Windowed wafer assemblies having interposers

Windowed wafer assemblies having interposers are described. A described example integrated circuit (IC) package includes first and second dies, where at least one of the first or second dies includes an optical window with a light transmittance wavelength range between 0.1 micrometers and 1.0 millimeter, and an interposer die between the first and second dies, where the interposer die is coupled to the first die at a first surface of the interposer to form a first bonded interface, where the interposer is coupled to the second die at a second surface of the interposer die to form a second bonded interface, where the second surface is opposite the first surface, where the first and second bonded interfaces form a sealed cavity of the IC package that is at least partially formed by the optical window, and where the interposer die includes electrical routing.