A complete surface acoustic wave band gap is found experimentally in a two-dimensional square-lattice piezoelectric phononic crystal etched in lithium niobate. Propagation in the phononic crystal is studied by direct generation and detection of surface waves using interdigital transducers. The complete band gap extends from $203\text{to}226\mathrm{MHz}$, in good agreement with theoretical predictions. Near the upper edge of the complete band gap, it is observed that radiation to the bulk of the substrate dominates. This observation is explained by introducing the concept of the sound line.

We report the development of a quick process for fabricating lithium niobate ridge waveguides with smooth walls, aspect ratios larger than 500 and side-wall verticality of 88°. The method is based on optical grade dicing, and allows the fabrication of ridges with a top width of 1 µm and a depth of more than 500 µm. Smart-cut ridge waveguides and strongly confined proton exchanged ridge waveguides are demonstrated. We show that the method enables the fabrication of ridge waveguides with propagation losses as low as 0.5 dB cm⁻¹. A new fabrication process is thus proposed for the fabrication of optical components with enhanced acousto-optic, electro-optic or nonlinear interactions. The high aspect ratios open opportunities towards the development of 3D photonic components in thin films of LiNbO₃, and towards hybrid integration of LiNbO₃ components.