Molecular adsorbed layer formation on cooled mirrors and its impacts on cryogenic gravitational wave telescopes

Abstract

Cryogenic mirrors have been introduced to the KAGRA gravitational wave telescope in Japan, and are also planned to be used in next-generation gravitational wave telescopes to further improve their sensitivity. Molecular gases inside vacuum chambers adhere to cold mirror surfaces because they lose their kinetic energy when they hit cryogenic surfaces. Finally, a number of adsorbed molecules form an adlayer, which will grow with time. The growing adlayer functions as an optical coating and changes the properties of the underlying mirror, such as reflectance, transmittance, and absorption, which are carefully chosen to maximize the detector sensitivity. The adlayer possibly affects the gravitational wave detector sensitivity. In order to characterize these changes, a high-finesse Fabry--Perot cavity was introduced to a KAGRA cryostat and the finesse of the cavity was monitored for 35 days under cryogenic conditions. We confirmed that the molecular adlayer was formed on a cold mirror and caused an oscillation in the finesse. The real and imaginary parts of the refractive index of the adlayer were $1.26 \pm 0.073$ and $2.2 \times 10^{-7} \pm 1.3 \times 10^{-7} $, respectively. These are considered to be that of $\mathrm{H_2O}$ molecules. The formation rate of the molecular adlayer was 27 $\pm$ 1.9 $\mathrm{nm/day}$. In this paper, we describe theoretical and experimental studies of the formation of a molecular adlayer on cryogenic mirrors. Furthermore, the effects of a molecular adlayer on the quantum noise and the input heat to the test mass are also discussed.