guide is schematically presented in Fig. S1a. As the name "MIM" implies, a structural symmetry exists with respect to the mirror plane that cuts through the center of the SiO 2 layer and is parallel to the x-y plane. As a result, supported SPP modes in the guide can be classified into two major types, each with its own distinctive eigenstates [1][2][3][4][5]. One is symmetric (S), and the other is antisymmetric (AS). In a symmetric mode, the electric-field distribution is symmetric around the mirror plane, while in an anti-symmetric mode, the distribution is anti-symmetric, as shown in Fig. S2. Figure S1b shows the existing AS-and S-modes supported by the proposed MIM gap plasmon waveguide structure with thickness h between 50 and 300 nm. The width of the waveguide w is related to h through the equation w = (500 nm/200 nm) × h, and the frequency of interest is 360 THz. In this figure, the fundamental AS mode (or AS1) does not exhibit any cut-off for h between 50 and 300 nm, and theoretically, AS1 can still be supported in the waveguide even if h of the SiO 2 layer becomes infinitesimally small [1]. We are most interested in the fundamental AS1 mode because it achieves the best confinement of energy, which can be clearly observed in Fig. S2. A more detailed description of this mode is provided in Section 2. The other two antisymmetric modes supported in this geometry are the AS2 and AS3 modes. Those modes are oscillatory along the y-direction and originate from the finite width of SiO 2 (Fig. S2). They are cut off when h becomes smaller than 85 nm and 185 nm, respectively (Fig. S1b). The symmetric modes, S1 and S2, are shown in the region of low effective refractive index. The cross-sectional field profiles of S1 and S2 modes show resemblance to monopole and dipole distributions, respectively, as seen in Fig. S2. The S1 mode does not have cut-off, and the S2 mode has cut off when h becomes smaller than 88 nm.

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