Silica as a key material for advanced gravitational wave detectors E. Cesarini a,b, ⁎, M. Lorenzini b , G. Cagnoli b,c , F. Martelli a,b , F. Piergiovanni a,b , F. Vetrano a,b a Università di Urbino, via S. Chiara 27, 61029 Urbino, Italy b INFN, Istituto Nazionale di Fisica Nucleare, Sez. di Firenze via G. Sansone 1, 50019 Sesto Fiorentino (FI), Italy c ITIS L. e A. Franchetti, Piazza San Francesco 1, 06012 Città di Castello, Italy abstract article info Article history: Received 18 June 2010 Received in revised form 21 October 2010 Available online 24 February 2011 Keywords: Gravitational wave detectors; Thermal noise; Loss angle; Fused silica; Multi-layer coating The thermal noise coupling with the displacement of the mirrors of a gravitational wave interferometric detector is a limit to its sensitivity in a range of frequencies from about 10 Hz to few hundreds of Hz; fused silica proved to be a very suitable material to reduce this source of noise for the first generation of detectors. The future advanced detectors are planning to make use of fused silica mirrors and suspending elements in a monolithic arrangement: in these conditions, the main contribution to the thermal noise will come from the amorphous multilayered coating deposited on the mirrors. This paper is focused on the multiple advantages provided by the use of fused silica material in a present day interferometric detector; a new suspension for measurements of the acoustic attenuation in fused silica is presented together with the loss angle dependance on frequency and aspect ratio. Furthermore we report on the status of the art of the research activity on fused silica wires production and characterization and coating thermal noise accomplished by the Firenze–Urbino Virgo group. © 2011 Elsevier B.V. All rights reserved. 1. Introduction The detection of GW is one of the most challenging objectives of the present day experimental physics. The prediction of the existence of space–time waves emitted by quadrupole deformation of gravity fields comes from the Einstein's general relativity, and in the 1970s an indirect proof of the generation of such waves from a spinning pair of compact stars was provided by astronomical observation done by Hulse and Taylor [1]. Very compact and rapidly evolving objects of astrophysical origin can emit a huge amount of energy via gravitational radiation, allowing a detection of their emission on Earth. Nonetheless, directly detecting such waves is an extremely difficult task, due to their intrinsic weakness. A typical distance variation induced by a gravitational wave reaching the Earth is ΔL/ L ∼ 10 −21 . Many efforts have been done to directly detect GW and data have been taken until now by a first generation of ground-based interferometers. These interferometric detectors use the laser time of flight variation between two suspended mirrors to track the metric change induced by a passing gravitational wave. Presently a network of ground-based interferometers, Virgo (Italy–France) [2], LIGO (USA) [3] and GEO600 (Germany–UK) [4] is fully operative; while no direct detection happened until now, several upper limits have been fixed [5]. In order to upgrade the sensitivity of the detectors, one has to investigate all the noise contributions to the interferometers sensitivity, trying to find suitable techniques to reduce them further. In the middle frequency range (10–200 Hz), the limiting factor is the thermal noise due to thermal excitation of mechanical degrees of freedom of the test masses (mirrors) and their suspension system, that results in a spurious contribution, like a displacement noise to the output of the detectors. 2. The thermal noise limit According to the Fluctuation–Dissipation theorem [6], in a linear and dissipative system, thermally activated equilibrium fluctuations are determined by the mechanical dissipative characteristics, so that the fluctuation power spectral density is related to the mechanical impedance Z: S 2 ω ð Þ = 4k B T ω 2 R 1 Z ω ð Þ   ð1Þ Each degree of freedom of a homogeneous body can be regarded as a simple harmonic oscillator, thanks to the harmonic decomposition in normal modes, the fluctuation spectrum of one dimensional harmonic oscillator with losses is: S 2 x ω ð Þ = 4k B T mω ϕω ð Þω 2 0 ω 2 −ω 2 0   2 + ϕω ð Þ 2 ω 4 0 ð2Þ where m is the equivalent mass of the harmonic oscillator, ω 0 is the resonance frequency and the quantity ϕ(ω), called loss angle, is the key feature in modeling the dissipation in a linear system. The loss angle at the resonance frequency is proportional to the ratio among Journal of Non-Crystalline Solids 357 (2011) 2005–2009 ⁎ Corresponding author at: Università di Urbino, via S. Chiara 27, 61029 Urbino, Italy. Tel.: +39 055 4572598; fax: +39 055 4572676. E-mail address: elisabetta.cesarini@uniurb.it (E. Cesarini). 0022-3093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.12.067 Contents lists available at ScienceDirect Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol