Growth of epitaxial LiNbO₃ films on sapphire substrates and their nonlinear optical properties

Epitaxial LiNbO₃ films, i.e., LiNbO₃(0001)/Al₂O₃(0001), LiNbO₃(0001)/Al₂O₃(1120), and LiNbO₃(1120)/Al₂O₃(1120) films, were grown by the pulsed laser deposition. By varying the growth conditions, it was possible to control various growth behaviors, i.e., twin-ratios and crystal orientations. The nonlinear optical properties of the films including the second harmonic generation (SHG) and electro-optic (EO) properties were investigated. The SHG patterns of LiNbO₃(0001)/Al₂O₃(0001) films were strongly dependent on their twin-ratios. In order to analyze them, the contributions of the structural twins and polarization domains were taken into account. The linear EO property of the LiNbO₃(0001)/Al₂O₃(0001) film was found to be as good as that of single cystal. However, the LiNbO₃(1120)/Al₂O₃(1120) film shows a very large quadratic EO behavior with an effective coefficient of 2.38×10^-15 m²/V².