  The action of a nanoscopic spherically symmetric refractive index profile on
a focused Gaussian beam may easily be envisaged as the action of a
phase-modifying element, i.e. a lens: Rays traversing the inhomogeneous
refractive index field n(r) collect an additional phase along their trajectory
which advances or retards their phase with respect to the unperturbed ray. This
lens-like action has long been understood as being the mechanism behind the
signal of thin sample photothermal absorption measurements [1, 2], where a
cylindrical symmetry and a different lengthscale is present. In photothermal
single (nano-)particle microscopy, however, a complicated, though
prediction-wise limited, electrodynamic (EM) scattering treatment was
established [3] during the emergence of this new technique. Our recent study
extended [4] this EM-approach into a full ab-initio model describing the
reality of the situation encountered and showed for the first time that the
mechanism behind the signal, despite its nanoscopic origin, is also the
lens-like action of the induced refractive index profile only hidden in the
complicated guise of the theoretical Mie-like framework. The diffraction model
proposed here yields succinct analytical expressions for the axial PT signal
shape and magnitude and its angular distribution, all showing the clear
lens-signature. It is further demonstrated, that the Gouy-phase of a Gaussian
beam does not contribute to the relative photothermal signal in forward
direction, a fact which is not easily evident from the more rigorous EM
treatment. The model may thus be used to estimate the signal shape and
magnitude in photothermal single particle microscopy.
