Silver Coating Maintains Reflectance

Aaron J. Hand

Although the reflectors behind the xenon flashlamps that pump the Nd:glass amplifiers in the National Ignition Facility's 192 laser beams might seem at most a secondary concern, their reliability is important.

Lawrence Livermore National Laboratory has developed a coating whose high reflectance and durability make it suitable to coat mirrors for telescopes, photocopiers and bathrooms, among others.

Silver has been the material of choice because it reflects well from 400 to 1000 nm over a broad range of angles. But bare silver tarnishes and corrodes in normal atmospheres, potentially reducing amplifier performance over time. Researchers at Lawrence Livermore -- with help from Viratec Thin Films Inc. in Faribault, Minn. -- have developed a silver coating that protects the surfaces from tarnishing.

For the National Ignition Facility, the reflectors must have an average reflectance of >95 percent for angles of 0° to 60°, and >90 percent for angles of 60° to 80°, for a wavelength range of 400 to 1000 nm. They also need to withstand 24,000 intense flashes over 30 years in normal atmospheric conditions.

To achieve this, the new coating designed by Jesse Wolfe consists of adhesion and passivation layers on each side of the sputtered silver, improving mechanical and chemical durability; a layer of silicon nitride, which improves mechanical durability and protects from corrodants; and metal oxide layers that return reflectance to acceptable levels.

Researchers are working out a means of measuring the coating's long-term reliability. The coating has survived exposure to 20,717 xenon flashes for four months in "100 percent Livermore air -- smog and all," said Norman Thomas, an optical engineer working on the project. The coating has maintained a reflectance of 95 to 98 percent and has shown no change in appearance or optical properties. It also survives salt fog and humidity.

Commercial industry has already shown interest in the development, Thomas said. It is rugged enough for military and space applications, protecting mirrors from desert sands, for example. But it also could protect mirrors in photocopiers and projection televisions, and could be used as a first-surface reflector on bathroom mirrors.