Investigation of damage mechanisms of KTiOPO4 crystals by use of a continuous-wave argon laser.

We have developed a simple method that uses cw irradiation from an argon laser to measure the damage threshold of KTP crystals. Our experimental results show that there are two types of damage in KTP crystal, depending on the polarization of the incident laser beam. One type of optical damage is the appearance of gray tracks, including both dark tracks and orange dots, when the pump polarization is perpendicular to the z axis. The other type is invisible damage when the polarization is parallel to the z axis. In addition, we have also observed weak photorefractive two-wave mixing in KTP crystals in each of these polarization states. After a systematic analysis we have concluded that the first type of damage is due to the formation and absorption of Ti3+ and Fe3+ centers and the second is due to the drift of K+ ions. The weak photorefractive effect is the result of screening of the electric field by the drifted K+ ions.