Abstract
We analyze parametric instabilities of microcavity polartions in the presence of one-dimensional periodic potentials. Instabilities appear in a certain window of pump momentum orientations with respect to the potential so that the momentum projection onto the direction of periodicity is large enough. Ring-shaped domains of parametrically amplified perturbations form a periodic chain in momentum space. Independently from the orientation of the pump momentum, the chain maintains its orientation along the direction of periodicity of the system. In the physical space the amplified structures emerge as stripes modulated along the direction of periodicity and of finite width in the orthogonal direction.