Sol-gel antireflective coatings for astronomical optics

摘要

Sol gel coatings are an important alternative to conventional broadband dielectric antireflection (AR) coatings. Originally developed in the early 1940s for AR coating of lens elements, interest in this technology has been rekindled with the advent of high energy laser systems. Fusion laser designs push the damage threshold limits of conventional dielectric films. In addition to these laser applications, sol gel AR coatings are important for broadband applications in the ultraviolet, visible or near infrared. Repeatable and predictable broadband coatings of this type are frequently required astronomical instruments. Lens size does not impose severe limitations for this technology, an important aspect when considering conventional dielectric AR coatings. Sol gel AR coatings typically have a minimum reflectivity per surface on the order of 0.1% and behave as a near perfect quarterwave coating for substrates with refractive indices between 1.38 and 1.60. Sol gel coatings can be removed from glass elements without damage or repolishing of substrates thereby eliminating the risk of a spoiled coating run. These coatings simplify changes for different wavelength requirements. Optical glass, fused silica and many types of crystalline elements are appropriate substrates for sol gel AR coatings. Recent technical development has improved mechanical durability of standard sol gel coatings on many substrates, allowing them to be more easily cleaned. Sol gel coatings have been used successfully in astronomical instruments, and it appears that they may find additional application as their properties become more widely understood.