The propagation characteristics in a new microstructured single-core holey fiber-based plasmonic sensor are investigated using a finite element method. The fiber is specifically designed for sensing analytes with small refractive index values, like water solutions. The proposed structure is made by a silica core with a small air hole in the center, surrounded by six air holes placed at the vertices of a hexagon and four or five smaller air holes between some large air holes, and further enclosed by gold and water layers. The presence of the four small holes impedes the resonant interaction (at 0.623 μm) between one of the pair of twofold degenerate core modes with a plasmon mode and introduces two new core modes in resonance with the plasmon modes when the phase matching (at 0.618 μm) or loss matching (at 0.632 μm) conditions are satisfied. The addition of such four small air holes to a previously studied sensor structure produces a stronger transmission loss () of a core guided mode at the resonant coupling due to efficient interaction with a plasmon mode near the loss matching point in the red part of the visible spectrum (0.632 μm). The advantages of the configuration with five small air holes are a better spectral resolution, a smaller value of the FWHM parameter, a higher value of the signal-to-noise ratio, and a higher amplitude sensitivity. Our sensors are capable of detecting large ranges of refractive indices with accuracy of refractive index units.
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