Paratellurite Nanowires as a Versatile Material for\nTHz Phonon Polaritons

Polaritons,\ni.e., hybrid quasi-particles of light and matter resonances,\nhave been extensively investigated due to their potential to enhance\nlight–matter interactions. Although polaritonic applications\nthrive in the mid-infrared range, their extension to the terahertz\n(THz) range remains limited. Here, we present paratellurite (α-TeO<sub>2</sub>) nanowires, a versatile material acting as a platform for\ndifferent types of phonon polaritons. Utilizing synchrotron infrared\nnanospectroscopy from 10 to 24 THz, we uncover the polaritonic properties\nof α-TeO<sub>2</sub> nanowires, showcasing their dual functionality\nas both a Fabry–Pérot cavity and a waveguide for surface\nphonon polaritons. Furthermore, near-field measurements with a free-electron\nlaser as a THz source reveal a localized optical contrast down to\n5.5 THz, an indication of hyperbolic bands. Our findings complement\nthe repertoire of polaritonic materials, with significant implications\nfor advancing THz technologies.