  Entangled two-photon absorption (ETPA) has recently become a topic of lively
debate, mainly due to the apparent inconsistencies in the
experimentally-reported ETPA cross sections of organic molecules. In this work,
we provide a thorough experimental study of ETPA in the organic molecules
Rhodamine B (RhB) and Zinc Tetraphenylporphirin (ZnTPP). The goal of this
contribution is twofold: on one hand, it seeks to reproduce the results of
previous experimental reports and, on the other, it aims to determine the
effects of different temporal correlations -- introduced as a controllable
time-delay between the photons to be absorbed -- on the strength of the ETPA
signal. In our experiment, the samples are excited by entangled pairs produced
by type-I SPDC, with a spectral distribution centered at 810 nm. Surprisingly,
and contrary to what was expected, the time delay did not produce in our
experiments any systematic change in the cross-sections when monitoring the
ETPA signal using a transmission measurement scheme. As a plausible cause of
this unexpected result, we argue that the photon-pair flux, typically-used in
these experiments, is not sufficient to promote the two-photon absorption
process in these molecules. This suggests that the actual absorption
cross-section values are lower than those previously reported, and therefore do
not lead to a measurable ETPA effect for the transmission method.
