Spectroscopic studies of pulsed-laser-induced damage sites in heated CaF2 crystals

Brian J. Bozlee; Gregory J. Exarhos; Randy W. Teel

doi:10.1117/12.344449

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7 April 1999 Spectroscopic studies of pulsed-laser-induced damage sites in heated CaF₂ crystals

Brian J. Bozlee, Gregory J. Exarhos, Randy W. Teel

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Proceedings Volume 3578, Laser-Induced Damage in Optical Materials: 1998; (1999) https://doi.org/10.1117/12.344449
Event: Laser-Induced Damage in Optical Materials: 1998, 1998, Boulder, CO, United States

Abstract

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 T_2g CaF₂ mode frequency. The T_2g 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); https://doi.org/10.1117/12.344449

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