PE-Layers and Proton Diffusion Profiles in LiNbO3 Investigated with Fourier-IR and Second Harmonic Generation

Deep proton-exchanged (PE) layers in congruent LiNbO3 with thickness up to 10 μm are studied by Fourier-IR spectroscopy, mode-coupling, and interference techniques. Effective diffusion constants, depending on the amount of Li buffering in the benzoic melt and crystal cut, as well as the relation between layer thickness and the absorption of the fundamental (v01) and the next higher O-H vibrational transition (v02) are measured. Based upon the H/Li exchange ratio in the layer an OH oscillator strength for v01 offosc = 1.6 × 10−2 is found. This allows to determine the refractive index change per OH concentration by combining frequency doubling (SHG) and absorption measurements in OH profiles in the bulk of LiNbO3. Resulting values for H indiffusion Δne/ΔCH = (2.9 ± 0.6) × 10−23 cm3 and Li outdiffusion Δne/ΔCli = 3.4 × 10−23 cm3 are about the same. Extrapolation of these Δn values to the large [H]/[Li] variation in the PE-layer, leads to much larger Δn values than observed, pointing to a strongly nonlinear mechanism.