Composite diamond assemblies

A composite diamond assembly comprising: a polycrystalline diamond wafer 4 and a crystalline diamond plate 2 mounted within a circular opening of the diamond wafer. The diamond wafer is bonded to the diamond plate by a brazed joint between the opposed diamond faces. The joint may comprise complementary tapered/wedge shaped side wall surfaces or a stepped side wall surfaces (see figure 2). The brazing joint may comprise Au/Sn, Cu/Ag, Cu/Ag/Ti, Au/Ge, Au/Si or Au/Ta. The composite diamond assembly body may be used as a large area diamond window or for a thermal substrate. Optical outcoupling structures such as microlenses, a diffraction grating, a Fresnel lens, a convex surface, a solid immersion lens and antireflective coating may be provided.