  The CYGNO experiment aims to build a large ($\mathcal{O}(10)$ m$^3$)
directional detector for rare event searches, such as nuclear recoils (NRs)
induced by dark matter (DM), such as weakly interactive massive particles
(WIMPs). The detector concept comprises a time projection chamber (TPC), filled
with a He:CF$_4$ 60/40 scintillating gas mixture at room temperature and
atmospheric pressure, equipped with an amplification stage made of a stack of
three gas electron multipliers (GEMs) which are coupled to an optical readout.
The latter consists in scientific CMOS (sCMOS) cameras and photomultipliers
tubes (PMTs). The maximisation of the light yield of the amplification stage
plays a major role in the determination of the energy threshold of the
experiment. In this paper, we simulate the effect of the addition of a strong
electric field below the last GEM plane on the GEM field structure and we
experimentally test it by means of a 10$\times$10 cm$^2$ readout area
prototype. The experimental measurements analyse stacks of different GEMs and
helium concentrations in the gas mixture combined with this extra electric
field, studying their performances in terms of light yield, energy resolution
and intrinsic diffusion. It is found that the use of this additional electric
field permits large light yield increases without degrading intrinsic
characteristics of the amplification stage with respect to the regular use of
GEMs.
