Dynamics of green-light induced infrared absorption in KTP and PPKTP

Due to high nonlinearity, good mechanical and structural properties, KTP became an important material for nonlinear optics applications. Periodically poled KTP (PPKTP) is becoming a popular material for visible light generation by frequency doubling of near-infrared lasers. It is known, that higher intensities of the visible radiation in the green-blue spectral region induce absorbing centers in KTP, so called gray tracks, associated with the capture of free-electron by the Ti4*-Oxygen vacancy complex and free-hole capture by Fe impurities [I]. These centers give rise to broad and partially overlapping absorption bands around 5 lOnm and 420nm, respectively. Illumination of KTP with green-light. (532nm) also induces absorption in the near-infrared region, so called green-induced infrared absorption (GRIIRA), which at least partly should be related to the above mentioned color-centers. In any case, GRIIRA is a very sensitive to the crystalline quality and stoichiometry of the nonlinear crystals. In this work we report on investigations of the differences in GRIIRA dynamics in KTP and PPKTP. The GRIIRA dynamics has been investigated using a common-path interferometer sensitive to the thermal lens induced by weak absorption of the near-infrared light [2]. Three laser beams, the probe at 633nni (He-Ne), the pump at 970nm ViSapphire) and gray-track inducing beam at 532nm from Q-switched and frequency-doubled Nd:YAG (pulse length of 5ns) have been propagating colinearly in the sample. All beams were polarized parallel to~the crystal z- axis. The 970nm pump beam bad a power of 800mW and was mechanically chopped. The modulation induced on the 633nm probe beam due to pump absorption was measured using a lockin amplifier. The maximum peak intensity of the graytrack-inducing 532nm beam was 320 MW/cm'. The repetition rate of the Q-switched Nd:YAG could be varied from single-pulse operation up to 20 Hz. KTP and PPKTP samples originated from the same flux-grown wafers. The probe absorption without 532nm light illun~ination was the same of about at10-4cm in KTP and PPKTP, showing that material properties have not been modified by poling. The GRIIRA increased exponentially in KTP and PPKTP with the time constant of 1.3s while illuminating samples with 320MW/cm' at 532nm (see Fig.1). GRlIRA was consistently larger by 30-40% in PPKTP. The magnitude of induced absorption after lOmn of illumination with 532nm light showed saturation behavior as a function of the green light intensity. This can be due to the limited concentration of available crystal defects participating in gray-track creation or due to increasing GRIIRA relaxation rates at higher gray-track density. The absorption showedmultiexponential relaxation at room temperature with the time constants of about 2mn,