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7 April 1999 Spectroscopic studies of pulsed-laser-induced damage sites in heated CaF2 crystals
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Based upon previous studies of temperature dependent defect state formation in high energy alpha particle irradiated CaF2 analogous temperature dependent effects were expected to occur when heated crystals were exposed to Q- switching emission from a pulsed Nd:YAG laser. It was shown in the (alpha) -irradiation work that optimum clustering of calcium atoms to form colloids would occur at a critical temperature. In recent work irradiation of isothermally heated crystals at fluences on the order of 1 J/cm2 produced both catastrophic mechanical damage to the crystal as well as localized defect states within the lattice. Below 200 degrees C the mechanical damage was severe, as evidenced by extensive fractures radiating from the damage center along the crystal cleavage planes. However, above 200 degrees C, these fracture lines revealed the presence of CaF2 lattice forbidden bands associated with calcium colloids in addition to a blue shift of the principal T2g CaF2 mode frequency. The T2g line shift signifies the presence of compressive stress in the CaF2 lattice induced by the presence of the calcium colloids. Forbidden phonon line intensities varied with Raman probe wavelength, and the associated Raman excitation profile followed the wavelength response of the optical absorption band at 580 nm, which is ascribed to colloid resonance absorption. The presence of colloid was detected at all temperatures, and the extent of colloid formation was found to be only weakly dependent upon the crystal temperature. This lack of sensitivity may be ascribed to highly located heating of the crystal surface during the laser pulse, which could supersede the effect of ambient temperature. Pulsed Raman studies of thin CaF2 section sat laser fluences just below the damage thresholds did indeed indicate temperature excursions within the laser footprint. For samples held at temperatures above 200 degrees C, the color centers were found to absorb more strongly in the 400-500 nm and NIR regions, Probably indicating greater aggregation of F centers in the crystal.
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Brian J. Bozlee, Gregory J. Exarhos, and Randy W. Teel "Spectroscopic studies of pulsed-laser-induced damage sites in heated CaF2 crystals", Proc. SPIE 3578, Laser-Induced Damage in Optical Materials: 1998, (7 April 1999);

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