Figure 1: Design and performance of silicon nanoantennas.

(a) SEM image of the array of nanostructures fabricated in Si on a silicon-on-insulator substrate. Each nanoantenna consists of two identical disks with a diameter of 220 nm, a height of 200 nm and a 20-nm gap in between. Scale bar, 2 μm. (b) SEM top-view and (c) lateral-view images of a single nanoantenna, indicated in the rectangle in a. Scale bar, 100 nm. (d) Numerical calculation results showing a scattering resonance at λ=860 nm (vertical black line) and a low absorption cross section for the Si dimer in PMMA. Note that the absorption curve is multiplied by a factor of 10. (e) Near-field distribution map for the silicon structure excited at resonance, showing good confinement of the electric field in the gap. Note that the maximum enhancement value is 5.5. Scale bar, 100 nm. (f) Experimental 2D normalized Raman map, showing enhanced signal coming from the molecules close to the nanoantennas. (g) SERS enhancement factors obtained for each individual nanoantenna shown in f, after volume normalization (see Methods, SERS mapping section, for details). The error bars show half the difference between the minimum and the maximum Raman intensity value in each nanoantenna. The dashed line corresponds to (E_max/E₀)⁴, from e.