  We present a theoretical study on the impact of an active optical layer on
the emission properties of an ultrathin luminescent film. While the study can
be generalized to any material, we focus here on a simple layered medium
composed of a conjugated polymers (CPs) thin film, a zinc oxide layer (ZnO) and
a sapphire substrate. The study spreads throughout variable aspects including
the effect of the structure parameters on the CP luminescence and radiation
pattern and more specifically the influence of the absorption and emission
properties of the active layer. Comparing between the passive and active layer
cases, the obtained results show that an enhancement of the CP luminescence of
more than 20 times can be obtained by using an optically active underlying
layer. The results can be explained in terms of photon recycling where the
optically active layer acts as a photon reservoir and a secondary light source
for the ultra thin film. This general concept is of a special interest for
ultra-trace chemosensor.
