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Pulsed top-hat beam thermal lens: a simple and sensitive tool for in situ measurement on ultraviolet dielectric components

 
: Li, B.C.; Martin, S.; Welsch, E.; Thielsch, R.; Heber, J.; Kaiser, N.

Exarhos, G.J. ; Society of Photo-Optical Instrumentation Engineers -SPIE-, Bellingham/Wash.:
Laser-Induced Damage in Optical Materials 1999
Bellingham/Wash.: SPIE, 2000 (SPIE Proceedings Series 3902)
ISBN: 0-8194-3508-2
pp.470-478
Annual Boulder Damage Symposium <31, 1999, Boulder/Colo.>
English
Conference Paper
Fraunhofer IOF ()
lanthanum compounds; laser ablation; laser beam effect; light absorption; magnesium compounds; melting; Optical films; optical material; optical multilayer; photothermal effect; thermal blooming; thermal stress; thermoelasticity

Abstract
A mode-mismatched surface thermal lens technique with pulsed top-hat beam excitation and near field detection scheme is developed to measure in situ the thermoelastic response of ultraviolet dielectric coatings and bulk materials under excimer laser (193 nm or 248 nm) irradiation. The thermal lens technique is demonstrated to be not only convenient for an accurate determination of the laser-induced damage threshold (LIDT), but also sensitive to measure the thermoelastic response of dielectric coatings irradiated with fluences much below the LIDT, and hence, to carry out time resolved predamage investigation. The minimum detectable surface displacement of approximately 0.002 nm is achieved with a simple experimental configuration. Nonlinear absorption of ultraviolet dielectric materials and coatings are demonstrated. The surface thermal lens technique is also a convenient technique to distinguish different damage mechanisms: thermal stress induced damage or melting induced damage, depending on the thermoelastic properties of the substrate.
Hence, this technique allows to indicate qualitatively the relative contribution of linear and nonlinear absorption as possible causes for laser damage. Moreover, the nonlinear effect in laser conditioning of a LaF(3)MgF(2) highly reflective dielectric coating has been observed experimentally.

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