Fluorescence studies on optical coatings

Higher laser powers are pushing the limits of common coating materials in various applications, from frequency conversion processes to space based LIDAR systems. Towards further ultraviolet material optimizations, luminescence spectroscopy may be employed to identify responsible energy levels as possible precursors for laser induced damage. Hafnium dioxide and aluminum oxide deposited by reactive ion beam sputtering are considered in the present study as a basis. Selected characterization methods are employed to determine the optical properties of the single layers of about four quarter wave optical thicknesses at 1064 nm. The luminescence induced by 193 nm excimer irradiation with about 2 mJ pulsed energy at 200 Hz repetition rate was evaluated by fiber coupled spectrophotometry. Detected aluminum oxide emission bands describe band gap intermediate states representing structural imperfections. Hafnium dioxide exhibits luminescence around 2.8 eV of photon energy that can be attributed to atomic and molecular interstitial defects and oxygen vacancies. Process variations like the supply of oxygen by the primary source keep the luminescence characteristics of hafnium dioxide unaffected. Luminescence transformations resulted from simultaneous ion bombardment of the growing layers associated to increased degrees of micro crystallinity. Luminescence spectroscopy can be considered as a process optimization tool for dual ion beam sputtering towards for example enhanced laser induced damage thresholds.