Figure 9.

Local intensity enhancement $M_1={\left(\frac{E_{loc}}{E_0}\right)}^2$ in a logarithmic scale in a plane through the centers of the Ag spheres and perpendicular to the incident wavevector $\overrightarrow{k}$ versus incident polarization: (a) 0°; (b) 30°; (c) 60° and (d) 90°. The incident wavelength was 514.5 nm in all cases. The arrows represent the different polarizations. In (e), the SERS enhancement factor $M=M_1^2$ is shown as a function of the incident polarization $\alpha$ for a point in the nanogap located at the dimer axis $\delta =0.5$ nm away from one spherical surface, and the fit (solid line) to a ${\cos }^4\alpha$ dependence. In (f), $M$ averaged over all points $\delta =0.5$ nm outside the Ag sphere surface versus $\alpha$ and fit to a ${\cos }^4\alpha$ dependency are shown. The radius $R=45$ nm corresponds to the average size of the Ag nanoparticles used in the experiment while the separation distance $d=5.5$ nm corresponds to the diameter of a hemoglobin molecule. Reprinted with permission from [54].