The previous chapter discussed the role of surface plasmon polaritons (SPPs) to increase the efficiency of various linear and nonlinear processes. One important example is the case of second-harmonic generation (SHG) from bare metallic surfaces, for which the nonlinearity is associated with the asymmetry of potentials confining the electron movement at metallic boundaries subjected to a strong orthogonal electric field. Similarly to surface-enhanced Raman spectroscopy (SERS), which greatly benefits from the use of SPPs to obtain 1010−1011 enhancement factors, making it an indispensable tool for modern spectroscopy and sensing technology [1,2], surface-enhanced SHG (SE SHG) can be also used for chemical and biological sensing, or as a method to characterize surface roughness, as even small corrugations lead to higher field localization and increased SHG [3].