Abstract: Light wave diffraction device formed of a dielectric layer (4), a reflecting structure (12) arranged on one lower face (10) of said layer, and a diffraction grating (8) arranged on one face (6) of said layer. The height (H) of the layer is selected so as to substantially satisfy the resonance condition for a leaky mode in said layer for at least one given incident wave having a wavelength &lgr; and a determined angle of incidence. Next, the diffraction grating has a spatial period selected so that the incident wave is diffracted essentially along the first negative diffraction order and at an angle of diffraction different to the angle of incidence (outside the Littrow condition). In particular, the invention provides that either the angle of incidence, or the angle of diffraction is high, i.e. relatively close to 90° from the direction perpendicular to the diffraction grating.