Tip-sample electromagnetic interaction in the infrared: Effective polarizabilities, retarded image dipole model and near-field thermal radiation detection

TL;DR

This paper investigates how a tip in infrared near-field microscopy interacts with samples, introducing effective polarizabilities that incorporate retardation effects, and demonstrates that near-field signals relate to the sample's optical properties and local density of states.

Contribution

It introduces a model with effective dipole polarizabilities including retardation, linking near-field signals to electromagnetic density of states and optical properties, and analyzes heated probes for surface spectroscopy.

Findings

Effective polarizabilities account for retardation effects.

Near-field signals relate to electromagnetic density of states.

Heated probes enable local surface spectroscopy.

Abstract

We analyse how a probing particle modifies infrared electromagnetic near fields. The particle, assimilated to both electric and magnetic dipoles, represents the tip of an apertureless scanning optical near-field microscope (SNOM). We show that the interaction can be accounted for by ascribing to the particle effective dipole polarizabilities that add the effect of retardation to the one of the image dipole. Apart from these polarizabilities, the SNOM signal expression depends only on the fields without tip perturbation, shown to be closely related to the electromagnetic density of states (EM-LDOS) and essentially linked to the sample's optical properties, so that measuring local spectra of heated samples is equivalent to performing a local surface spectroscopy. We also analyse the case where the probing particle is hotter. We evaluate in this case the impact of the effective polarizabilities on the tip-sample near-field radiative heat transfer. We also show that such an heated probe above a surface also performs a surface spectroscopy. The calculations agree well with available experimental data.