Mechanical loss of the reflective coating and fluorite at low temperature

Abstract

In some future interferometric gravitational wave detector projects, LCGT and EGO, the mirrors will be cooled so as to reduce the thermal noise. In order to evaluate the thermal noise of such cryogenic interferometers, the mechanical loss of the reflective coating of the mirrors was measured at low temperature. The measured loss angles were about 4 × 10−4 and constant between 4 K and 300 K. This result implies that the thermal noise of the coating is proportional to the square root of the temperature. The cooling mirror suppresses the thermal noise of the coating loss effectively, which satisfies the goal sensitivity of the future projects (h ∼ 3 × 10−24 Hz−1/2 at 100 Hz). The Q-values of fluorite, which is a candidate for the mirror substrate of the cryogenic interferometer, were also measured at low temperature. The measurement showed that the Q-value below 12 K is at least 107. The fluorite mirrors must be cooled down to less than 5 K if they are to be used in future projects.

High mechanical Q-factor measurements on calcium fluoride at cryogenic temperatures

“…Yamamoto et al presented in [19] a Q-factor of several 10 7 below 12 K which corresponds pretty well to our observations although the test samples have completely different geometries (Yamamoto et al: ∅ 100 mm × 3 mm, here: ∅ 75 mm × 75 mm). Also a decreasing Q-factor with decreasing temperature below 15 K could be observed.…”

“…They found a highest value of 4.5 × 10 7 . Yamamoto et al [19] Article published by EDP Sciences and available at http://www. [20].…”

A new method of probing mechanical losses of coatings at cryogenic temperatures

“…It has been applied for data at room temperature [21,22], as well as at low temperatures [23][24][25]. In these experiments, dependence of the mechanical loss of the dielectric coating with frequency has sometimes been considered [21], but, anyway, the frequency range of interest never goes above a few kilohertz in those applications.…”

Titania-doped tantala/silica coatings for gravitational-wave detection

“…Study of multilayer dielectric optical coatings is an ongoing research project in the gravitational-wave detection community [6,[11][12][13][14][15][16][17][18][19][20].…”