Nonlinear Susceptibility and Second Harmonic Generation in Proton- Exchanged Lithium Niobate Waveguides

Lithium niobate (LiNbO3) is one of the most commonly used optical crystals because of its excellent electro-optic and nonlinear optical properties. Ease of waveguide fabrication and amenability to periodic poling has made lithium niobate highly attractive for quasi-phase-matched guided wave interactions such as second harmonic generation (SHG) and optical parametric amplification/oscillation [1]. Currently, the annealed proton exchange (APE) technique has become a very convenient way of fabricating waveguides in LiNbO3. Proton exchange (PE) in LiNbO3 is an attractive technique for nonlinear frequency conversion, because PE LiNbO3 waveguides have high photorefractive damage threshold. Our works, reviewed and summarized in [2], have shown that the seven different crystallographic phases HxLi1-xNbO3, the α, κ1, κ2, β1, β2, β3 and β4 phases, can be realized in PE LiNbO3 layers depending on exchange and annealing conditions. However, the relationships between the phase composition and the resulting nonlinear optical properties of PE LiNbO3 waveguides were not well known. For the design, fabrication and optimization of nonlinear optical devices is it essential to determine the nonlinear optical susceptibility of PE waveguides consisting of the different HxLi1-xNbO3 phases and correlate it to the processing conditions and their optical properties.