  The contrast formation mechanism in Nanoscale Infrared Spectroscopy (Nano-IR
or AFM-IR) is analyzed for the boundary between two layers with different light
absorption, thermal and elastic parameters. Analytical results derived in the
decoupling approximation for low frequency limit show that the response
amplitude is linearly proportional to the intensity of the illuminating light
and thermal expansion coefficient. The spatial resolution between two
dissimilar materials is linearly proportional to the sum of inverse light
adsorption coefficients and to the effective thermal transfer length. The
difference of displacements height across the T-shape boundary ("thermo-elastic
step") is proportional to the difference of the adsorption coefficients and
inversely proportional to the heat transfer coefficient. The step height
becomes thickness-independent for thick films and proportional to h^2 in a very
thin film
