Vacuum and cryogenic compatible black surface for large optical baffles in advanced gravitational-wave telescopes
What can one use when an optical black coating is required for stray light suppression but black paint is not suitable? This paper describes a niche application where a black coating is needed for a laser interferometer serving as a gravitational wave detector. The coating must have low reflectance in the near IR, survive cryogenic temperature cycling, and be compatible with an ultra high vacuum environment (~1e-7 Pa). The parts to be coated are large (~ 80 cm) optical baffles with irregular shapes. Historically an electroless plating process which produces a thin layer of “black nickel” has been used. Such a process was considered by this same Japanese group in 2004, but rejected in favor of a diamond like carbon (DLC) coating which had lower outgassing by two orders of magnitude. However, the thickness of the DLC layer has to be adjusted for a specific wavelength, incidence angle, and polarization to achieve the best performance. DLC is deposited by a plasma process, which makes it very difficult to achieve the required uniformity in thickness for large complex-shaped surfaces.
In this Optical Materials Express article, the group takes on again the “black nickel” technique, developing further an electroless nickel-phosphorus-tungsten plating which has surface microstructure to absorb and trap stray light, while keeping the overall surface area low enough to hold the outgassing to an acceptable level (even if still higher than DLC). Compared to the DLC, the reflectivity is lower and relatively insensitive to wavelength and incidence angle. The outgassing is several times lower than “black nickel.” It can be readily deposited on large irregular surfaces.