Optical helicity density in the thermal near field of twisted bilayer van der Waals materials
Phys. Rev. B 114, 045414 – Published 10 July, 2026
DOI: https://doi.org/10.1103/sdrc-g2ls
Abstract
Twisted van der Waals (vdW) bilayers can support tunable surface/hyperbolic phonon polaritons (S/HPhPs) depending on the interlayer twist angle. S/HPhPs can be thermally excited and significantly modify the thermal near field. A photonic topological 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 twisted bilayer vdW materials is identified and investigated. To evaluate the OHD, a coherence-matrix method is derived 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 of the vdW bilayer at different twist angles. A strong correlation between the OHD and TTA is observed. This behavior is associated with polariton canalization and the asymmetric polariton modal response. In this study, we provide insights into the analysis of angular momentum in near-field thermal radiation from twisted vdW structures.
