Vacuum and cryogenic compatible black surface for large optical baffles in advanced gravitational-wave telescopes

This paper reports on the optical properties, outgassing rate, and cryogenic performance of surface finishing we have adopted for large optical baffles absorbing stray light in KAGRA, an advanced interferometer for detecting gravitational waves. The surface finishing is based on an electroless nickel-phosphorus-tungsten (NiPW) plating, applicable to large surface area up to ~ 800 mm in diameter, and achieves less than 3% total reflectance against a light beam at 1064 nm with a reasonable scattering distribution ~ 0.05/sr. The outgassing rate from the black coating meets our requirements of 3×10−7 Pa · m3 s−1 m−2. The black coating can tolerate low temperature down to 12 K, and can be installed close to cold mirrors indispensable for the future interferometers.