J.A.-C., A.I.F.T.-M., E.T.-G. and G.A.-P. acknowledge support from the Severo Ochoa program of the government of the Principality of Asturias (nos. PA-22-PF-BP21-100, PA-21-PF-BP20-117, PA-23-PF-BP22-046 and PA-20-PF-BP19-053, respectively). J.M.-S. acknowledges financial support from the Ramón y Cajal Program of the Government of Spain and FSE (RYC2018-026196-I), and the project PCI2022-132953 funded by MCIN/AEI/10.13039/501100011033 and the EU “NextGenerationEU/PRTR”. P.A.-G. acknowledges support from the European Research Council under Consolidator grant no. 101044461, TWISTOPTICS and the Spanish Ministry of Science and Innovation (State Plan for Scientific and Technical Research and Innovation grant number PID2022-141304NB-I00). A.Y.N. acknowledges the Spanish Ministry of Science and Innovation (grants PID2020-115221GB-C42, PID2023-147676NB-I00) and the Basque Department of Education (grant PIBA-2023-1-0007). L.H. and J.M.D.T. acknowledge financial support by Gobierno de Aragón via grant E13_23R with European Social Funds (Construyendo Europa desde Aragón). L.H. and J.M.D.T. thank the CSIC Interdisciplinary Thematic Platform (PTI) on Quantum Technologies (PTI-QTEP). Authors acknowledge the use of instrumentation as well as the technical advice provided by the National Facility ELECMI ICTS, node «Laboratorio de Microscopias Avanzadas (LMA)» at «Universidad de Zaragoza». Parts of this research were carried out at the ELBE Center for High-Power Radiation Sources at the Helmholtz–Zentrum Dresden–Rossendorf e. V., a member of the Helmholtz Association. M.O., L.M.E., and S.C.K. acknowledge the financial support by the Bundesministerium für Bildung und Forschung (BMBF, Federal Ministry of Education and Research, Germany, Project Grant Nos. 05K19ODB and 05K22ODA) and by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany’s Excellence Strategy through Würzburg-Dresden Cluster of Excellence on Complexity and Topology in Quantum Matter—ct.qmat (EXC 2147, project-id 390858490). T.G.F. was supported through the National Science Foundation through grant 2236807. S.D. was supported by the Office of Naval Research MURI grant N0001-23-1-2567. K.D.-G. acknowledges support by the Army Research Office under grant W911NF-21-1-0119. R.K. acknowledges support by the National Aeronautics and Space Administration (NASA) grant NASA 80NSSC22K1201. A.S.S. acknowledges support by the Office of Naval Research Petersen grant N00014-23-1-2676 while J.D.C. was supported by the Office of Naval Research grant N00014-22-1-2035 and by the Department of Energy—Basic Energy Sciences under Grant DE-FG02-09ER4655. G.C., N.S.M., S.W., M.W., and A.P. were supported by the Max Planck Society.

The vast repository of van der Waals (vdW) materials supporting polaritons offers numerous possibilities to tailor electromagnetic waves at the nanoscale. The development of twistoptics—the modulation of the optical properties by twisting stacks of vdW materials—enables directional propagation of phonon polaritons (PhPs) along a single spatial direction, known as canalization. Here we demonstrate a complementary type of directional propagationofpolaritonsbyreportingthevisualization ofunidirectional ray polaritons (URPs). They arise naturally in twisted hyperbolic stacks with very different thicknesses of their constituents, demonstrated for homostructures of α-MoO3 and heterostructures of α-MoO3 and β-Ga2O3. Importantly, their ray-like propagation, characterized by large momenta and constant phase, is tunable by both the twist angle and the illumination frequency. Apart from their fundamental importance, our findings introduce twisted asymmetric stacks as efficient platforms for nanoscale directional polariton propagation, opening the door for applications in nanoimaging, (bio)-sensing, or polaritonic thermal management.

Open Access funding enabled and organized by Projekt DEAL.

Álvarez-Cuervo, J. et al.

Peer reviewed