The present work goes beyond the state of the art in lithography techniques for fabricating nanophotonic devices, allowing for the first time, to produce 3D nanophotonic circuits inside optical crystals, which up to now where only feasible in twodimension only at the crystal surface, but not inside its volume and in 3D. This Copernican twist may now allow to fabricate photonic devices inside optical crystals which where previously undreamt-of. The 3D nanolithography technique could allow to transfer concepts for nanophotonics and ultrafast laser optics fields to the fields of solid state rare-earth doped laser crystals and classical optics. The impact could be very high since it could represent a new fabrication platform to exploit long awaited for concepts such as those of photonic crystal nanolasers, sensors, optical fibers and lasers in general, which so far have been impossible to fabricate due to the impossibility of fabricating large-scale, high quality, three-dimensional, optical nano-materials. Furthermore, due to the particular physical and chemical properties of these crystals to resist extreme environments, the new nanophotonic engineered crystals could be used to develop a new platform for photonic devices capable of operating in extreme environments, where standard silicon nanophotonics and plasmonics are of no use.