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

A mode-mismatched surface thermal-lens technique with pulsed top-hat beam e xcitation and a near-field detection scheme are developed to measure in sit u the thermoelastic response of ultraviolet dielectric coatings to excimer- laser (193- or 248-nm) irradiation. The thermal-lens technique is demonstra ted to be not only convenient for accurate determination of the laser-induc ed damage threshold (LIDT) but also sensitive to measurement of the thermoe lastic response of dielectric coatings irradiated with fluence far below th e LIDT, and hence is shown to be appropriate for time-resolved predamage in vestigation. A minimum detectable surface displacement of approximately 0.0 02 nm is achieved with a simple experimental configuration. Nonlinear absor ption as well as the nonlinear effect in laser conditioning of a LaF3/MgF2 highly reflective dielectric coating are observed for what is believed to b e the first time. (C) 1999 Optical Society of America OCIS codes: 120.6810, 140.3440, 310.6870.