Mirrors Coating

• Develop crystalline oxide mirror coatings
• Highly uniform coatings (~ 0.1%) over large areas such as on 300mm wafers
• Explore layer transfer and wafer bonding processes
• 4 ✕ reduction in coating loss compared to SiO2 and TiO2:Ta2O5 in Adv. LIGO/Virgo (for 12 cm beam)

Highly-reflective coatings are obtained by stacking of many layers, usually using two materials with different refractive index n

➢ optical layer thickness: quarter of a wavelength (= higher n → thinner layer)

Example: amorphous SiO₂ (n=1.45) and Ta₂O₅ (n=2.05)

➢ ~38 layers needed for highly reflective ETMs of R = 99.9995%

➢ light intensity reduces with every layer pair

Coating thermal noise (CTN)is alimiting noise source and needs to be reduced. It is:

• frequency dependent (high at low frequency)
• lower for larger beams
• temperature dependent
• determined by elastic properties / mechanical loss factor (coating and substrate)
• research aims to improve SiO₂/Ta₂O₅ through doping, nanolayers, replacing Ta₂O₅ by other materials like GeO₂, amorphous Si, etc. 

To improve the telescope performance, two approaches are followed in parallel for the mirrors. First, the diameter (d) of the mirror bodies is increased since noise scales with . The second path is to significantly improve the CTN by lowering the temperature of operation as well as to significantly lower (~ factor of 10) the mechanical losses of the coatings.

ET-HF:

○ Same wavelength, substrate material and temperature as LIGO and Virgo detectors

○ Challenge: Upscaling coating diameter by almost a factor of two (bubble- and defect-free)

ET-LF:

○ Active research areas within extended EMR region: nanolayers, multi-material coatings and crystalline coatings

Proposals for joint ET-relevant R&D activities / new business applications: To be filled in on the base of the companies’ proposals

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