20 April 1998 Surface preparation and analysis for highest ultraviolet laser damage resistivity of fluoride materials
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Abstract
The ultraviolet laser damage resistivity of bulk fluoride optical materials is mostly limited by defects related to the preparation of the surface. Cleavage of crystals produces atomically flat terraces with highest damage thresholds(< 40J/cm2 for ns-pulses at 248nm) while cleavage steps and tips result in a dramatic local reduction of damage resistivity. Conventional polishing introduces contamination, scratches and dislocations yielding damage thresholds of typically 1 O to 20 J/cm2. Advanced preparation techniques like chemo-mechanical polishing and precision grinding provide surfaces with a damage threshold uniform over large areas that is close to that of cleavage terraces. In all cases the damage threshold is determined by light absorption of defect-induced electronic states energetically located in the band gap of the insulator. Band gap states in CaF2 and other fluoride crystals have been investigated by ultraviolet photoelectron spectroscopy, luminescence spectroscopy and photoacoustic absorption measurements. Surface quality was monitored by scanning electron and scanning force microscopy. These techniques allow the detection of minute absorptions and changes in surface structure and it is shown that not only surface preparation but even simple exposure of the surfaces to the atmosphere results in irreversible degradation. Laser damage thresholds obtained for differently prepared surfaces are related to their structural and electronic properties and the primary mechanisms of energy uptake from the laser light and damage due to the thermoelastic response of the crystal are discussed.
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Michael Reichling, Michael Reichling, } "Surface preparation and analysis for highest ultraviolet laser damage resistivity of fluoride materials", Proc. SPIE 3244, Laser-Induced Damage in Optical Materials: 1997, (20 April 1998); doi: 10.1117/12.306977; https://doi.org/10.1117/12.306977
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