Oxide coatings for one micrometer laser fusion systems

Refractory metal oxides are commonly used for 1 μm high energy laser coatings. Recent improvements have increased the resistance to damage by a factor of two for both high reflector coatings (HRCs) and antireflection coatings (ARCs). Most of the data reported apply to coatings made with silica and titania. HRCs can be improved by the addition of a half-wave silica overcoat. Attempts to improve the threshold further by modifying the electric field distribution within the coatings have produced mixed results. The fields at the coating interfaces seem to play a major role. The major factor affecting the threshold of ARCs is the glass surface. A surface with low scatter offers a significant improvement over a conventionally polished surface. A half-wave silica barrier layer between the substrate and the antireflection coating also improves the damage resistance. Laser calorimetry has helped to reduce bulk absorption to a level where it is no longer a factor in laser damage. Optical microscopy and transmission electron microscopy indicate that isolated defects play a major role in the damage mechanism. (Auth.)