Concrete slab on pressure treated lumber
It depends what will be the load over the hollow are below but in any case it is not going to be enough to just use steel mesh. You are going to need to place re-bar into the slab as well that has its structure designed by an engineer to provide the proper strength to carry the slab weight plus the load weight over the hollow space. 

Keep in mind as well that this slab will need to have some considerable thickness to accommodate the engineered form of the re-bar. It may turn out that the final design involves a thinner slab that is supported by integrally poured in webbing below.