  We introduce a new, highly sensitive, and simple heterodyne optical method
for imaging individual non-fluorescent nano-objects. A two orders of magnitude
improvement of the signal is achieved compared to previous methods. This allows
for the unprecedented detection of individual small absorptive objects such as
metallic clusters (of 67 atoms) or non-luminescent semiconductor nanocrystals.
The measured signals are in agreement with a calculation based on the
scattering field theory from a photothermal-induced modulated index of
refraction profile around the nanoparticle.
