Modeling Of Evanescent Coupling Between Parallel Optical Nanowires

Modeling of evanescent coupling between parallel nanowires is of special importance for applications such as evanescent-coupling-based devices, direct interconnection with external optical systems through fiber tapers, and energy exchange and control between two nanowires. In this paper, evanescent coupling between parallel nanowires is investigated using Three-Dimensional Finite-Difference Time-Domain (3D-FDTD) method. Silica, tellurite and silicon nanowires are used as typical materials in the simulation. Basic properties including the coupling efficiency and the transfer length for light exchange between the nanowires are obtained. The overlapping-length-dependent coupling efficiency shows an oscillating behavior similar to that in weakly coupled systems. Compared with weakly coupled waveguides, strongly coupled nanowires show much smaller transfer lengths without sacrificing high coupling efficiencies. Meanwhile, the minimum coupling efficiency is considerably higher than zero. High coupling efficiency can be obtained between different types of optical nanowires (index difference), such as coupling between the silica nanowires and the tellurite nanowires, coupling between the tellurite nanowires and the silicon nanowires, and coupling between the silica nanowires and the silicon nanowires. The polarization-dependent coupling properties, the supermode-cutoff-like behavior, and the end-to-end coupling properties are also investigated. The optical nanowires are promising building blocks for mocro- or nanoscale photonic devices, suggesting possibilities for reducing the size of optical waveguide coupling devices, as well as for achieving high-efficiency interconnection between nanowires and external optical systems. Coupling properties of nanowires demonstrated in this work may provide valuable references for practical applications of optical nanowires.