Fig. 5: Scaling of tip-enhanced optical signals with tip-NPoM distance.

Numerical calculations of normalized SFG (a) and DFG (b) enhancement factor distributions on the mid-gap horizontal plane of the NPoM gap for different tip-NPoM distances Δz_t. Bottom facet of the gold nanoparticle is depicted by a white dashed line. First column displays a top-view schematics of the tip-NPoM configuration and the illumination considered. c Simulated tip-height dependence of SFG (\({{\mathcal{F}}}_{{\rm{SFG}}}^{{\nu }_{{\rm{s}}}}({\vec{{\rm{r}}}}_{{\rm{hs}}})\), blue curve), DFG (\({{\mathcal{F}}}_{{\rm{DFG}}}^{{\nu }_{{\rm{s}}}}({\vec{{\rm{r}}}}_{{\rm{hs}}})\), green) and Stokes (\({{\mathcal{F}}}_{{\rm{S}}}^{{\nu }_{{\rm{s}}}}({\vec{{\rm{r}}}}_{{\rm{hs}}})\), light green) enhancement factors at the frequency of the vibrational mode ν_s. Inset illustrates the three normalized enhancement factors after smoothing the numerical results with a Whittaker algorithm. d Sidebands signals (area under the peaks) extracted from the spectra shown in Fig. 3(b) as a function of effective tip-NPoM distance \(\Delta {z}_{{\rm{t}}}^{* }\). Signal intensity measured on the driven ν_s upper sideband is shown in blue (\({I}_{{\rm{SFG}}}^{\,{\nu }_{{\rm{s}}}}\)). The intensity measured on the ν_s lower sideband is shown in dark green (\({I}_{{\rm{S}}}^{\,{\nu }_{{\rm{s}}}}+{I}_{{\rm{DFG}}}^{\,{\nu }_{{\rm{s}}}}\)). Inset illustrates the normalized behavior of the measured signals after linear background suppression