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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.

Optical antennas have generated much interest in recent years due to their ability to focus optical energy beyond the diffraction limit, benefiting a broad range of applications such as sensitive photodetection, magnetic storage, and surface-enhanced Raman spectroscopy. To achieve the maximum field enhancement for an optical antenna, parameters such as the antenna dimensions, loading conditions, and coupling efficiency have been previously studied. Here, we present a framework, based on coupled-mode theory, to achieve maximum field enhancement in optical antennas through optimization of optical antennas’ radiation characteristics. We demonstrate that the optimum condition is achieved when the radiation quality factor (Q rad) of optical antennas is matched to their absorption quality factor (Q abs). We achieve this condition experimentally by fabricating the optical antennas on a dielectric (SiO2) coated ground plane (metal substrate) and controlling the antenna radiation through optimizing the dielectric thickness. The dielectric thickness at which the matching condition occurs is approximately half of the quarter-wavelength thickness, typically used to achieve constructive interference, and leads to ∼20% higher field enhancement relative to a quarter-wavelength thick dielectric layer.

Using first-principles theory and experiments, chemical contributions to surface-enhanced Raman spectroscopy for a well-studied organic molecule, benzene thiol, chemisorbed on planar Au(111) surfaces are explained and quantified. Density functional theory calculations of the static Raman tensor demonstrate a strong mode-dependent modification of benzene thiol Raman spectra by Au substrates. Raman active modes with the largest enhancements result from stronger contributions from Au to their electron-vibron coupling, as quantified through a deformation potential. A straightforward and general analysis is introduced to extract chemical enhancement from experiments for specific vibrational modes; measured values are in excellent agreement with our calculations.