  The role of relativistic surface plasmons (SPs) in high order harmonic
emission from laser-irradiated grating targets has been investigated by means
of particle-in-cell simulations. SP excitation drives a strong enhancement of
the intensity of harmonics, particularly in the direction close to the surface
tangent. The SP-driven enhancement overlaps with the angular separation of
harmonics generated by the grating, which is beneficial for applications
requiring monochromatic XUV pulses.
