A reflection metal diffraction grating has a high diffraction efficiency for diffracting femtosecond mode laser pulses, and includes a substrate with a set of lines having a pitch A. The substrate is made of metal or covered with a metal layer, and the grating includes a thin film of dielectric material having a thickness, the dielectric film covering the metal surface of the lines of the grating, the grating being suitable for receiving a pulsed electromagnetic lightwave in a femtosecond mode. The thickness of the dielectric thin film is lower than 50 nm, and is suitable for reducing by a third order factor at least the maximum of the square of the electric field of the electromagnetic lightwave on the metal surface and in the metal layer of the substrate as compared to the square of the electric field at the surface of a metal grating not having a dielectric thin film.