  Electromagnetically induced transparency (EIT) is an optical phenomenon which
allows a drastic modification of the optical properties of an atomic system by
applying a control field. It has been largely studied in the last decades and
nowadays we can find a huge number of experimental and theoretical related
studies. Recently a similar phenomenon was also shown in quantum dot molecules
(QDM) , where the control field is replaced by the tunneling rate between
quantum dots. Our results show that in the EIT regime, the optical properties
of QDM and the atomic system are identical. However, here we show that in the
strong probe field regime, i.e., "coherent population trapping" (CPT) regime,
it appears a strong discrepancy on the optical properties of both systems. We
show that the origin of such difference relies on the different decay rates of
the excited state of the two systems, implying in a strong difference on their
higher order nonlinear susceptibilities. Finally, we investigate the optical
response of atom/QDM strongly coupled to a cavity mode. In particular, the
QDM-cavity system has the advantage of allowing a better narrowing of the width
of the dark state resonance in the CPT regime when compared with atom-cavity
system.
