Fig. 1. Diagram of the stepped tip and active substrate model

Fig. 2. (a) SEM image of the whole stepped tip; (b) partial SEM images of tips with curvature radii of (b) 5 nm, (c) 10 nm, and (d) 40 nm

Fig. 3. Model of the stepped tip with silver film and a silver-nanoparticle active substrate

Fig. 4. Electric field distributions of the stepped tip with silver film and the silver-nanoparticle active substrate in x-z plane. (a) Three-dimensional electric field; (b) near-field electric field intensity

Fig. 5. Model and electric field distributions of a conical tip with silver film and a silver-nanoparticle active substrate structure in x-z plane. (a) Diagram of simulation model; (b) three-dimensional electric field; (c) near-field electric field intensity

Fig. 6. Model and electric field distributions of a stepped tip with silver film and a glass substrate structure in x-z plane. (a) Diagram of simulation model; (b) three-dimensional electric field; (c) near-field electric field intensity

Fig. 7. Model and electric field distributions of a stepped tip with gold film and a glass substrate structure in x-z plane. (a) Diagram of simulation model; (b) three-dimensional electric field; (c) near-field electric field intensity

Fig. 8. Model and electric field distributions of a conical tip with gold film and a glass substrate structure in x-z plane. (a) Diagram of simulation model; (b) three-dimensional electric field; (c) near-field electric field intensity

Fig. 9. Electric field intensity E and enhancement factor G_EF as functions of incident angle θ under the conditions of r=5 nm, t=25 nm, h =300 nm, d=55 nm, and g=2 nm

Fig. 10. Electric field intensity E and enhancement factor G_EF as functions of the curvature radius r under the conditions of θ=45°, t=25 nm, h=300 nm, d=55 nm, and g=2 nm

Fig. 11. Electric field intensity E and enhancement factor G_EF as functions of silver-film thickness t under the conditions of θ=45°, r=5 nm, h=300 nm, d=55 nm, and g=2 nm

Fig. 12. Electric field intensity E and enhancement factor G_EF as functions of silver-film height h under the conditions of θ=45°, r=5 nm, t=25 nm, d=55 nm, and g=2 nm

Fig. 13. Electric field intensity E and enhancement factor G_EF as functions of silver-nanoparticle diameter d under the conditions of θ=45°, r=5 nm, t=25 nm, h=300 nm, and g=2 nm

Fig. 14. Electric field intensity E and enhancement factor G_EF as functions of the gap g between tip and silver nanoparticle under the conditions of θ=45°, r=5 nm, t=25 nm, h=300 nm, and d=55 nm