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Most of the optical components of MegaJoule laser working in transmission are coated with an antireflective sol-gel layer. The colloidal silica is used for lenses made of silica since its refractive index is close to 1.22 so that we can build antireflective layers at 1 ω (1 053 nm) if their thickness is 216 nm and antireflective layer at 3 ω (351 nm) 72 nm.
These coatings are deposited by dip coating and are quite fragile mechanically. The coated components are then immersed into vapor of ammonia to harden them and to increase their mechanical resistance against wiping, cleaning, handling or procedures for maintenance. This processing causes a layer shrinkage in thickness which is sometimes accompanied by crazing for thicker layers and longer exposition times. We want to study this crazing effect in order to reduce it by optimizing the parameters of deposition and treatment. As a fast way to evaluate the crazing process we used video optical microscopy and a scanning stage at the sample position
microscopy, image analysis, diffusion measurement, sol-gel, thin films
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