A method of making GaP window slabs having largest dimensions of greater than 4 inches and GaAs IR window slabs having largest dimensions of greater than 8 inches, includes slicing and dicing at least one smaller GaAs or GaP single crystal boule, which can be a commercial boule, to form a plurality of rectangular slabs. The slabs are ground to have precisely perpendicular edges, which are polished to be ultra-flat and ultra-smooth, for example to a flatness of at least λ/10, and a roughness Ra of less than 10 nanometers. The slab edges are then aligned and fused via optical-contacting/bonding to create a large GaAs or GaP slab having negligible bond interface losses. A conductive, doped GaAs or GaP layer can be applied to the window for EMI shielding in a subsequent vacuum deposition step, followed by applying anti-reflection (AR) coatings to one or both of the slab faces.