Crystalline mirrors for gravitational waves detectors – MICRONG

The first observation of gravitational waves from coalescing binary systems formed by binary black holes and neutron stars opened the field of gravitational astronomy. Further events and new sources of gravitational waves will be detected with the improvements of Virgo and LIGO and the beginning of the operation of KAGRA in Japan. In parallel, the next generation of facilities dedicated to gravitational wave astronomy are being prepared in Europe and the United States. They will provide a unique tool to probe the laws of fundamental physics and cosmology. Current and future detectors are limited by the thermal noise affecting the interferometer mirrors. The origin of this fundamental noise lies in the optical coatings used to obtain the high reflectivity and low optical losses required. Today, these high quality optical coatings with a large surface (330 mm) are based on amorphous materials. In this project, we propose to develop new coatings based on single crystal AlGaAs multilayers. These materials, in addition to being of very good optical quality, have a thermal noise three times lower than the materials used for the Virgo and LIGO mirrors. Their application to large mirrors used in gravitational wave detectors requires further improvement of their optical quality and size. To achieve this goal, this project brings together the expertise in gravitational wave detectors and low-noise optics of two of the CNRS laboratories participating in the Virgo experiment with the expertise in III-V materials of CEA-LETI, the company RIBER, and III-V Lab, a private R&D organization jointly created by Nokia, Thales and CEA. The final objective will be to demonstrate the ability to produce mirrors based on crystalline coatings of 200 mm diameter and to prepare the next step for 300 mm mirrors. The growth on 200 mm diameter AlGaAs crystalline coatings with very low thermal noise will be developed by III-V Lab, with the support of RIBER which will contribute with new tools for monitoring the growth process developed in collaboration with CNRS. The growth will be performed on GaAs substrates as well as on Germanium substrates. The transfer of AlGaAs multilayers on silica, sapphire or silicon optical substrates will be handled by CEA-LETI. The optical characterizations of the final mirrors will be performed at CNRS/IP2I-LMA, the laboratory that hosts the characterization equipment developed for the Virgo project. Finally, the measurement of the thermal noise of thin films will be developed at CNRS/LAPP, the laboratory that built the detection system for the Virgo interferometer. The project brings together some of the best expertise available in France to achieve the goal of making the first 200 mm AlGaAs mirror and pave the way for the realization of mirrors of this type for gravitational wave detectors. This is a truly interdisciplinary project involving teams from different fields of physics and engineering, from the public and private sectors, which are necessary for the success of the project. This consortium also guarantees a wide exploitation of the technologies developed and the results obtained in other fields such as the development and mass production of advanced GaAs-based devices for communications, 3D sensors or LIDARs.