Study of blue light induced infrared absorption in KTiOPO4 and its isomorphs

Potassium Titanyl Phosphate (KTiOPO4 or KTP) is one of the most used crystals in nonlinear optics. Because of its properties, such as the high transparency in the range from 0.35m to 4.3m, the large angular and thermal acceptances and the high nonlinear coefficient, it is broadly used for second harmonic generation (SHG) in intracavity or extracavity configurations and as an active medium for optical parametric oscillators (OPO) and generators (OPG). In the last decades many other isomorphs of KTP with improved optical or electrical features have been synthesized, such as KTA, RTA or RTP. This work aims at studying an effect that arises in a large variety of nonlinear crystals: a reversible photochromic damage that limits SH conversion efficiency called Blue Light Induced Infrared Absorption (BLIIRA). Effects of BLIIRA have been reported before for CW exposure in different materials [43,44], and for exposure to pulses in KTP, Ce:KTP and RKTP [19,24,42]. Here we study this damaging process in different crystals from the KTP family, namely KTP, RKTP, KTA, RTP and RTA, showing how the exposure to picoseconds pulses at 400nm changes the absorption of IR light. We have recorded the dynamics and the effects of BLIIRA under different conditions in order to better understand the mechanisms behind it; in particular we have tested crystals at different temperatures and under the exposure to different pulse length and average power of the damaging beam. The observation of the damaging for long exposures to blue pulses has shown similar features for KTP and RKTP in accordance with previous publications, while in some of the other materials it has been possible to observe effects that inhibit the growth of the absorption coefficient during illumination. The comparison between BLIIRA in KTP isomorphs has shown various features depending on conductivity and on the chemical composition. Measurements at higher temperatures are characterized by faster relaxation dynamics for the absorption coefficient. The study of the damage with different pulse length and average power of the absorption-inducing beam has showed new features of BLIIRA among the crystals of the KTP family.