  We show that a graphene sheet perforated with micro- or nano-size antidots
have prominent absorption resonances in the microwave and terahertz regions.
These resonances correspond to surface plasmons of a continuous sheet
"perturbed" by a lattice. They are excited in different diffraction orders, in
contrast to cavity surface plasmon modes existing in disconnected graphene
structures. The resonant absorption by the antidot array can essentially exceed
the absorption by a continuous graphene sheet, even for high antidot
diameter-to-period aspect ratios. Surface plasmon-enhanced absorption and
suppressed transmission is more efficient for higher relaxation times of the
charge carriers.
