Sensitivity curves beyond the Advanced detectors 3

Mirror coatings thermal noise: setting the goal for ET aLIGO ET-HF ET-LF Mirrors (Coatings) 1/5.5 from T 1/1.7 from beam diameter 1/√x from j Reduction: times @ 300 K times @ 10 K 2.8 8.2 For the ET requirements: x > 2.7 @ 300K x > 0.8 @ 10k The 1.7 factor may increase further if the LG33 mode replaces the TEM00 in the cavities

Parameters for the different generations of GWD 300 K 10 K 20 K

Strategies to reduce the losses in coatings Optimum annealing Blending of oxides (TiO2, Ta2O5, SiO2, HfO2, ZrO2) Nano-layering Oxygen free amorphs (silicon... But absorption is high) Structural investigation: searching for the relaxation mechanisms and a way to deactivate them

Structural investigation tools - 1 Atomic distribution, small and medium range order X-ray and electron diffraction (length scale up to ~1 nm) Extended x-ray Absorption Fine Structure (EXAFS) (length scale up to ~1 nm) Scanning diffraction using 2 nm x-ray beam size (length scale 0.5 nm to 3 nm) Structure dynamics Raman scattering spectrometry: [0.1, 5] THz (vibration of structures with dimensions in the range [0.1, 1] nm) Brillouin scattering spectrometry: [5, 100] GHz (medium range order)

Structural investigation tools - 2 Chemical state and stoichiometry X-ray Photoelectron spectroscopy (XPS) Nuclear Magnetic Resonance (NMR) Rutherford Back Scattering (RBS) Modelling So far the simulation is the only way to “see” the relaxing structures Prediction of mechanical losses are based on barrier height distribution g(V) and on the deformation potential g. Structural data can be used to check the simulation

Open questions (not an exhaustive list!) Does annealing increase or reduce losses? And does it at any temperature? What are the relaxing structures in the different optical coatings? Why does fused silica have a very deep minimum of losses at room temperature? And why the other oxides don’t? How close to reality are the barrier heights distributions simulated by molecular dynamics?