  The optical response of traditional semiconductors depends on the laser
excitation power used in experiments. For two-dimensional (2D) semiconductors,
laser excitation effects are anticipated to be vastly different due to
complexity added by their ultimate thinness, high surface to volume ratio, and
laser-membrane interaction effects. We show in this article that under laser
excitation the optical properties of 2D materials undergo irreversible changes.
Most surprisingly these effects take place even at low steady state excitation,
which is commonly thought to be non-intrusive. In low temperature
photoluminescence (PL) we show for monolayer (ML) MoSe2 samples grown by
different techniques that laser treatment increases significantly the trion
(i.e. charged exciton) contribution to the emission compared to the neutral
exciton emission. Comparison between samples exfoliated onto different
substrates shows that laser induced doping is more efficient for ML MoSe2 on
SiO2/Si compared to h-BN and gold. For ML MoS2 we show that exposure to laser
radiation with an average power in the $\mu$W/$\mu$m$^2$ range does not just
increase the trion-to-exciton PL emission ratio, but may result in the
irreversible disappearance of the neutral exciton PL emission and a shift of
the main PL peak to lower energy.
