LIGO Document G1700998-v1

Optical properties of silicon for cryogenic interferferometric gravitational wave detectors

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The next generation of interferometric gravitational wave detectors (GWDs), like Cosmic Explorer or Einstein Telescope, will likely be cryogenic in order to minimise the displacement thermal noise of the arm cavity mirrors. Fused silica core optics, like the ones currently employed at room temperature, would not be suitable in a cryogenic environment: silica exhibits too high a mechanical loss at cryogenic temperatures, that would counteract any gains in thermal noise. Silicon shows an excellent mechanical loss at low temperatures both in its crystalline form (c-Si) and as an amorphous film (a-Si). Silicon optics, with c-Si test masses and a-Si films employed in coatings, offer a promising cryogenic-compatible alternative to silica, albeit at the price of moving the light source from 1064 nm to a wavelength where silicon is transparent, like 1550 nm or 2000 nm. Basic R&D is still needed to understand the optical properties of silicon at the level required for GWDs. We report on the optical absorption of magnetic Czochralski c-Si and ion-beam sputtered a-Si films. We investigate how absorption correlates with deposition parameters and other physical properties of the films. We also report on bulk scattering of magnetic Czochralski c-Si, potentially qualifying this material as suitable substrate for optics that transmit the detector's main beam.
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Amaldi 12 held on 09 Jul 2017 in Hilton Hotel, Pasadena, CA

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