Optical properties of high-quality oxide coating materials used in gravitational-wave advanced detectors

High-reflection interference mirrors for current gravitational wave detectors (aLIGO, Advanced Virgo, KAGRA) are made of high-quality oxide multi-layers deposited by ion beam sputtering (IBS) at the Laboratoire des Mat\ue9riaux Avanc\ue9s (LMA). For this task, LMA uses a large IBS custom-made machine (the grand coater GC) able to deposit very uniform coatings over very large surfaces, with diameter of some tens of cm. We report for the first time about the optical characterization by spectroscopic ellipsometry of oxide coatings deposited by the GC under strictly the same conditions used for the production of interference mirrors. We have investigated oxide materials like silica (SiO2), tantala (Ta2O5) and titania-doped tantala (Ti:Ta2O5), providing for each material a broad-band (190\u20131700 nm) accurate determination of the complex index of refraction, with particular attention to wavelengths used in interferometers. Particular focus has been dedicated to the influence of Ti-doping on tantala coating. The doping induces a red-shift of the optical gap and an increase of the NIR refractive index. Furthermore, doping induces a decrease of the so-called Urbach energy, consistent with the well-known reduction of the internal friction in these kind of systems

High-reflection coatings for gravitational-wave detectors: State of the art and future developments

We report on the optical, mechanical and structural characterization of the sputtered coating materials of Advanced LIGO, Advanced Virgo and KAGRA gravitational-waves detectors. We present the latest results of our research program aiming at decreasing coating thermal noise through doping, optimization of deposition parameters and post-deposition annealing. Finally, we propose sputtered Si3N4as a candidate material for the mirrors of future detectors

Dichroic and anti-reflective coatings for astronomical instrumentation

International audienceDue to the increase of astronomical projects and of their instruments, the request of large optics with higher optical performances does not stop growing. An important step in the manufacture of these optics is the deposition of high-precision optical coatings. To answer to this request we developed coatings working at different angles of incidence and spectral ranges on large surface: - anti-reflective coatings for large lenses with strong curvatures, - dichroic coatings with sharp transition for large optics. Main results will be presented on the basis of several examples of realization. © (2018) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only

High uniformity IBS coatings for the world’s largest Fabry-Perot etalon of the VTF instrument

International audienceThe first large Fabry-Perot etalon (Ø35 cm) of the VTF instrument was coated successfully using IBS technique. The High Reflective (HR) coatings need to meet the reflectivity specifications (95 +/- 1%) over the entire wavelength range 520-870 nm and the entire aperture (Ø25 cm) and also preserve the plate's flatness and airgap uniformity between the two platesto be better than 3 nm RMS. The change of the figure error of the individual faces after HR coating was exceptionally small: For plate 1 (upper) it changed from 1.7nm RMS before coating to 2.12 nm after coating, no change at all for plate 2 (lowe

Photometric calibration of an in situ broadband optical thickness monitoring of thin films in a large vacuum chamber

International audienceTo improve the in situ monitoring of thin films at the Laboratoire des Matériaux Avancés, a broadband optical monitoring of the coated thin films was developed and installed in the biggest ion-beam sputtering machine in the world. Due to the configuration of the coating machine and the chamber strain under vacuum, a standard calibration procedure is impossible and a double-beam optical system is not suitable. A novel theoretical and practical solution to calibrate the measurements was found and is described in this paper. Some relevant results achieved thanks to this technique are discussed as well

Large and extremely low loss: the unique challenges of gravitational wave mirrors

International audienceThis paper describes the making of large mirrors for laser interferometer gravitational wave detectors. These optics, working in the near infrared, are among the best optics ever created and played a crucial role in the first direct detection of gravitational waves from black holes or neutron star fusions