Fig. 1. Fabrication diagrams of narrow-gap AuNF Moire-grating. (a) Single exposure by dual-beam interference; (b) double exposure by angle-twisted dual-beam interference; (c) 1D grating structure by single exposure; (d) Moire-grating structure with double exposures by angle-twisted dual-beam interference; (e) cross-section views of AuNF Moire-gratings with different AuNF thicknesses along Moire pattern direction and widths of narrow-gap slits; (f) narrow-gap AuNF Moire-grating structure with physical object image shown in inset

Fig. 2. Micrographs of narrow-gap AuNF Moire-gratings. (a)‒(k) Microscopic morphologies of narrow-gap AuNF Moire-gratings under different α; cross-section micrographs; (l) perpendicular and (m) parallel to Moire fringe directions

Fig. 3. Resonant absorption law of narrow-gap AuNF Moire-grating. (a) Experimentally measured and (b) numerically simulated reflectance spectra as a function of α (circularly polarized light excitation); SPR and LSPR optical field distributions of AuNF Moire-gratings excited by (c) s-polarization and (d) p-polarization components with excitation wavelengths of 633 nm and 785 nm

Fig. 4. Polarization dependence of narrow-gap AuNF Moire-gratings. (a) Polarization definition diagram; (b)-(l) variation of reflectance spectra with θ under different α

Fig. 5. Raman detection limits for CV molecules on different substrates excited by 633 nm laser. (a) Si substrate; (b) AuNF substrate; (c) AuNF 1D-grating substrate; AuNF Moire-grating substrates under (d) p-polarization and (e) s-polarization excitations

Fig. 6. Raman detection limits for CV molecules on different substrates excited by 785 nm laser. (a) Si substrate; (b) AuNF substrate; (c) AuNF 1D-grating substrate; AuNF Moire-grating substrate under (d) p-polarization and (e) s-polarization excitations

Fig. 7. SERS polarization dependence characteristics and homogeneity of narrow-gap AuNF Moire-grating substrate. Polarization dependence under (a) 633 nm and (b) 785 nm excitation resonance wavelength matching conditions ; polarization dependence under (c) 633 nm and (d) 785 nm excitation resonance wavelength mismatch conditions; (e) evolution of Raman characteristic peak intensity of CV molecule at 1617 cm^-1 with molecular concentration under incident conditions of p-polarization and s-polarization with excitation wavelength of 633 nm ; (f) SERS uniformity test of AuNF Moire-grating substrates