In this paper, we investigate optical absorption enhancements
in graphene by introducing nano-gratings to excite graphene
surface plasmons. We first show that the absorption rate can be
well described within the temporal coupled mode theory, a simple
phenomenological model which only requires two parameters to
predict the resonant absorption rate. We then show that one
parameter only relates to the electron relaxation time of graphene
and the other parameter connects with the geometric structure of
the device. Complete absorption condition is reached when the
two parameters are equal. Furthermore, it is demonstrated that
the absorption spectrums can be tuned by varying the Fermi
energy of graphene and the grating period.