New coatings on new substrates for low mechanical loss mirrors

1. New coatings on new substrates for low mechanical loss mirrors 1. Mechanical loss measurement bench 2. New high index coating materials 3. New substrates Task M4

2. New coatings on new substrates for low mechanical loss mirrors • Mirror thermal noise is predicted to be the limiting noise in the most sensitive band around 100 Hz • Much of this thermal noise comes from the optical coatings of the interferometer mirrors • The thermal noise is caused by mechanical loss in the system

3. 1. Mechanical loss measurement bench • 3 years ago, no metrology at LYON, R&D on low mechanical coatings was very slow • Delivery of the bench in April 2004 thanks to a collaboration with INFN Perugia • Quick and low cost measurement for the search of new coatings at LMA • Measurement at low frequency around 100 Hz • Freely decaying resonant motion of the membrane • HeNe laser reflected by the membrane

4. 1. Mechanical loss measurement bench : mechanical loss : resonant frequency

5. 1. Mechanical loss measurement bench Thin membranes of silica (Suprasil 311) 45 mm x 5 mm x 110 µm with

6. 2. New high index coating materials First generation Virgo mirrors (stack of SiO2 / Ta2O5): • The high index material is highly responsible for the mirror mechanical losses • Search for a new low mechanical loss, high index coating material

7. 2. New high index coating materials   Thickness is 500 nm Before annealing It is not easy to lower the mechanical loss without increasing the absorption A low stressed coating is not necessarely a low mechanical loss coating

8. 2. New high index coating materials  With Ta2o5 : Ti Absorption is unchanged : 0.5 ppm Refractive index is high : 2.1 Our best high index coating material : Advanced LIGO requirement : Advanced LIGO goal : Optimization of the Ti concentration Mirrors SiO2/Ta2O5:Ti ** on 3 inch diameter substrates sent to LIGO for mechanical loss measurement Mirrors SiO2/Ta2O5:Ti ** (standard and optimized) on membranes for LMA mechanical loss measurement Search for other binary oxydes, ternary compounds

9. 3. New substrates • silicon membranes from Glasgow • 500 nm Ta2O5:Ti « formula 5 » coating • Good adhesion if membrane previously annealed

10. 3. New substrates 68 mm x 10 mm x 100 µm Si membrane • Mechanical loss measurements made at LMA • To be compared with Glasgow measurements

11. 3. New substrates • 3" diameter 1“ thick  silicon substrate • (HB)15 deposited on it after annealing • H: Ta2O5:Ti « formula 5 » • B: SiO2 • Good adhesion of the mirror

12. 3. New substrates • 3" diameter 1.7 mm thick  silica substrate • etched diffraction grating • (HB)15 deposited on it • H: Ta2O5:Ti « formula 5 » • B: SiO2 • Good adhesion of the mirror •  not measurable because of chipping • coating will be made again soon

13. Conclusion • LMA has developed new high index coating materials, enabling a  reduction of the mechanical losses by a factor of 1.5 with good optical performances. • The research for new coating materials has been made easier thanks to a mechanical loss measurement bench developed in the laboratory. This research is still going on with Advanced Virgo and Advanced LIGO R&D. • Coatings have been successfully deposited on a new substrate, silicon, which has interesting properties at low temperature. • A silica substrate, with an etched diffraction grating on it, has also been coated with a low mechanical loss mirror.