Recently ,The experiment of Au-enhanced reflective film stacks in the range from near ultraviolet to infrared band have been successfully completed by researchers from Nanjing Institute of Astronomical Optics & Technology ,National Astronomical Observatories, CAS. The experimental scheme was promoted and carried out firstly by associate researcher Jinfeng Wang and engineer Jie Tian who from large-aperture optical technology laboratory. The mentioned reflective film stacks have advantages in wide bandwidth ,high reflectance, low film stress and excellent environmental stability ,and the service life is comparable with dielectric mirrors. All of these characters make it perfectly applied to reflection mirrors of large astronomical telescope and large-scale optical instrument .
The aluminum reflective coatings are always used on primary mirror of large astronomical telescope. It have high reflection efficiency in infrared ,visible and ultraviolet. The Al coating and glass substrate can well adhered with each other, which make aluminum mirror serving for several years. However, there are apparent defects, the spontaneous radiation in infrared band impose restrictions on application in this band, besides, Al mirror have low reflection efficiency which lower than silver coating for 8% in visible band .
Silver is the excellent material for metal reflective film, the reflection efficiency of which is higher than aluminum mirror in visible and infrared band, and, low polarization effect be induced when used tilted. But, there are notable weaknesses: low reflectivity in ultraviolet band , poor adhesion with glass substrate , and the poorer environmental stability than Al coating, namely, it is easy to vulcanize and acidize in the air ,which can change the mirror surface. Therefore, the service life is the key factor that perplex the use in primary mirror of telescope .
Gold is used in infrared band as reflective coatings ,gold coating have very high reflection efficiency in infrared band ,it is quite stable ,having no reaction with acid or alkali in the air .But its reflection efficiency drop rapidly when wavelength less than 600nm ,which make it is not available in visible and ultraviolet band .
The all-dielectric mirror ,which have high reflection efficiency and long service life, is coated with alternating dielectric oxide of high refractive index and low refractive index. However, its reflection bandwidth ,which is related to film thickness , is confined .The thick film bring about high stress, leading to the mirror surface topography changing and film cracking .
The Au-enhanced reflecting mirror made the problems mentioned above perfectly solved, which make it meet usable requirements of wide bandwidth, high efficiency, low stress and long service life. The researchers from NIAOT will continue further study in large-aperture and low-temperature film forming, which could be applied to the primary mirror of prospective Giant splicing telescope in our country .
