Negative refraction inspired polariton lens in van der Waals lateral heterojunctions

Controlling the phase of free-space waves and surface-confined polaritons is fundamentally important for many applications such as wave-front shaping, metasurface arrays, beam-steering surfaces, and optical modulators. Conventional lenses rely on the curvature of bulk transparent dielectrics to achieve gradual phase accumulation to convert plane waves into converging spherical waves. However, the same concept is not suitable for in-plane polariton waves, due to the large momentum mismatch between polariton modes in different effective index (neff) media. Here, we propose an approach for planar-polariton refractive optics, by constructing curved lateral heterojunctions with one side hosting positive neff polaritons and negative neff polaritons on the other side. The magnitude of neff maintains momentum matching, but the sign difference across the curved interface results in phase accumulation. We further demonstrate a proof of concept of this polariton lensing in a real material system, comprising lateral heterojunctions of 2D h-BN and graphene on 3D SiC, with lens resolutions 100 times smaller than the illumination wavelengths. Our results applying the concepts of in-plane negative refraction with phase modulation to design optical functionality open new opportunities for polariton wave-front control.