Spontaneous Symmetry Breaking in Diffraction.

We demonstrate spontaneous symmetry breaking in the diffraction of a laser-driven grating with memory in its nonlinear response. We observe, experimentally and theoretically, asymmetric diffraction even when the grating and illumination are symmetric. Our analysis reveals how diffracted waves can spontaneously acquire momentum parallel to the lattice vector in quantities unconstrained by the grating period. Our findings point to numerous opportunities for imaging, sensing, and information processing with nonlinear periodic systems, which can leverage a much richer diffractive response than their linear counterparts.