Revisiting the Dipole Model for a Thermal Infrared Near-Field Spectroscope

TL;DR

This paper reexamines the dipole model for thermal infrared near-field spectroscopy, revealing new insights into the roles of scattering and direct emission in near-field signals, which impacts interpretation of experimental data.

Contribution

It provides a revised analysis of the dipole approximation, showing conditions under which scattered power is zero and emphasizing the importance of direct emission in near-field imaging.

Findings

Scattered near-field power is zero when the nanoparticle is heated relative to its environment.

Direct emission from the tip significantly influences near-field signals.

Results deviate from previous models, refining the understanding of near-field interactions.

Abstract

We determine the scattered near-field and directly emitted power of a heated spherical nanoparticle above a sample within the framework of fluctuational electrodynamics using the dipole approximation. Our results deviate from previously obtained results. Additionally, we show that in a configuration where the nanoparticle is heated with respect to its environment, the scattered power of the near field of the sample is strictly zero. Only when the sample is heated or the temperature of the surroundings of the sample is lowered is there a contribution from the scattered power. Our results indicate that for the interpretation of near-field imaging setups as the thermal infrared near-field spectroscope not only the scattering of the near field but also the direct emission of the tip plays a role.