Reducing the suspension thermal noise of advanced gravitational wave detectors

The international network of gravitational wave detectors is currently undergoing sensitivity upgrades (aLIGO, aVIRGO and GEO-HF) which will lead to the first detection and subsequent observation of a rich variety of astrophysical sources. To obtain a factor of 10 improvement in the strain sensitivity at low frequencies requires the use of ultralow mechanical loss materials and monolithic fused silica suspensions, optimized mirror coatings and the development of cutting edge techniques to super-polish and figure the interferometer optics. The possibility of applying incremental upgrades to the second generation detectors can be realized by making small but significant changes to the suspensions and/or optical mirror coatings. This includes the use of longer suspensions to increase the dissipation dilution, the development of techniques to reduce the surface loss in fused silica suspensions and methods to lower the mechanical loss from the metal springs used to support the test mass. Such upgrades can potentially improve the strain sensitivity by a factor of 2.5. Looking beyond 2015, the development of techniques to further improve the sensitivity by one order of magnitude are discussed. The third generation detectors will be located underground and will be operated at cryogenic temperatures. At low temperatures, silicon is a particularly promising candidate material as it displays good thermal conductivity, high tensile strength and zero thermal expansion coefficient at 120 K, 18 K and T -> 0 K.