Annealed proton exchanged optical waveguides in lithium niobate: differences between the X- and Z-cuts

Abstract

Abstract This article summarizes results and assessments of our systematic fabrication and characterization of proton exchanged (PE) and annealed proton exchanged (APE) waveguides study in lithium niobate. This study focused on different behavior of crystallographically diverse X(1 1 2 0) and Z(0 0 0 1) substrate cuts during waveguides fabrication, and differences in characteristics of the resulting waveguides. Non-toxic adipic acid was used as a proton source, and the waveguides properties were defined by a mode spectroscopy (waveguides characteristics) and neutron depth profiling (NDP, lithium concentration and distribution), infrared vibration spectra and elastic recoil detection analysis (ERDA, concentration and depth distribution of hydrogen). It was discovered that the X-cuts structure is more permeable for moving particles (lithium and hydrogen ions), which leads to a higher effectiveness of the PE process within the X-cut. The explanation of this phenomenon is based on the fitting X-cuts orientation towards cleavage planes of lithium niobate crystal. Higher content of interstitial hydrogen in the X-cuts then prevents lithium from free movement during the post-exchange annealing in direction to the surface of samples, and so causes a typical step-like shape of the depth concentration profiles of lithium within the X-cuts. A free transport of lithium within the Z-cuts is being reflected in a gradient shape of the lithium depth concentration profiles and extraordinary refractive index, as well the last but not least, in a trouble-free good reproducibility of the waveguides fabrication within the Z-cuts.