Topological darkness in van der Waals materials

Abstract

For decade, researchers face numerous challenges trying to embed two-dimensional (2D) van der Waals (vdWs) materials into optoelectronic devices. One such key problem is the small phase accumulation (∆φ ~ 0.01π) inside 2D material due to its atomic thickness (t ~ 0.7 nm), while the majority of optoelectronic devices require ∆φ ~ π. As a result, current efficiency of 2D photonic devices limited by about 1%. Here, we provide a solution to this task through topological darkness in vdW materials.In this work, we utilize the concept of topological phase singularity points in reflection to acquire the desired optical phase change ∆φ ~ π in atomically thin materials. These points arise at zero-reflection since at zero amplitude the phase is undefined and thanks to excitonic resonances obtain topological charge in contrast to classical Brewster zero-reflection phenomena. As a consequence of topology, optical phase exhibit rapid optical phase change ∆φ ~ π, which we experimentally observed by spectroscopic ellipsometry for 2D PdSe2, MoS2, WS2, and graphene. Hence, the proposed topological approach provide an indispensable route to manipulate optical phase in vdW-based devices. For demonstration, we created a label-free biological sensor with the record phase sensitivity of 7.5·104 degrees per refractive index change using topological phase singularity of atomically thin PdSe2. Additionally, we describe topological phenomena of phase singularities in reflection: annihilation of topological charges and high-order topological charges.

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Georgy Ermolaev
Moscow Institute of Physics and Technology
Russia

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