  Here we study subwavelength gratings for coupling into graphene plasmons by
means of an an- alytical model based on transformation optics that is not
limited to very shallow gratings. We consider gratings that consist of a
periodic modulation of the charge density in the graphene sheet, and gratings
formed by this conductivity modulation together with a dielectric grating
placed in close vicinity of the graphene. Explicit expressions for the
dispersion relation of the plasmon po- laritons supported by the system, and
reflectance and transmittance under plane wave illumination are given. We
discuss the conditions for maximising the coupling between incident radiation
and plasmons in the graphene, finding the optimal modulation strength for a
conductivity grating.
