Atomic force infrared spectroscopy reveals the nature of vibrational epsilon-near-zero polariton modes in thin films. At frequencies near the vibrational bands of a dielectric thin film, the real part of its dielectric function can approach zero. This enables polariton modes with a strong, spatially and spectrally localized optical absorption, potentially useful for photothermal medicine, sensing, or photovoltaics. However, practical characterization methods are needed for identifying these modes, which can be “bright” or “dark,” i.e., excitable by far-field or near-field illumination, respectively. Timur Shaykhutdinov and coworkers demonstrate the potential of atomic force infrared spectroscopy in this context. Far-field infrared light is shined on a SiO₂ film probed with a gold-coated atomic force microscope (AFM) tip. The thermal dilatation of the film is measured as a function of the incident wavelength. The obtained spectrum reflects the optical absorption that peaks when polaritons are excited. Upon comparison with far-field reflectance measurements and with calculations, the nature of the excited mode can be addressed. Such analysis shows the dominant role of a far-field-excited Berreman mode and of a tip plasmon near-field-excited confined epsilon-near-zero mode in films of 100 nm and 2 nm thickness, respectively. 

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