In-line rainbow trapping based on plasmonic gratings in optical microfibers

In-line rainbow trapping is demonstrated in an optical microfiber with a plasmonic grating. The dispersions of x- and y-polarized surface plasmon polariton (SPP) modes are analyzed in detail by the 3D finite element method (FEM). In this system, the incident light is coupled from an optical microfiber into a graded grating. The plasmonic structure shows strong localization as the dispersion curve approaches cut-off frequency. Gradually increasing the depth or width of the grating elements ensures that the cut-off frequency of the SPP mode varies with the position along the microfiber. Near-infrared light at different frequencies can be trapped in different spatial positions. The in-line rainbow trapping is important for potential applications including optical storage, slow light, optical switch and enhanced light-matter interactions in fiber integrated devices and highly integrated optical circuits.