20 April 1998 Photothermal mapping of defects in the study of bulk damage in KDP
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Abstract
Interest in producing high-damage-threshold KH2PO4 (KDP) and DKDP for frequency conversion and optical switching applications is driven by the requirements of the National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory (LLNL). At present only the best crystals meet the NIF system requirements at the third harmonic and only after a laser conditioning process. Neither the mechanism for damage in bulk DKP nor the mechanism for conditioning is understood. As part of a development effort to increase the damage thresholds of DKP and DKDP, we have been developing techniques to pinpoint the locations where damage will initiate in the bulk material. After successfully developing a diagnostic tool that will find these locations, we will use other measurement techniques to determine how these locations differ form the surrounding materials and why they cause damage. This will allow crystal growers to focus their efforts during the growth process in improving damage thresholds. Previously we reported that there was a low correlation between defects in crystals located using light scatter and the sites where damage would initiate. Damage, when it occurs, is almost certainly associated with a localized heating of the crystal which results in mechanical damage to or chemical decomposition of the crystal lattice. If this heating occurs at fluences below the damage threshold, we should be able to measure this heating and predict the locations where damage will initiate prior to damaging the crystal. As a result, we are developing photothermal techniques to probe the bulk material to look for heating due to localized absorption and to determine the correlation between localized heating and the initiation sites for damage.
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Bruce W. Woods, Ming Yan, Jim J. DeYoreo, Mark R. Kozlowski, Harry B. Radousky, Zhouling Wu, "Photothermal mapping of defects in the study of bulk damage in KDP", Proc. SPIE 3244, Laser-Induced Damage in Optical Materials: 1997, (20 April 1998); doi: 10.1117/12.307033; https://doi.org/10.1117/12.307033
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