  Graphene has been recognized as an attractive two-dimensional material for
fundamental research and wide applications in electronic and photonic devices
owing to its unique properties. The technologies to modulate the properties of
graphene are of continuous interest to researchers in multidisciplinary areas.
Herein, we report on the first experimental observation of the layer-to-layer
compression and enhanced optical properties of few-layer graphene nanosheets by
applying the irradiation of energetic ion beams. After the irradiation, the
space between the graphene layers was reduced, resulting in a tighter contact
between the few-layer graphene nanosheet and the surface of the substrate. This
processing also enhanced the interaction between the graphene nanosheets and
the evanescent-field wave near the surface, thus reinforcing the
polarization-dependent light absorption of the graphene layers (with 3-fold
polarization extinction ratio increment). Utilizing the ion-irradiated graphene
nanosheets as saturable absorbers, the passively Q-switched waveguide lasing
with considerably improved performances was achieved, owing to the enhanced
interactions between the graphene nanosheets and evanescent field of light. The
obtained repetition rate of waveguide laser was up to 2.3 MHz with a pulse
duration of 101 ns.
