The propagation in a new microstructured plasmon optical fiber specifically designed for sensing of water dissolved chemicals is investigated using a finite element method. The fiber is made by a silica core with a small air hole in the center of the structure, surrounded by six air holes placed at the vertices of a hexagon, and further enclosed by gold and water layers. In order to enhance the sensitivity, the structure is designed to have the phase matching point corresponding to the maximum of the power fraction for a core guided mode in the water and gold layers and to a minimum in the glass layer, and vice versa for the plasmon mode. This way, near the phase matching point there is a strong interaction between the core and plasmon modes, causing a splitting in the real part of the propagation constant and also a shift of the imaginary part of the effective index toward the higher wavelengths. The real part of the group refractive index shows a minimum (maximum for the group velocity) and a very small value of the imaginary part of the group refractive index near the phase matching point for the degenerate core mode. When the analyte refractive index is increased by 0.001 RIU, the phase matching point is shifted by 4 nm toward longer wavelengths, with a corresponding sensitivity better than .
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