Nonlinear Fiber Optics
The ideal form of a compact all-optical frequency conversion in a photonic circuit is a monolithical intracavity and resonant nonlinear fiber scheme. However, integrating nonlinear crystalline material that provides significant gain and realizes enhanced second-order nonlinearity (χ(2)) remains challenging to date. This challenge is due to the inherent conflict in achieving crystallization of nonlinear crystals in a rather unstable state, while attaining a sufficient thermal stability for glass fiber drawing. In addition, existing nano-fabrication techniques, such as the e-beam and focused ion beam, are not necessarily suitable for fabricating such large-scale three-dimensional metamaterials throughout the fiber. We therefore propose and demonstrate, for the first time, a new platform toward an enhanced χ(2), where a large-scale harmonic crystal (Si4+:γ-Al2O3) is monolithically integrated in a hybrid crystal–glass metamaterial fiber cavity. Through a comprehensive nano-scale investigation, along with nonlinear optical measurement, we confirmed a detailed growth mechanism for a non-centrosymmetric harmonic crystal directly derived from a centrosymmetric sapphire template. The key to this accomplishment lies in the development of a simple and scalable laser-based fiber drawing that involves the interplay of the inter-crystalline layer forming, the crystal core phase separation, and considerable defective centers. The proof-of-concept developed in this study can be applied to any nonlinear optical fiber comprised of hybrid materials, depending on the practical applications.
