Fig. 1. (a) Schematic illustration of the model STM junction used for electromagnetic simulations. The tip is modeled as a truncated cone with a radius of 30 nm, a height of 300 nm, and a semi-cone angle of 15°. (b) Local electric field enhancement M_z = |E_z|/|E₀| at position (0, 0, 1.2 nm) as a function of incident photon energy for different decoupling layer dielectric constants ε_r. (c) Spatial distribution of the local electric field enhancement M_z = |E_z|/|E₀| in the xz plane (with y = 0) for different dielectric constants ε_r. (d) Instantaneous induced surface charge density at the surfaces of the tip and the substrate for ε_r = 1 (upper panel) and ε_r = 9 (lower panel). The density values are normalized to the maximum value of ε_r = 1. In (b)–(d), the thickness of the decoupling layer is set to t_d = 1 nm, and the distance between the tip apex and the top surface of the decoupling layer is set to d_vac = 0.6 nm.

Fig. 2. (a) Electric field enhancement M_z = |E_z|/|E₀| along the z axis (with x = 0 and y = 0) for different decoupling layer dielectric constants ε_r. (b) Non-normalized and (c) normalized electric field enhancement M_z = |E_z|/|E₀| along the x axis (with y = 0 and z = 1.2 nm). The gap distance is d_gap = 1.6 nm, and the decoupling layer thickness is t_d = 1 nm.

Fig. 3. Electric field enhancement with and without a dielectric layer with different thicknesses t_d and dielectric constants ε_r. Electric field enhancement |E_z|/|E₀| distribution in the xz plane (y = 0) for (a) ε_r = 1 and (c) ε_r = 9 with different decoupling layer thicknesses. (b) Enhancement and (d) FWHM of the electric field enhancement |E_z|/|E₀| along the x axis (y = 0 and z = t_d + 0.2 nm) as functions of decoupling layer thickness for four different dielectric constants.

Fig. 4. Schematics showing the configurations of (a) a vertical dipole and (d) a horizontal dipole in the model STM junctions. In (a), the shape of the tip and substrate are the same as in Fig. 1(a). In (d), a spherical protrusion with a radius of 0.5 nm is superimposed at the apex of the tip shaft. The quantum efficiency for a vertical dipole on (b) Ag substrate and (c) W substrate and a horizontal dipole on (e) Ag substrate and (f) W substrate with different decoupling layer thicknesses and dielectric constants. In (a)–(f), the vertical dipole–tip distance is 0.4 nm, and the vertical dipole–decoupling-layer distance is 0.2 nm. For a vertical dipole in (a)–(c), the dipole is placed right under the tip apex, while for a horizontal dipole in (d)–(f), the dipole is placed 0.5 nm away laterally from the tip apex.

Fig. 5. Influence of the dielectric constant of the decoupling layer on the TEPL and TERS intensities of a single dipole emitter. (a) TEPL intensity and (b) TERS intensity as functions of the decoupling layer thickness for different dielectric constants. (c) and (d) show the TEPL and TERS images for ε_r = 1 and ε_r = 9. In (c) and (d), the molecule is approximated as a vertical dipole, the gap distance is 1.6 nm, the thickness of the decoupling layer is 1 nm, and the plane for simulation of the photon image is z = 1.2 nm.