Electromagnetic response nonreciprocity induced by spin-orbit coupling in transdimensional plasmonic systems

We explore the optical properties of ultrathin transdimensional (TD) plasmonic film systems of variable thickness by studying the electron spin-orbit coupling (SOC) effect on their electromagnetic (EM) response. In the TD regime, the plasma frequency of the system shows thickness-dependent spatial dispersion (nonlocality) in ultrathin films but becomes constant in thicker ones [1,2]. With SOC considered, the plasma frequency becomes complex, a feature persisting only in noncentrosymmetric TD systems. We find that the complex nonlocal plasma frequency leads to an anisotropic and nonreciprocal EM response tensor, where off-diagonal terms (causing anisotropic and nonreciprocal EM response) depend on the electron spin orientation and EM wave propagation direction relative to the in-plane symmetry axis of a noncentrosymmetric quasi-2D lattice. By applying an external perpendicular magnetostatic field one can verify the anisotropic and nonreciprocal EM response behavior, in which case the effect can be reversed by flipping the direction of the field. These findings can lead to a new controllable approach for tailoring the optical properties of the TD plasmonic systems. [1] I.V.Bondarev & V.M.Shalaev, Opt. Mater. Express 7, 3731, 2017; [2] D.Shah et al., Nano Lett. 22, 4622, 2022.