properties of anisotropic materials, enabling diverse applications in photonics, biomedicine, and magnetism. Anisotropic particles are also prevalent in systems such as ice grains, liquid crystal droplets, and ferromagnetic particles. Nanostructures supporting hyperbolic phonon-polaritons hold significant promise for infrared applications due to their strong anisotropic optical response. However, previous experiments primarily explored isotropic or uniaxial nanostructures, with eigenmode theories limited to isotropic particles, restricting the understanding and applicability of anisotropic particles. Here, localized phonon resonances in the mid-infrared spectral region in biaxial nanoparticles with three distinct axial permittivities are observed. Using a novel femtosecond-pulsed laser ablation method, α-molybdenum trioxide nanoparticles are synthesized with tunable, high-Q-factor mid-infrared resonances. Additionally, a comprehensive theoretical framework is derived for anisotropic nanoparticles, which aligns exceptionally well with the experimental results. The findings uncover the physics of polaritons in biaxial nanoparticles, including both fundamental and higher-order modes, paralleling the significant shift in isotropic plasmon-polariton research toward nanostructure resonators in the visible range. The research paves the way for a new generation of tunable, multispectral, anisotropic, and directional mid-infrared nanoresonators, opening new possibilities for mid-infrared imaging, sensitive photonic devices, and biomarkers.