Fig. 4: Calculated multispectral enhancement factors inside NPoM cavities.

Spectra of the simulated maximum intensity enhancement factor inside the NPoM cavity with a static Pt tip at position P1 in the infrared (a) and visible (c) frequency ranges for different tip-NPoM distances. Spectra of the NPoM cavity in absence of a tip are depicted by a gray area. Charge distributions of the NPoM cavity modes S₁₁, L₀₁ and S₀₂ are sketched near their resonance frequencies. Panels (b, d) depict the spatial distribution of the intensity enhancement factor at different frequencies: \({\omega }_{{\rm{IR}}}^{{\nu }_{{\rm{s}}}}/(2\pi )=48\) THz in (b) and \({\omega }_{+}^{{\nu }_{{\rm{s}}}}/(2\pi )=430\) THz in (d). Top maps show a vertical cross-section of the distribution of the intensity enhancement factor inside the NPoM gap for a tip at a distance of Δz_t = 20 nm from the NPoM cavity. Maps below show the intensity enhancement factor distribution on the mid-gap horizontal plane of the NPoM gap for tip-NPoM distances of Δz_t = {20, 100, 300} nm and for the NPoM cavity without tip case