Replacing traditional refractive lenses with metalenses that have ultrathin and flat architecture could make imaging systems more compact. However, the space between the lens and image is usually the dominating factor in determining the bulkiness, and it cannot be shortened just by using thinner lenses. Here, we propose a pancake metalens that folds the optical path at will using a metacavity consisting of a spin-dependent bifacial metasurface and a mirror. We first verify the asymmetric...