Emerging optical spin and chirality by three-dimensional evanescent field coupling

Optical spin and chirality play key roles in engineering photonic emission and light-matter interactions. Here we show that three-dimensional 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 \ensuremath{\pi}/2 between two coupled modes. For a directional coupler with 2 \ifmmode\times\else\texttimes\fi{} 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 spins and chiralities 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.