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

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.