  The combination of plastic scintillators with Silicon Photo-Multipliers
(SiPMs) is widely used for detecting radiation in high-energy astrophysics,
particle physics, neutrino physics, or medical physics. An example of
application for this kind of detectors are Compton polarimeters such as POLAR-2
or LEAP, for which a low-Z material is needed for the Compton effect to be
dominant down to as low energy as possible. Such detectors aim to measure low
energy Compton depositions which produce small amounts of optical light, and
for which optimizing the instrumental optical properties consequently
imperative.
  The light collection efficiency of such a device was studied with a focus on
the POLAR-2 GRB polarimeter, in which the conversion of incoming $\gamma$-rays
into readable signal goes through the production and collection of optical
light, which was to be optimized both through measurements and simulations. The
optical elements of the POLAR-2 polarimeter prototype module were optically
characterized and an optical simulation based on Geant4 was developed to fully
model its optical performances. The results from simulations were used to
optimize the design and finally to verify its performance. The study resulted
in a detector capable of measuring energy depositions of several keV. In
addition an important finding of this work is the impact of the plastic
scintillator surface roughness on the light collection. It was found that a
plastic scintillator with a higher scintillation efficiency but made of a
softer material, hence with a rougher surface, was not necessarily the best
option to optimize the light collection. Furthermore, in order to optimize the
optical crosstalk between different channels, a production technique for very
thin ($\sim$150$~\mu$m) and reusable silicone-based optical coupling pads,
which can be applied for other experiments, was developed.
