We design novel photonic crystal slab heterostructures, substituting the air in the holes with materials of refractive index higher than n=1. This can be achieved by infiltrating the photonic crystal slab (PCS) with liquid crystal, polymer or nano-porous silica. We find that the heterostructures designed in this way can have quality factors up to Q=106. This high-Q result is comparable with the result of previously reported designs in which the lattice is elongated in one direction. Unlike conventional heterostructures, our design does not require nanometre-scale changes in the geometry. Additionally, infiltrated PCS can be constructed at any time after PCS fabrication.