Fano enhancement of SERS signal without increasing the hot spot intensity

Abstract: Plasmonic nanostructures enhance nonlinear response, such as surface enhanced Raman scattering (SERS), by localizing the incident field into hot spots. The localized hot spot field can be enhanced even further when linear Fano resonances (FR) take place in a double resonance scheme. However, hot spot enhancement is limited with the modification of the vibrational modes, the break-down of the molecule and the tunnelling regime. Here, we present a method which can circumvent these limitations. Our analytical model and solutions of 3D Maxwell equations show that: enhancement due to the localized field can be multiplied by a factor of $10^2$ to $10^3$. Moreover, this can be performed without increasing the hot spot intensity which also avoids the modification of the Raman modes. Unlike linear Fano resonances, we create a path interference in the nonlinear response. We demonstrate on a single equation that enhancement takes place due to cancellation of the contributing terms in the denominator of the SERS response.