Improved profiles of photonic crystals in Lithium Niobate by truncating tapered bottoms

Lithium niobate (LiNbO₃, LN) has found extensive applications because of its unique optical properties, such as acousto-optic, electro-optic, piezoelectric, ferroelectric and nonlinear optic effects. Conventionally, a waveguide in a LN crystal is fabricated using metal diffusion, ion exchange, or proton exchange. Waveguides in LN are used for high-speed modulation or for nonlinear devices such as parametric oscillators and amplifiers. Thin LN films can also be separated from bulk and transferred to other substrates such as silicon and gallium arsenide by crystal ion slicing. The refractive indices and the electro-optical coefficient of the fabricated thin films have already been investigated. Suspended waveguides have been widely applied to silicon-on-insulator structures because they are easily fabricated with processing techniques similar to those of integrated circuit design. However, it is difficult to fabricate such structures in LN. One main challenge is the difficulty of etching LN. Here, we show a method to fabricate suspended slab waveguides and photonic crystals (PCs) in LN by combining ion implantation, focused ion beam milling and selective wet etching techniques. The method does not involve wafer bonding or crystal ion slicing and is monolithic. Lattice damage can be introduced to a buried thin layer of a certain depth beneath sample surface by ion implantation, resulting in a considerable wet etching selectivity to bulk material. Profiles of PCs can be improved dramatically by truncating the cone-shaped bottoms. Our method can effectively be applied to bulk LN wafers, which are typically of much higher quality than epitaxial LN.