A mode-mismatched surface thermal lens technique with pulsed top-hat beam excitation and near field detection scheme is used to measure in situ the thermoelastic response of ultraviolet dielectric coatings under excimer laser (248nm, 25ns) irradiation with fluences from below to above damage threshold (LIDT). At fluences below LIDT, the thermal lens amplitude is caused by the surface displacement of the optical coating sample, due to the coating absorption and the thermal expansion of the substrate. Measurements are made on highly reflective (HR) dielectric coating on quartz and copper substrates. For copper substrate HfO2/SiO2 HR mirror, the damage of the coating is caused by thermal stress due to the strong thermal expansion of the copper substrate. For quartz substrate sample, on the other hand, the thermal stress caused by the thermal expansion of the substrate is relatively weak and the coating is damaged by the defect-absorption induced melting and evaporation of the coating. The copper substrate sample therefore shows lower LIDT than the quartz substrate sample. For HfO2/SiO2 HR coating on copper sample, we also observed a decrease of thermal diffusion rate with increasing the fluence from below to above LIDT, which indicates that delamination occurs at interface of the coating/substrate or/and between different coating layers.
