We present combined experimental and numerical work on light-matter interactions at nanometer length scales. We report novel numerical simulations of near-field infrared nanospectroscopy that consider, for the first time, detailed tip geometry and have no free parameters. Our results match published spectral shapes of amplitude and phase measurements even for strongly resonant surface phonon-polariton (SPhP) modes. They also verify published absolute scattering amplitudes for the first time. A novel, ultrabroadband light source enables near-field amplitude and phase acquisition into the far-infrared spectral range. This allowed us to discover a strong SPhP resonance in the polar dielectric SrTiO3 (STO) at approximately 24 micrometer wavelength of incident light.
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