Towards a silicon monolithic suspension

The thermal noise of the main optics is a fundamental limit to the sensitivity of present and future gravitational waves interferometers. To reduce it, the next generation of interferometers will adopt larger mirrors and cryogenic solutions. To reach this goal, it is important to identify suitable materials for substrates and suspensions. That means not only materials with good thermal, mechanical and optical properties, but also materials that allow extended substrates and thin, long and strong fibers. Sapphire is a good candidate and it is already used in the KAGRA experiment, but it is hard to work and polish and is very expensive. Moreover, the production of substrates larger than 20 cm in diameter presents many difficulties. A very promising alternative candidate is silicon. With present work, we report the status of the R&D project on silicon monolithic suspension, the results obtained so far on bers produced with three methods and the strategies to improve them. We also present a protocol to characterize silicon bulks under cryogenic conditions before installation and the control system to reduce the seismic noise on the suspended substrates.