| Popis: |
In this time and wavelength regime optical coatings typically fail from thermal damage. Unlike catastrophic damage resulting from dielectric breakdown, one finds disparity among the thresholds for one or more of the following damage characteristics: delamination, cracking, flow, erosion, perforation, ion emission, and light emission. Comparison of the different thresholds provides useful information on how damage proceeds, and how the absorbed energy is distributed in the coating layer. Comparison of multithreshold damage data at the two different wavelengths permits evaluation of the importance of impurities which absorb preferentially at one of the wavelengths, such as water. Comparing thresholds at the two wavelengths after N/1 conditioning provides detection of preferentially absorbing impurities which are laser-desorbed. Complementary information may be obtained from laser calorimetry. The following materials, prepared as single-layer, antireflection coatings, were evaluated: NaF, SiO x , Al 2 O 3 , ZnS, As 2 S 3 , and Si. Samples of the first three were prepared in ultrahigh vacuum, in addition to the usual standard vacuum preparation. Thresholds are expressed in terms of the theoretical energy per unit area in the coating layer in order to permit a practical intercomparison of materials for multilayer coating design. |
