Repulsive shield between polar molecules

We propose and analyze a technique that allows to suppress inelastic collisions and simultaneously enhance elastic interactions between cold polar molecules. The main idea is to cancel the leading dipole-dipole interaction with a suitable combination of static electric and microwave fields in such a way that the remaining repulsive van-der-Waals-type potential forms a repulsive shield with controllable range. We analyze in detail the elastic and inelastic scattering cross sections, and outline a method towards efficient evaporative cooling of polar molecules. Furthermore, we show that this setup is suitable for the realization of three-dimensional crystalline structures.