Optical loss study of molecular layer for a cryogenic interferometric gravitational-wave detector

The detection of gravitational waves provides us with a deep insight into the Universe. In order to increase the number of detectable gravitational-wave sources, several future gravitational-wave detectors will operate with cryogenic mirrors. Recent studies, however, showed that residual gas molecules inside the vacuum chamber adhere to the cryogenic mirror surface and form a molecular layer which grows with time. This growing molecular layer introduces an additional optical loss in the mirror which can decrease the detector's performance. We theoretically estimate the optical loss by the molecular layer in a cryogenically operated gravitational-wave detector. The impact on a cryogenic gravitational-wave detector is discussed based on the results of the optical loss estimation.