  We analyze how a probing particle modifies the infrared electromagnetic near
field of a sample. The particle, described by electric and magnetic
polarizabilities, represents the tip of an apertureless scanning optical
near-field microscope (SNOM). We show that the interaction with the sample can
be accounted for by ascribing to the particle dressed polarizabilities that
combine the effects of image dipoles with retardation. When calculated from
these polarizabilities, the SNOM signal depends only on the fields without the
perturbing tip. If the studied surface is not illuminated by an external source
but heated instead, the signal is closely related to the projected
electromagnetic local density of states (EM-LDOS). Our calculations provide the
link between the measured far-field spectra and the sample's optical
properties.We also analyze the case where the probing particle is hotter than
the sample and evaluate the impact of the dressed polarizabilities on
near-field radiative heat transfer. We show that such a heated probe above a
surface performs a surface spectroscopy, in the sense that the spectrum of the
heat current is closely related to the local electromagnetic density of states.
The calculations agree well with available experimental data.
