Various metal-insulator-metal- (MIM-) type plasmonic waveguides and gratings are investigated numerically. Three gratings are treated: one is formed by alternately stacking two kinds of MIM waveguides, another by periodic changes in the dielectric insulator materials of an MIM waveguide, and the other by a periodic variation of the air core width in an MIM waveguide. The dispersion property of each MIM waveguide of which the grating consists is analyzed using the implicit Yee-mesh-based beam-propagation method. It is shown that the third one has a relatively large effective index modulation of the guided mode with a simple grating structure, while maintaining a low propagation loss. Further examination is given to modifications of this grating structure. The transmission characteristics are examined using the frequency-dependent implicit locally one-dimensional FDTD method. We discuss how the modified grating structure affects the bandgap of the transmission characteristics. 1. Introduction Recently, metal-insulator-metal- (MIM-) type plasmonic waveguides have received considerable attention, since compact optical circuits may be realized [1, 2]. The alternative effective index modulation of an MIM waveguide leads to a plasmonic waveguide Bragg grating that is one of the basic building blocks for small size plasmonic circuits. Three gratings have been mainly investigated: one is formed by alternately stacking two kinds of MIM waveguides (Figure 1(a)) [3], another by periodic changes in the dielectric insulator materials of an MIM waveguide (Figure 1(b)) [4], and the other by a periodic variation of the air core width in an MIM waveguide (Figure 1(c)) [5, 6]. We have numerically studied the sidelobe suppression of the latter one [7]. It is found that apodized and chirped gratings are quite effective in reducing the sidelobes. In addition, we have proposed a plasmonic microcavity offering a tunable resonance wavelength with varying an air core width. Note, however, that the characteristics of the above-mentioned three structures have not been compared in terms of an effective index modulation that is quite important to design gratings. Figure 1: Configurations of plasmonic gratings. (a) grating 1: formed by alternately stacking two kinds of MIM waveguides, (b) grating 2: formed by periodic changes in the dielectric insulator materials of an MIM waveguide, and (c) grating 3: formed by a periodic variation of the air core width in an MIM waveguide. In this paper, we compare the basic characteristics of several MIM waveguides of which gratings are composed.
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