Pulsed top-hat beam thermal-lens measurement for ultraviolet dielectric coatings

A mode-mismatched surface thermal-lens technique with pulsed top-hat beam excitation and a near-field detection scheme are developed to measure in situ the thermoelastic response of ultraviolet dielectric coatings to excimer-laser (193- or 248-nm) irradiation. The thermal-lens technique is demonstrated to be not only convenient for accurate determination of the laser-induced damage threshold (LIDT) but also sensitive to measurement of the thermoelastic response of dielectric coatings irradiated with fluence far below the LIDT, and hence is shown to be appropriate for time-resolved predamage investigation. A minimum detectable surface displacement of approximately 0.002 nm is achieved with a simple experimental configuration. Nonlinear absorption as well as the nonlinear effect in laser conditioning of a LaF(3)/MgF (2) highly reflective dielectric coating are observed for what is believed to be the first time.