Overview of Advanced LIGO Coating Status Sheila Rowan University of Glasgow for the Core Optics Working Group Plenary technical session LSC meeting, Livingston, LA 17th March 2004

Coating Development Specifications for Test Masses Parameter Sapphire goal Sapphire requirement Fused Silica goal Fused Silica requirement Mechanical loss 2 x 10-5 6 x 10-5 1 x 10-5 3 x 10-5 Optical Absorption 0.5 ppm 1 ppm 0.2 ppm Thermal expansion 5 x 10-6/K < 2 x 10-5/K >1 x 10-6/K 5 x 10-7/K < 2 x 10-6/K >1 x 10-7/K Birefringence 1 x 10-4 rad 2 x 10-4 rad - Scatter5 2 ppm Thickness uniformity 10-3 (over 21.5 cm diameter) 10-2 (over 33.0 cm diameter) 10-2 (over 30.0 cm diameter) ITM HR transmission 5 x 10-3 ±2.5 x 10-4 ETM HR transmission 5 ppm 10 ppm Test Mass HR matching 1 x 10-2 AR reflectivity 200 ±20 ppm

Adv LIGO Coating Requirements Mechanical loss Fused silica : f < 3 x 10-5 (goal 1 x 10-5) Sapphire: f < 6 x 10-5 ( goal 2 x 10-5 ) These numbers are guides, thermal noise will depend on other parameters also. Source of requirements on all parameters influencing thermal noise: Brownian thermal noise (Nakagawa/Gretarsson) Thermoelastic noise (Braginsky/Fejer) AdvLIGO sensitivity modeling with BENCH Optical absorption Fused silica: 0.5 ppm (goal 0.2 ppm) Sapphire: 1 ppm (goal 0.5 ppm)

Results for Initial LIGO Silica/Tantala Coating Silica substrates were coated with layers of alternating silica and tantala, similar to the initial LIGO coating Q factors were measured by exciting resonances in the samples and recording the subsequent decay Fused silica substrates of 2 different aspect ratios used: ‘Thick’ samples (3” dia. x 1” thick): c=(2.8 ± 0.7) x 10-4 ‘Thin’ samples (3” dia. x 0.100” thick): butterfly = 2.7 x 10-4 drumhead = 3.1 x 10-4

Reminder of Previous Results Measurements on several SiO2/Ta2O5 coatings with different amounts of layers (2 to 60) and with various layer thickness in different combinations (λ/4 - λ/4; λ/8 - 3λ/8; 3λ/8 - λ/8) Concluded that: Substrate / coating interface is not a significant source of loss. Coating layer interfaces are not a dominant source of loss Ta2O5 has a higher mechanical loss than SiO2 fsilica = (4±3)x10-5 + f (2.7±0.9)x10-9 ftantala = (4.2±0.4)x10-4 + f (4±9)x10-10

Status of experiments Measured mechanical loss (and optical absorption) of a number of alternate coatings: eg: Al203/Ta2O5 SiO2/Al203 Nb2O5/ SiO2 Mechanical loss results are broadly similar to SiO2/Ta2O5 – no significant benefits from using coatings listed above. Most promising results to date: Ti-Doped (3%) Ta2O5 / SiO2 Reduction in coating loss: c= (1.8 +/- 0.1) x 10-4 c= (1.5 +/- 0.7) x 10-4

Program Overview Driver is development of low mechanical loss coating Optical and thermo-mechanical properties will be studied during development, (vendors, Caltech, Stanford), but will not drive program until mechanical loss is better understood and/or a low mechanical loss coating is developed Coating vendors selected: LMA/Virgo in Lyon, France CSIRO in Sydney, Australia Next phase of coating development has begun

Current results and immediate coating development plans CSIRO: First ‘calibration’ coating studies of SiO2/Ta2O5 coating from CSIRO: fc = (4.4 +/- 0.6) x 10-4 fbutterfly = (4.1 +/- 0.1) x 10-4 fdrumhead = (5.0 +/- 0.1) x 10-4 ‘secondary ion-bombardment’ used in deposition process (Nb: coating appeared to show variations in thickness across surface) LMA/VIRGO: Currently working on: a) depositing single layers of coating materials for study of mechanical loss and thermo-mechanical properties b) depositing doped SiO2/Ta2O5 coating on sapphire substrate

Coating development plans Dopants Continue with studies of effects of doping on mechanical loss SiO2/Ta2O5 doped with 3% Ti showed a reduction in mechanical loss without sacrificing n, Y, or optical absorption. Alternate high index materials HfO2/SiO2 sample produced – mechanical loss to be measured shortly Annealing Systematic studies of effects of annealing on coating mechanical loss Ion bombardment of substrate during coating Controls density and stress of coatings – investigate effect on mechanical loss

R & D Milestones Start Coating Development January 2004 Substrate Material Downselect June 2004 Coating Material Downselect June 2005 LASTI’s ETM Delivered January 2006