Finite dipole model for extreme near-field thermal radiation between a tip and planar SiC substrate

•The finite dipole model (FDM) is developed to compute extreme near-field thermal radiation between a tip and a substrate.•The FDM explains the spectral redshift of the surface phonon polariton (SPhP) resonance peak observed in the experiment.•The effects of tip geometry on thermal infrared near-field spectroscopy has been analyzed.•The SPhP resonance peak may split in the extreme near-field regime.

Recent experimental studies have measured the infrared (IR) spectrum of tip-scattered near-field thermal radiation for a SiC substrate and observed up to a 50cm−1 redshift of the surface phonon polariton (SPhP) resonance peak [1] ;  [2]. However, the observed spectral redshift cannot be explained by the conventional near-field thermal radiation model based on the point dipole approximation. In the present work, a heated tip is modeled as randomly fluctuating point charges (or fluctuating finite dipoles) aligned along the primary axis of a prolate spheroid, and quasistatic tip-substrate charge interactions are considered to formulate the effective polarizability and self-interaction Green's function. The finite dipole model (FDM), combined with fluctuational electrodynamics, allows the computation of tip-plane thermal radiation in the extreme near-field (i.e., H/R≲1H/R≲1, where H is the tip-substrate gap distance and R is the tip radius), which cannot be calculated with the point dipole approximation. The FDM provides the underlying physics on the spectral redshift of tip-scattered near-field thermal radiation as observed in experiments. In addition, the SPhP peak in the near-field thermal radiation spectrum may split into two peaks as the gap distance decreases into the extreme near-field regime. This observation suggests that scattering-type spectroscopic measurements may not convey the full spectral features of tip-plane extreme near-field thermal radiation.