Another function of the metal grating is to excite the graphene
surface plasmonic wave. Because the wavevectors between a graphene
plasmonic wave and an incident TM wave are in huge mismatch,
they cannot couple to each other unless to introduce the
grating to compensate the mismatch. Specifically, when a TM wave
with angular frequency x vertically incidents on the grating, its
nth diffraction order will gain an extra wavevector 2np/l with n
being an integer and l the grating period. Then, the nth order of ด, but the bandwidth still decreases which is in accordance with
Fig. 2(a).