Topologically enhanced optical helicity density in the thermal near field of twisted bilayer van der Waals materials

Abstract

Twisted van der Waals (vdW) bilayers can support tunable surface/hyperbolic phonon polariton (S/HPhP) depending on the interlayer twist angle. S/HPhPs can be thermally excited and significantly modify the thermal near field. A photonic topological phase transition occurs at a critical twist angle where the polariton dispersion switches from hyperbolic to elliptical. Because the twist angle governs the polariton modes, it is intrinsically linked to the optical helicity density (OHD) of the near-field thermal emission. In this work, a relationship between the OHD of near-field emission and the twist angle of bilayer twisted vdW materials is discovered and investigated. To evaluate the OHD, a 3$\times$3 coherence matrix method is obtained from the fluctuation-dissipation theorem (FDT), which provides a complete description of the thermal electromagnetic field of the twisted bilayer, and a formalism for OHD based on the polarization matrix is employed. The topological transition angle (TTA) is determined by calculating the polariton dispersion relation of the vdW bilayer at different twist angles. A strong correlation between OHD and TTA is observed, which can be attributed to polariton canalization and confined group velocity, leading to enhanced polariton directionality. This study provides new insights into the analysis of angular momentum in near-field thermal radiation from twisted vdW structures.