  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.
