Unidirectional ray polaritons in twisted asymmetric stacks
José Álvarez-Cuervo, Martin Obst, Saurabh Dixit, Giulia Carini, Ana I. F. Tresguerres‐Mata, C. Lanza, Enrique Terán-García, G. Álvarez-Pérez, L F Álvarez-Tomillo, Katja Diaz‐Granados, Ryan A. Kowalski, A. S. Senerath, Nicole Mueller, Lucía Herrer, J. M. De Teresa, Sören Waßerroth, J. Michael Klopf, Thomas E. Beechem, Martin Wolf, Lukas M. Eng, Thomas G. Folland, Aitana Tarazaga Martín-Luengo, Javier Martín‐Sánchez, Susanne C. Kehr, Alexey Y. Nikitin, Joshua D. Caldwell, Pablo Alonso‐González, Alexander Paarmann
Abstract
Abstract 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 propagation of polaritons by reporting the visualization of unidirectional ray polaritons (URPs). They arise naturally in twisted hyperbolic stacks with very different thicknesses of their constituents, demonstrated for homostructures of $$\alpha$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>α</mml:mi> </mml:math> -MoO 3 and heterostructures of $$\alpha$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>α</mml:mi> </mml:math> -MoO 3 and $$\beta$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>β</mml:mi> </mml:math> -Ga 2 O 3 . 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.