Mirrors Coating

Key challenges in a nutshell

  • 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)

Short description of the technology

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

State of the Art: technology in existing gravitational wave detectors / TRL

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. 

Intended use in the frame of the Einstein Telescope

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.

Improvements needed: Technological challenge for the Einstein Telescope


○ 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)


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

Economic perspectives of participation beyond the ET applications

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

Related projects and labs


Ongoing and future procurements


Technology contact

Jean-Pierre Locquet
KU Leuven – jeanpierre.locquet@kuleuven.be

Business Development contacts

Peter Gier
AGIT DE – p.gier@agit.de

Dirk Kalinowski
NMWP DE – dirk.kalinowski@nmwp.de

Michel Stassart
Skywin BE – michel.stassart@skywin.be

Maxime Corvilain
POM Limburg BE – maxime.corvilain@pomlimburg.be

René Kessen
LIOF NL – rene.kessen@liof.nl