Fig. 1. Geometry of a conical silver tip with grating-assisted light coupling.The excitation light is focused onto the diffractive grating. R is the cross-sectional radius of the tip body gradually decreasing from 1 μm to 20 nm. The tip apex is modeled as a hemisphere with a radius of r.

Fig. 2. (a) Sketch map of SPPs excitation using a planar grating; (b) Dispersion relationship of SPPs and the grating coupling; (c) Reflection obtained from the field monitor located above the surface without grating; (d) Re(H_y) distribution of the grating-coupled SPPs generation and propagation along the silver-air interface with excitation wavelength at λ=632.8 nm and θ=28.5°.

Fig. 3. Effective indices n_eff in real part (a) and imaginary part (b) of guided SPP modes versus the radius of cylindrical silver guide R. (c) Sketch map of a silver tip removing the diffracting grating. (d–j) Transverse modes intensity distributions of guided modes for R=1 μm.

Fig. 4. (a) Transverse mode intensity distributions of the grating-coupled SPPs at R=1 μm; (b–g) Transverse mode intensity distributions of the hybrid mode at R=800, 750, 600, 350, 230, and 20 nm, respectively. (h) Electric field intensity distribution at the tip apex. (i) Transverse electric field intensity distribution at 1 nm below the tip apex.

Fig. 5. Adiabatic parameter δ of TM₀₁ mode versus R.

Fig. 6. (a) Gap-mode configuration with the gap distance d=2 nm. (b) Electric field intensity and polarization distributions in the x-z plane. (c) Electric field intensity distribution in the x-y plane at 1 nm below the tip apex. (d) Comparison of electric field enhancement factor between the non-gap mode (Fig. 4(i)) and gap mode located at 1 nm below the apex of the silver tip.