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23 March 2000 Onset of laser plume formation at 248 nm on cleaved-single crystal NaCl: evidence for highly localized emissions
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Proceedings Volume 3935, Laser Plasma Generation and Diagnostics; (2000)
Event: Symposium on High-Power Lasers and Applications, 2000, San Jose, CA, United States
When a wide bandgap insulator is exposed to UV laser radiation at fluences close to the damage threshold, many laser pulses may be required before the onset of damage. Typically, damage is accompanied by the formation of a highly localized, bright, fluorescent plume. In this work, we explore the onset of damage and plume formation by imaging the laser-induced fluorescence from cleaved, single crystal NaCl exposed to pulsed 248-nm laser radiation (KrF excimer). Subsequent observations of the resulting surface topography are made by scanning electron microscopy. The onset of plume fluorescence is accompanied by localized fracture, usually associated with cleavage steps but originating some microns beneath the surface. Strong laser interactions are typically confined to the fractured region, which grows from pulse to pulse. We interpret these results in terms of absorption by deformation- induced defects along cleavage steps. Deformed material, produced by pushing a small metal sphere against the surface, damages at especially low fluences, consist with this interpretation. Indirect evidence suggests that thermal effects are localized along dislocation cores, which serve as recombination centers for laser-induced excitations.
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J. Thomas Dickinson, Y. Kawaguchi, Mary L. Dawes, and Steve C. Langford "Onset of laser plume formation at 248 nm on cleaved-single crystal NaCl: evidence for highly localized emissions", Proc. SPIE 3935, Laser Plasma Generation and Diagnostics, (23 March 2000);

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