Thin layers of in-plane anisotropic materials can support ultraconfined polaritons, whose wavelengths depend on the propagation direction. Such polaritons hold potential for the exploration of fundamental material properties and the development of novel nanophotonic devices. However, the real-space observation of ultraconfined in-plane anisotropic plasmon polaritons (PPs)-which exist in much broader spectral ranges than phonon polaritons-has been elusive. Here we apply terahertz nanoscopy to image in-plane anisotropic low-energy PPs in monoclinic Ag2Te platelets. The hybridization of the PPs with their mirror image-by placing the platelets above a Au layer-increases the direction-dependent relative polariton propagation length and the directional polariton confinement. This allows for verifying a linear dispersion and elliptical isofrequency contour in momentum space, revealing in-plane anisotropic acoustic terahertz PPs. Our work shows high-symmetry (elliptical) polaritons on low-symmetry (monoclinic) crystals and demonstrates the use of terahertz PPs for local measurements of anisotropic charge carrier masses and damping.

Central European Institute of Technology Brno University of Technology Brno Czech Republic

CIC nanoGUNE BRTA and Department of Electricity and Electronics UPV EHU Donostia San Sebastián Spain

CIC nanoGUNE BRTA Donostia San Sebastián Spain

Departamento de Física Aplicada Escuela de Ingeniería de Gipuzkoa Universidad del País Vasco Donostia San Sebastián Spain

Departamento de Física Facultad de Ciencia y Tecnología Universidad del País Vasco Bilbao Spain

Donostia International Physics Centre Donostia San Sebastián Spain

IKERBASQUE Basque Foundation for Science Bilbao Spain

Institute of Physical Engineering Brno University of Technology Brno Czech Republic

Materials Physics Center CSIC UPV EHU Donostia San Sebastián Spain

Max Planck for Chemical Physics of Solids Dresden Germany

State Key Laboratory of Surface Physics and Department of Physics Institute for Nanoelectronic Devices and Quantum Computing Zhangjiang Fudan International Innovation Center Fudan University Shanghai China

Terahertz Technology Innovation Research Institute National Basic Science Center Terahertz Science and Technology Frontier Terahertz Precision Biomedical Discipline 111 Project Shanghai Key Lab of Modern Optical System University of Shanghai for Science and Technology Shanghai China

Theory of Condensed Matter Cavendish Laboratory University of Cambridge Cambridge UK

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