Study on Formation of Small Polarization Domain Inversion for High-Efficiency Quasi-Phase-Matched Second-Harmonic Generation Device

Abstract

One of the fabrication methods for a coherent blue light source is a quasi-phase-matched second-harmonic generation (QPM-SHG) device. The key technology for producing an efficient QPM-SHG device is fabrication of the periodic domain inversion in approximately 3 µm range. With the lift-off method used widely, fabrication of fine periodic electrodes is difficult. Recently, to overcome these problems, groups from Stanford University and Osaka University have proposed the following. A LiNbO3 (LN) crystal surface with a periodic resist pattern is filled with a liquid electrode or metal electrode, and periodic domain inversion is obtained when a high voltage is applied throughout the crystal surface through the electrode. We call this technique the “full cover electrode (FCE) method”. It has been proved that this method has a design flexibility which does not exist in the conventional lift-off method, since there are many parameters, such as the resist pattern form with a rectangular section or corrugated section, the sizes of the intervals and periods, the thickness of the resist, and the dielectric constant of the resist. These parameters will have a big influence on periodic domain inversion. The above-mentioned results were reported under specially fixed conditions. Can these conditions be the best state? To what extent will fine periodic domain inversion be obtained? These questions have not been answered. Thus we investigated “an extended concept of the FCE method”. At first, we determined the relationship between many resist parameters and electric-field distribution. In the case of appropriately setting several parameters, when a periodic electrode line width is designed to be as large as possible, it was proved that the ratio of the electric-field intensity directly under an electrode to that directly under a resist is larger than that obtained by the lift-off method. This result can be obtained for a fine periodic domain inversion. Experimentally, it was found that the domain-inversion shape depended on a large value of E/ER. It was found that a domain-inversion form approximated to an electrode form can be obtained with a large value of E/ER, and uniformity over the whole surface and sufficient polling current can be secured, which is difficult in the lift-off method. We proved that the FCE method has useful features which do not exist in the lift-off method. Recently, we have fabricated a periodic domain inversion with a 4.0 µm period in a 500-µm-thick LT crystal.