Towards nonlinear photonic wires in lithium niobate

Abstract

The development of wafer-scale (3'' diameter) smart-cut lithium niobate (LN) single-crystal films of sub-micrometer thickness is reported. Z-cut LN wafers, implanted by high energy He-ions, are crystal-bonded to a SiO₂ layer on another Z-cut LN handle sample. The bonded pair of samples splits along the He-implanted layer by appropriate annealing. As this fabrication method is similar to the process widely used for silicon-on-insulator (SOI) fabrication, the resulting material is called LNOI. Two different routes to develop periodically poled LNOI photonic wires are discussed. The first one starts with poling of planar LNOI samples; the photonic wires are fabricated afterwards by ICP-etching. The second one starts with the fabrication of LNOI photonic wires; they are "locally" poled afterwards. As both approaches were not yet successful, a PPLN-substrate was ion beam sliced to generate a planar periodically poled LNOI sample directly. Using planar LNOI samples as starting material, high-quality photonic wires have been developed. The smallest structure has a cross-section of ~ 1 x 0.7 μm² only. Its optical properties with mode distributions, waveguide propagation losses, and group index were investigated. Moreover, the first periodically poled LNOI photonic wires were successfully fabricated, but not yet investigated optically. They are of great potential for second order nonlinear integrated optics.