Fig. 1. Schematic of Raman scattering in different structures. (a) Slot-waveguide coupling structure; (b) free space

Fig. 2. Electric field distributions of slot waveguide. (a) Mesh division of the 3D model; (b) 3D model distribution; (c) normalized mode distributions of quasi-transverse electric (TE) modes of PSW with width of 50 nm, PSW with width of 100 nm, DSW, and HPSW respectively, and the arrow is polarization direction of electric field; (d) normalized electric field distributions of the corresponding waveguide coupling Ag nanosphere structure on the XY plane at center of the nanosphere; (e) normalized

Fig. 3. Propagation characteristics of different waveguides. (a) Relationship between the Possel factor and the wavelength; (b) waveguide collection percentage as a function of wavelength

Fig. 4. Electric field distributions in free space. (a) Mesh division of COMSOL model; (b)~(e) normalized electric field distributions on XY plane corresponding to Fig. 2(d)

Fig. 5. Raman enhancement factors of different waveguides as functions of propagation distance. (a) F_ESM as a function of longitudinal propagation distance Z; (b) F_EA as a function of longitudinal propagation distance Z. The solid lines represent F_EA of backscattering and dashed lines represent F_EA of out-of-plane scattering