Emerging optical spin and chirality by 3D evanescent field coupling

Abstract: Optical spin and chirality play key roles in engineering photonic emission and light-matter interactions. Here we show that 3D evanescent coupling of guided modes by strongly confined waveguides can extrinsically produce optical spin and chirality. These emerging optical quantities are essentially derived from an intrinsic phase retardation of {\pi}/2 between two coupled modes. For a directional coupler with 2*2 input ports, the produced optical spin and chirality are inherently characterized by the side-locked and path-locked phenomena, respectively. We numerically demonstrate that these optical spin and chirality can be ubiquitously produced by higher-order mode coupling and other structural waveguides. These new quantities in optical waveguides may pave the way to exploit near-field photonic spin and on-chip chiral light-matter interactions.