Abstract
The announcement of the direct detection of gravitational waves (GW) by the LIGO and Virgo collaboration in February 2016 has removed any uncertainty around the possibility of GW astronomy. It has demonstrated that future detectors with sensitivities ten times greater than the Advanced LIGO detectors would see thousands of events per year. Many proposals for such future interferometric GW detectors assume the use of silicon test masses. Silicon has low mechanical loss at low temperatures, which leads to low displacement noise for a suspended interferometer mirror. In addition to the low mechanical loss, it is a requirement that the test masses have a low optical loss. Measurements at 1550 nm have indicated that material with a low enough bulk absorption is available; however there have been suggestions that this low absorption material has a surface absorption of >100 ppm which could preclude its use in future cryogenic detectors. We show in this paper that this surface loss is not intrinsic but is likely to be a result of particular polishing techniques and can be removed or avoided by the correct polishing procedure. This is an important step towards high gravitational wave detection rates in silicon based instruments.
Highlights
The direct detection of gravitational waves (GW) by the two LIGO detectors on the 14 September 2015 [1] opened the new era of GW astronomy
Recent work [10] has shown that the room temperature bulk optical absorption at 1550 nm of some commercially available material is at a level suitable for its use as a mirror substrate in a GW detector, and further investigations [11] indicate that the cryogenic absorption is similar
The work in this paper shows that reports of excess surface absorption in silicon are not due to an intrinsic absorption of the material
Summary
The direct detection of GW by the two LIGO detectors on the 14 September 2015 [1] opened the new era of GW astronomy. We choose these temperatures as they are the intended operating temperatures for a proposed upgrade to the aLIGO interferometers [12], known as Voyager, and for the low frequency part of the Einstein telescope (ET LF) detector [4], a European proposal for a 3rd generation detector Both these designs use silicon mirror substrates, cryogenically cooled, as test masses. The Voyager design will use radiative cooling to remove excess heat from the test mass, whereas the ET LF design looks to use conduction through the suspension elements. In the Voyager design it is envisioned that there will be a laser power of several kilowatts in the substrate of the input test mass This means a surface absorption of order 100 ppm would dump close to one watt from the laser into the test mass.
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