Measurement of the mechanical loss of a cooled reflective coating for gravitational wave detection

Abstract

We have measured the mechanical loss of a dielectric multilayer reflective coating (ion-beam sputtered SiO2 and Ta2O5) in cooled mirrors. The loss was nearly independent of the temperature (4 K ∼ 300 K), frequency, optical loss, and stress caused by the coating, and the details of the manufacturing processes. The loss angle was (4 ∼ 6) × 10 −4 . The temperature independence of this loss implies that the amplitude of the coating thermal noise, which is a severe limit in any precise measurement, is proportional to the square root of the temperature. Sapphire mirrors at 20 K satisfy the requirement concerning the thermal noise of even future interferometric gravitational wave detector projects on the ground, for example, LCGT.

Dislocation nucleation in bcc Ta single crystals studied by nanoindentation

“…The appearance of slip lines requires that the native Ta oxide film supposes no barrier for dislocations and fails via shear at slip edges [42]. Consistent with our observation, the 1-nm-thick native Ta oxide film is not expected to be a barrier for dislocations due to its thinness and the fact that the Young's modulus of Ta oxide (140 GPa) [43] is lower than that of Ta(189 GPa).…”

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.…”

“…For isotropic materials with a Poisson's ratio much lower than unity, it can be shown that the ratio between the energies can be simplified reduced to 3tcYc tsYs , where Y i denote the Young's modulus of element i, and t i its thickness [23,31]. Quartz is only slightly anisotropic, so this simplified model remains relevant.…”

“…Quartz is only slightly anisotropic, so this simplified model remains relevant. Furthermore, in the operating conditions, Young's modulus of a thin twolayer coating Y c can be simply calculated as an average of Young's modulus of both deposited materials, weighted by their respective thicknesses t 1 and t 2 [22,23]:…”

“…Given these values, the weighted average provides a Young's modulus of coating used in our experiments on both surfaces of the quartz substrates estimated by Y CrAu = 81 GPa. Similarly, in the case of the multi-layer dielectric coatings, The Young's modulus is estimated Y DC = 100 GPa, with Y Ta2O5 = 140 GPa and Y SiO2 = 72 GPa [23,37,38]. Finally, Young's modulus of the SC-cut quartz crystal substrate can be calculated from two the elastic coeffi-…”