LiNbO3 thin-film optical waveguide grown by liquid phase epitaxy and its application to second-harmonic generation

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Abstract
LiNbO3 thin‐film optical waveguides with high crystallinity and good surface morphology have been grown on 5 mol % MgO‐doped Z‐plate LiNbO3 substrates by liquid‐phase epitaxy using Li2O‐V2O5 flux. A small lattice mismatch less than 4×10−3 Å in the X axis, a small linewidth of 11s in an x‐ray rocking curve, and a perfect mirror surface are realized by choosing a melt composition. Ordinary refractive index no at a wavelength of 0.6328 μm is unchanged (2.287) with melts of different compositions, but extraordinary refractive index ne can be changed from 2.205 to 2.192 or less by increasing Li2O in the melt with V2O5 fixed at 40 mol %. The optical absorption loss at a wavelength of 0.5145 μm caused by V incorporated from the flux is significantly reduced from 25 to 1.6 dB/cm by O3 annealing for 1 h at 600 °C. Efficient second‐harmonic generation of TE0(1.064 μm) →TM0(0.532 μm) for a single‐poled film is observed by a temperature‐tuning method. The mechanism of self poling and domain inversion which were revealed by the present research is discussed.