Photonic guiding structures in lithium niobate crystals produced by energetic ion beams

A range of ion beam techniques have been used to fabricate a variety of photonic guiding structures in the well-known lithium niobate (⁠$LiNbO3$ or LN) crystals that are of great importance in integrated photonics/optics. This paper reviews the up-to-date research progress of ion-beam-processed $LiNbO3$ photonic structures and reports on their fabrication, characterization, and applications. Ion beams are being used with this material in a wide range of techniques, as exemplified by the following examples. Ion beam milling/etching can remove the selected surface regions of $LiNbO3$ crystals via the sputtering effects. Ion implantation and swift ion irradiation can form optical waveguide structures by modifying the surface refractive indices of the $LiNbO3$ wafers. Crystal ion slicing has been used to obtain bulk-quality $LiNbO3$ single-crystalline thin films or membranes by exfoliating the implanted layer from the original substrate. Focused ion beams can either generate small structures of micron or submicron dimensions, to realize photonic bandgap crystals in $LiNbO3$⁠, or directly write surface waveguides or other guiding devices in the crystal. Ion beam-enhanced etching has been extensively applied for micro- or nanostructuring of $LiNbO3$ surfaces. Methods developed to fabricate a range of photonic guiding structures in $LiNbO3$ are introduced. Modifications of $LiNbO3$ through the use of various energetic ion beams, including changes in refractive index and properties related to the photonic guiding structures as well as to the materials (i.e., electro-optic, nonlinear optic, luminescent, and photorefractive features), are overviewed in detail. The application of these $LiNbO3$ photonic guiding structures in both micro- and nanophotonics are briefly summarized.