  Photothermal microscopy has recently complemented single molecule
fluorescence microscopy by the detection of individual nano-objects in
absorption. Photothermal techniques gain their superior sensitivity by
exploiting a heat induced refractive index change around the absorbing
nano-object. Numerous new applications to nanoparticles, nanorods and even
single molecules have been reported all refering to the fact that photothermal
microscopy is an extinction measurement on a heat induced refractive index
profile. Here, we show that the actual physical mechanism generating a
photothermal signal from a single molecule/particle is fundamentally different
from the assumed extinction measurement. Combining photothermal microscopy,
light scattering microscopy as well as accurate Mie scattering calculations to
single gold nanoparticles, we reveal that the detection mechanism is
quantitatively explained by a nanolensing effect of the long range refractive
index profile. Our results lay the foundation for future developments and
quantitative applications of single molecule absorption microscopy.
