Amarie, S. and F. Keilmann
Physical Review B 83, p.45404 (2011)
The phonon-enhanced near-field response of polar materials is studied in theory and with a broadband midinfrared near-field microscope that generates spectra in real time. Absolute magnitude and phase spectra are determined for SiC, SiO2, and a-SiO2 at several demodulation orders. The data set is compared with results from two theoretical models of near-field interaction, point dipole and finite dipole. Only the latter produces acceptable agreement with a single parameter choice of all measured quantities (line shape in amplitude and phase, line position, and absolute scattering amplitude). This allows determining for the commercial metal tip used that (i) the dipole representing the near-field interaction has 600-nm effective length and that (ii) its near-field-induced far-field backscattering amplitude efficiency reaches 0.3% at phonon resonance, for the NA ≈ 0.45 objective used. The near-field phonon resonance is a robust and well-understood feature whose bright and sharp (Q ≈ 200) signatures specifically can highlight and can identify polar materials in the nanoscale imaging of heterogeneous composites.