The optical properties of an ultraviolet (266-nm) -induced transient absorption in normal and deuterated potassium dihydrogen phosphate crystals are discussed. Our main contribution has been to analyze this effect quantitatively, assess its effect on ultraviolet (UV) frequency-conversion applications, and examine sample variability. As observed previously [ Chem. Phys. Lett. 207, 540 ( 1993)], the transient absorption is broad (~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 find that the defect absorption decays over a wide variety of time scales, ranging from fractions of a second to several days, depending on the particular sample under study. The decay time does not appear to be correlated with deuteration or chemical impurities. The temperature-dependent decay rate has an Arrhenius behavior with a single activation energy of 0.51 ± 0.04 eV for all samples studied, and the decay rate appears to be mediated by a diffusional process as previously postulated. We determined the defect absorption cross section at its spectral peak to be 6 ± 3 × 10−18 cm2. Based on these and other measurements, we have numerically modeled previously published experimental data on fourth-harmonic frequency conversion of a Nd:YLF laser and found good agreement when the effects of transient absorption are included.
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