  The plane wave scattering and absorption by finite and infinite gratings of
free-space standing infinitely long graphene strips are studied in the THz
range. A novel numerical approach, based on graphene surface impedance,
hyper-singular integral equations, and the Nystrom method, is proposed. This
technique guarantees fast convergence and controlled accuracy of computations.
Reflectance, transmittance, and absorbance are carefully studied as a function
of graphene and grating parameters, revealing the presence of surface plasmon
resonances. Specifically, larger graphene relaxation times increases the number
of resonances in the THz range, leading to higher wave transmittance due to the
reduced losses; on the other hand an increase of graphene chemical potential
up-shifts the frequency of plasmon resonances. It is also shown that a
relatively low number of graphene strips (>10) are able to reproduce Rayleigh
anomalies. These features make graphene strips good candidates for many
applications, including tunable absorbers and frequency selective surfaces.
