Lightwave diffraction device formed of a dielectric layer (4), a mirror (12) arranged at the lower face (10) of said layer, a semi-reflective structure (13) arranged at the upper face (100) of said layer, and a diffractive structure (8) arranged in said layer or on its faces. The height (H) of the layer is chosen so as to substantially satisfy the resonance condition for at least one leaky mode propagating in said layer for at least one given incident wave having a determined wavelength.lamda. and a determined incidence angle .theta..sub.c. Next, the diffractive structure is arranged so that there is no propagating positive diffracted order, and so that all negative orders other than the -1.sup.st propagating order have zero or a relatively small diffraction efficiency, the reflected -1.sup.st order propagating in a direction non-parallel to the incident wave. This diffraction device allows a high diffraction efficiency of up to 100% for the -1.sup.st order.