Fig. 2: Tip-enhanced nanocavity SFG.

a Anti-Stokes side of the spectra of a BPT-filled NPoM cavity below an oscillating metallized scanning probe tip. The NPoM is formed by a 63 nm high faceted Au particle on top of an Au mirror that is functionalized with a monolayer of BPT molecules. Insets illustrate the positions P1 and P2 of the tip relative to particle, as well as illumination at VIS (orange) and IR (red) frequencies. Black spectrum is recorded for tip positioned at P1 under VIS illumination exclusively. Red and brown spectra were recorded under VIS and IR (2.2 mW tuned to the vibrational mode ν_b at 32 THz) illumination at positions P1 and P2, respectively. b Topography image of the NPoM cavity. Red and brown dots indicate the tip positions P1 and P2 where spectra shown in panel (a) were recorded. Maps of \({I}_{+}^{\,{\nu }_{{\rm{b}}}}\) (averaged over peak area) without (c) and with (d) IR illumination. VIS illumination is at 382 THz (785 nm) at a power of 200 μW. Acquisition time per spectra is 2 s. Tapping amplitude (TA) is 50 nm. e Simulated spatial distribution of the intensity enhancement factor at 32 THz \({F}_{{\rm{IR}}}^{{\nu }_{{\rm{b}}}}\) around the NPoM cavity. f Simulated \(\mathcal{F}_{\rm{aS}}^{\nu_{\rm{b}}}(\vec{\rm{r}}_{\rm{hs}})\) (pink line) and \(\mathcal{F}_{\rm{SFG}}^{\nu_{\rm{b}}}(\vec{\rm{r}}_{\rm{hs}})\) (blue line) enhancement factors along the dashed arrow depicted in (d). x_t is the lateral position of the tip with respect to the particle’s center. The vertical distance Δz_t = 20 nm between tip apex and NPoM is kept constant, as illustrated in the inset. Optical maps for other vibrational signals can be found in Supplementary Note 3