  The interaction of laser pulses with thin grating targets, having a periodic
groove at the irradiated surface, has been experimentally investigated.
Ultrahigh contrast ($\sim 10^{12}$) pulses allowed to demonstrate an enhanced
laser-target coupling for the first time in the relativistic regime of
ultra-high intensity $>10^{19} \mbox{W/cm}^{2}$. A maximum increase by a factor
of 2.5 of the cut-off energy of protons produced by Target Normal Sheath
Acceleration has been observed with respect to plane targets, around the
incidence angle expected for resonant excitation of surface waves. A
significant enhancement is also observed for small angles of incidence, out of
resonance.

Plasma Physics;Optics