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We report on electric-field poling of MgO:LiNbO3 (MgLN) using patterned electrodes. Investigation of electric properties of MgLN reveals that polarization switching causes reversible resistance change and that this phenomenon can explain the physical mechanism of random domain growth. Based on these results, we propose a domain-control method in MgLN single crystals by suppressing resistance reduction during the poling process. Using this method, short periodic structures consisting of submicron domain geometries have been achieved in X-, Y-, and Z-cut MgLN crystals. High performances of these periodic domain-inverted structures are also demonstrated by evaluating their potentials as second-harmonic generation devices.

We report on fabrication of a first‐order periodically domain‐inverted structure in −Z‐cut LiTaO3 using proton exchange followed by heat treatment. The expansion of the domain inversion width can be suppressed within the first‐order intervals by shortening the heat‐treatment time. To perform this suppression, we propose quick heat treatment, which features rapid rising rate and short soak time. This technique was used to study the formation process and the expansion characteristics of domain inversion during heat treatment. This produced a periodically domain‐inverted structure with interval of 4 μm and depth of 1.8 μm over a 10 mm length in the LiTaO3. The potential for this structure was evaluated by examining the characteristics of a first‐order quasi‐phase‐matched second‐harmonic generation device with a proton‐exchanged waveguide. We also discuss the nucleation and expansion of domain inversion, based on experimental studies.

A periodically domain‐inverted structure in LiTaO3 has been fabricated for quasiphase‐matched (QPM) second‐harmonic generation (SHG). Uniform proton exchange is achieved with pyrophosphoric acid. Using this technique with a Ta mask followed by heat treatment, the periodically domain‐inverted structure for quasiphase matching is realized over large areas. The effect of fabrication conditions on the domain‐inverted profile is investigated to form a periodic structure having a considerable depth and a short inversion period. Consequently, a third‐order QPM‐SHG device can be fabricated by using this periodically domain‐inverted structure and proton‐exchanged waveguide. To investigate period regularity of the domain‐inverted structure and nonlinearly of the inverted region, phasematching characteristics and conversion efficiency of this device are examined. The formation mechanism of the domain inversion is also discussed.

Characteristics of waveguides with periodically domain‐inverted regions in LiNbO3 and LiTaO3 for quasi‐phase‐matched (QPM) second harmonic generation are investigated. The domain‐inverted regions in LiNbO3 are formed by heat treatment using selective SiO2 mask and those in LiTaO3 are formed by heat treatment using selective proton exchange. The domain‐inverted region in LiTaO3 is fabricated up to 2.7 μm depth, which is twice as deep as that in LiNbO3. The channel waveguides incorporating the regions are fabricated using proton exchange by pyrophosphoric acid. The propagation loss (0.9 dB/cm) of LiTaO3 waveguide is much lower than that of LiNbO3 waveguide (2.9 dB/cm). The second harmonic power generated in LiTaO3 waveguide by third‐order QPM consequently has three time higher conversion efficiency, compared to that in LiNbO3 waveguide. In domain‐inverted LiNbO3 waveguide, we have observed optical damage. On the other hand, in domain‐inverted LiTaO3 waveguide, stable blue light (421 nm) has been obtained wi...