ULTRAVIOLET-INDUCED TRANSIENT ABSORPTION IN POTASSIUM DIHYDROGEN PHOSPHATE AND ITS INFLUENCE ON FREQUENCY-CONVERSION

The optical properties of an ultraviolet (266-nm) -induced transient a bsorption in normal and deuterated potassium dihydrogen phosphate crys tals are discussed. Our main contribution has been to analyze this eff ect quantitatively, assess its effect on ultraviolet (UV) frequency-co nversion applications, and examine sample variability. As observed pre viously [Chem. Phys. Lett. 207, 540 (1993)], the transient absorption is broad (approximately 200-700 nm) and is suggested to arise from an induced defect state resulting from the promotion of an electron into the conduction band by two-photon absorption. At room temperature we f ind that the defect absorption decays over a wide variety of time scal es, ranging from fractions of a second to several days, depending on t he particular sample under study. The decay time does not appear to be correlated with deuteration or chemical impurities. The temperature-d ependent decay rate has an Arrhenius behavior with a single activation energy of 0.51 +/- 0.04 eV for all samples studied, and the decay rat e appears to be mediated by a diffusional process as previously postul ated. We determined the defect absorption cross section at its spectra l peak to be 6 +/- 3 x 10(-18) cm2. Based on these and other measureme nts, we have numerically modeled previously published experimental dat a on fourth-harmonic frequency conversion of a Nd:YLF laser and found good agreement when the effects of transient absorption are included.