  Two-photon excited fluorescence (TPEF) is a standard technique in modern
microscopy but still affected by photo-damage of the probe. It was proposed
that TPEF can be enhanced by using entangled photons, but has proven to be
challenging. Recently it was shown that some features of entangled photons can
be mimicked with thermal light, which finds application in ghost imaging,
sub-wavelength lithography and metrology. Here, we utilize true thermal light
from a super-luminescence diode to demonstrate enhanced TPEF compared to
coherent light using two common fluorophores and luminescent quantum dots. We
find that the two-photon absorption rate is directly proportional to the
measured degree of second-order coherence, as predicted by theory. Our results
show that photon bunching can be exploited in two-photon microscopy with the
photon statistic providing a new degree of freedom.
