Here, C is the calibration factor that also accounts for OCP normalization of the measured SHG intensities and the Fresnel coefficients (C=3.1 x 10 -22 m 2 V -1 in our case, please see Supplementary Information Note S3), Esig,norm is the measured SHG amplitude normalized to the value obtained at OCP (the condition at which we calibrate to quartz, see Supplementary Information Note S3), jsig is the phase relative to the zero phase from the glass:air interface, and 𝜒 67!89 (:) is the third-order nonlinear susceptibility of the diffuse layer (1x10 -21 m 2 V -2 from experiment and theory) , which is invariant with ionic strength, pH, and surface composition . Eqn. 1 accounts for the ca. 5x larger nonlinear optical response we obtain from the nickel nanolayer when compared to a fused silica window, both at pH 13 and 1 M ionic strength. This experimentally determined factor of 5 is in excellent agreement with the computed factor of 4.5 in eqn. 7 of Nagy and Roy and the 1/2 term in eqn. 1 of Guyot-Sionnest et al. that account for metals' bulk magnetic dipole contribution to 𝜒 (;) . With eqn. 1 establishing Φ(0) !"! , the second-order nonlinear susceptibility is given by (see Supplementary Information Notes S4 and S5):