An overview is presented of recently developed light‐mediated methods for ferroelectric domain engineering of lithium niobate single crystals. These methods include light‐assisted poling, UV laser‐induced inhibition of poling, and all‐optical poling. In addition to the primary application of ferroelectric domain patterns, namely the realization of non‐linear optical devices, the ability of transferring a domain pattern into a topographical structure by domain selective etching allows also for surface structuring of lithium niobate. This intertwining between ferroelectric domain patterns and surface topography has been used to fabricate exquisite micro‐structures based on unusual domains generated purposefully by these light‐mediated methods.
Annealing of micro-structured lithium niobate substrates at temperatures close to, but below the melting point, allows surface tension to reshape preferentially melted surface zones of the crystal. The reshaped surface re-crystallizes upon cooling to form a single crystal again as it is seeded by the bulk which remains solid throughout the process. This procedure yields ultra-smooth single crystal superstructures suitable for the fabrication of photonic micro-components with low scattering loss.
The impact of UV laser irradiation on the distribution of lithium ions in ferroelectric lithium niobate single crystals has been numerically modelled. Strongly absorbed UV radiation at wavelengths of 244–305 nm produces steep temperature gradients which cause lithium ions to migrate and result in a local variation of the lithium concentration. In addition to the diffusion, here the pyroelectric effect is also taken into account which predicts a complex distribution of lithium concentration along the c-axis of the crystal: two separated lithium deficient regions on the surface and in depth. The modelling on the local lithium concentration and the subsequent variation of the coercive field are used to explain experimental results on the domain inversion of such UV treated lithium niobate crystals.
Ultraviolet laser irradiation of a lithium niobate +z polar surface enables the production of ridge waveguides. Ultraviolet laser induced inhibition of poling is used to define an inverted domain pattern which transforms into a ridge structure by differential etching in Hydrofluoric acid. The laser irradiation step also induces a refractive index change that provides the vertical confinement within the ridge structure. Furthermore, it was observed that poling-inhibition results in a significant enhancement of the refractive index contrast between the bulk crystal and the ultraviolet irradiated tracks.Lithium niobate (LN) because of its distinctive combination of inherent physical properties; has always been recognised as an appropriate platform for implementation of integrated optical circuits 1 . One of the suggested approaches which is quite regularly implemented to further improve the efficiency and compactness of such integrated optical circuits based on components such as modulators and resonators, is the use of ridge waveguides instead of conventional diffused 2 or proton-exchanged 3 waveguides. Such ridge waveguide superstructures provide better lateral confinement of the optical mode due to the much higher index contrast as compared to their conventional counterparts. Several different methods have been investigated for the production of ridge waveguides in LN. Most of them utilize an etching step that defines the ridge geometry and a separate waveguide fabrication step 4-6 such as the commonly used ionindiffusion or proton-exchange processes. Delineation of the ridge is achieved either via wet etching in an acid mixture containing hydrofluoric acid (HF) or through dry etching processes such as ion beam milling 7 or plasma etching 8 . Domain-selective acid-based wet etching has also been reported as an alternative approach for defining ridges in domain engineered LN 9 . Interestingly, ridge waveguides have also been fabricated by mechanical dicing of planar MgO:LN bonded on to LN 10 . The mandatory step required to induce the vertical refractive index (RI) contrast either precedes or succeeds the wet or dry etching process. In all of these approaches, a clean-room based photolithographic step is incorporated to define the ridge on an already produced planar waveguide or to define the waveguide on a prestructured ridge. Any alternative process that reduces the complexity of fabrication would be extremely well-suited for the implementation of the dense compact photonic circuits envisaged for future optoelectronic integration technologies. In this contribution we report on such an alternative method for the fabrication of ridge waveguides in z-cut LN. The method utilizes a continuous wave (c.w.) UV laser direct writing procedure that provides both the definition of the ridge pattern and the production of the necessary RI contrast for vertical confinement in a single step. Direct writing (DW) of waveguides in congruent LN has been demonstrated using c.w. UV laser light with writing wavelengths within the ran...
The ability to manipulate the size and depth of poling inhibited domains, which are produced by UV laser irradiation of the +z face of lithium niobate crystals followed by electric field poling, is demonstrated. It is shown that complex domain structures, much wider than the irradiating laser spot, can be obtained by partially overlapping the subsequent UV laser irradiated tracks. The result of this stitching process is one uniform domain without any remaining trace of its constituent components thus increasing dramatically the utility of this method for the fabrication of surface microstructures as well as periodic and aperiodic domain lattices for nonlinear optical and surface acoustic wave applications. Finally, the impact of multi exposure on the domain characteristics is also investigated indicating that some control over the domain depth can be attained.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.