An Overview of Research into Low Internal Friction Optical Coatings by the Gravitational Wave Detection Community

Abstract

The direct detection of gravitational waves by ground-based interferometric gravitational wave detectors in recent years has opened a new window of the universe, allowing the astrophysical observations of previously unexplored phenomena, such as the collisions of black holes and neutron stars. However, small thermodynamic fluctuations of the density of the thin films that compose the mirrors used within the gravitational wave detectors, such as the LIGO and Virgo detectors, give rise to noise which limits these instruments at their most sensitive frequencies. This "Brownian Thermal Noise" can be related to the inherent internal friction of the mirror materials through the fluctuation-dissipation theorem. Therefore, the improved sensitivity of gravitational wave detectors depends, to some extent, upon the development of optical thin films with low internal friction. The past two decades have therefore seen the growth of internal friction experiments undertaken within the gravitational wave detection community. This article attempts to summarize the results of these investigations and to highlight current research directions in order to foster a stronger dialogue with the larger internal friction and mechanical spectroscopy community.

Optimal Design of Coatings for Mirrors of Gravitational Wave Detectors: Analytic Turbo Solution via Herpin Equivalent Layers

“…Thermal noise in gravitational detectors is the most important limitation for their operation. We refer the reader to the works in [13][14][15] for an exhaustive description of the problem and the proposed solutions [16,17] in the operating observatory Virgo [1] and LIGO [2].…”

Effects of the annealing of amorphous Ta2O5 coatings produced by ion beam sputtering concerning the effusion of argon and the chemical composition

“…Actually, multilayers reflecting coatings and photonic crystals are conceived and realized combining films of different refractive index in stacked sequences. This possibility has been largely explored for the construction of the coatings of the most demanding optical components ever realized: the mirrors of the gravitational-wave (GW) detectors [8][9][10][11][12][13]. At the present state, the mirrors of the Advanced Virgo and Advanced LIGO detectors are large masses of silica (35 cm diameter, 40 kg mass) with a high reflecting coating composed of alternate layers of amorphous SiO 2 and Ta 2 O 5 [8,14,15].…”

On the performance limits of coatings for gravitational wave detectors made of alternating layers of two materials

“…Increasing the visibility distance of the operating GW detectors is a needed step to deploy the full potential of MMA. Thermal (Brownian) fluctuations in the high-reflectance (HR) coatings of the test-masses is presently the dominant noise source in interferometric GW detectors [5] setting their ultimate visibility distance in the (40-300) Hz band, where all recent detections have been made. Notably, efforts to reach and beat the quantum noise limit will be meaningful only after a significant reduction of coating thermal noise is achieved.…”