1 Cascina – October 19, 2011 ASPERA Forum Laurent Pinard Substrates, Polishing, Coatings and Metrology for the 2 nd generation of GW detector Laurent PINARD Laboratoire des Matériaux Avancés – Lyon - FRANCE

2 Cascina – October 19, 2011 ASPERA Forum Laurent Pinard  Introduction – The Virgo mirrors  Advanced Virgo : New mirrors  Substrates : new type of fused silica  Polishing : Crucial point = Flatness (Round trip losses) Cavity simulations, Spec definition, Foreseen polishing solutions (Corrective coating, Ion beam polishing)  Metrology : Absorption, Defects, Scattering, Flatness  Coating : Thickness uniformity, Thermal noise  Scenario for the 3 rd generation of GW detector (ET) Overview

3 Cascina – October 19, 2011 ASPERA Forum Laurent Pinard Introduction : The Virgo mirrors Mirrors : 35 cm diameter, 10 cm thick 20 kg LMA : in charge of the mirror coating

4 Cascina – October 19, 2011 ASPERA Forum Laurent Pinard  Microroughness : < 1 Å rms  ROC : /- 100 m  Few point defects  Flatness : < 8 nm RMS on  150 mm achieved : # 3-4 nm RMS Silica substrate polishing ( Gooch and Housego, ex General Optics (US)) Introduction : The Virgo mirrors

5 Cascina – October 19, 2011 ASPERA Forum Laurent Pinard Coating deposited by Ion Beam Sputtering Introduction : The Virgo mirrors

6 Cascina – October 19, 2011 ASPERA Forum Laurent Pinard Advanced Virgo  LIGO/VIRGO allow verification of the upper limit predictions  Event rate too small  Detector improvement needed Sensitivity x 10 = rate x 1000 Advanced Virgo approved in Dec 2009  Main impovement: - High power laser (200 W) - New optical configuration - Heavier mirrors (40 kg) - Monolithic suspension LMA : Mirror responsible ~ 5 M€ investment (25% of the total budget)

7 Cascina – October 19, 2011 ASPERA Forum Laurent Pinard Advanced Virgo : The Mirrors  In Virgo : Round-Trip Losses in the cavity = ppm Main Origin : Flatness defects having a period of 1 cm or more  For Advanced Virgo : Round-Trip Losses = 75 ppm (specif.) 25 ppm : Abs+Diff+T 50 ppm : Flatness defects specification very severe Substrate :  Low absorption Silica (Suprasil Heraeus)  Diameter = 35 cm,  Thickness = 20 cm, Weight = 40 kg  Unit Cost 130 k€ (without polishing) AdV IM

8 Cascina – October 19, 2011 ASPERA Forum Laurent Pinard Advanced Virgo : Mirror polishing spec. Goal : Define a spec. on Flatness for the IM and EM (losses 50 ppm) Measured maps at LMA or by the polisher PSD shape depends on the polisher ( f -n, n  [0;2]) Artificial maps generated with the PSD obtained, with different RMS flatness values Cavity simulation PSD(1D) Extraction

9 Cascina – October 19, 2011 ASPERA Forum Laurent Pinard ROC = 1420 m ROC = 1683 m 0.5 nm RMS = spécification Advanced Virgo : Mirror polishing spec.

10 Cascina – October 19, 2011 ASPERA Forum Laurent Pinard Advanced Virgo : Polishing - Solutions 2 solutions identified « Classical Polishing » (best as possible) + Corrective Coating (LMA) AdV Baseline Microroughness preserved « Classical Polishing » + Ion Beam Figuring LIGO solution Drawbacks : More expensive (factor 2-3) Microroughness # 1.5 Å rms

11 Cascina – October 19, 2011 ASPERA Forum Laurent Pinard Substrate in translation Interferometer Ion Source Robot mask Silica Target Sputtered Atoms Corrective Coating Developed in 2005/2006 at LMA Use of the IBS deposition chamber developed for Virgo in 2000 Add material (silica) to suppress holes and aim at a «perfect» plane Advanced Virgo : Polishing - Solutions

12 Cascina – October 19, 2011 ASPERA Forum Laurent Pinard Substrate  156 mm VIRGO type Before correction (  120 mm) 3.3 nm R.M.S. 16 nm P.V. After correction (  120 mm) 0.98 nm R.M.S. 10 nm P.V. Microroughness preserved (0,5 Å RMS) Defect at the center linked to the robot (mechanics) Corrective Coating Advanced Virgo : Polishing - Solutions

13 Cascina – October 19, 2011 ASPERA Forum Laurent Pinard New robot developed Corrective Coating Advanced Virgo : Polishing - Solutions

14 Cascina – October 19, 2011 ASPERA Forum Laurent Pinard  CC effect on the PSD : Flatten the spectrum in the low frequency domain up to a cutoff frequency fc fc Corrective Coating Advanced Virgo : Polishing - Solutions

15 Cascina – October 19, 2011 ASPERA Forum Laurent Pinard  Cutoff frequency : 50 m -1 (correction of defects with a period  2 cm), reasonably achievable with the CC  To reach 0.5 nm rms after CC, we can not start from any flatness  Simulations (1000) done with artificial maps obtained with the PSD for different RMS flatness (4 nm like Virgo, 1.5 nm) Corrective Coating Advanced Virgo : Polishing - Solutions

16 Cascina – October 19, 2011 ASPERA Forum Laurent Pinard 1% of simulations give losses < 50 ppm CC not sufficiently powerful Corrective Coating Advanced Virgo : Polishing - Solutions

17 Cascina – October 19, 2011 ASPERA Forum Laurent Pinard 96% of simulations give losses < 50 ppm Conclusion : Before CC, flatness at least lower than 1.5 nm rms, Obtained by « classical » polishing : challenge Other constraints for the polishers : - accuracy on the ROC (+/- 10m) - points defects Corrective Coating Advanced Virgo : Polishing - Solutions

18 Cascina – October 19, 2011 ASPERA Forum Laurent Pinard Ion Beam Figuring  Always a “classical” polishing phase  ROC et flatness corrected with a small ion beam Opposite to the Corrective Coating : Remove material (silica) instead of adding material. Advanced Virgo : Polishing - Solutions

19 Cascina – October 19, 2011 ASPERA Forum Laurent Pinard LIGO Substrate : 0.21 nm RMS - 2 nm PV (150 mm diam.) ROC accuracy +/- 10 m on 2 km Ion Beam Figuring Advanced Virgo : Polishing - Solutions Polisher : CoastLine Optics +ASML (US)

20 Cascina – October 19, 2011 ASPERA Forum Laurent Pinard  Based on a phase shifting interferometer (ADE Phase Shift)  MIRAU type interferometer  Working wavelength 1064 nm, Aperture 150 mm, Distortion correction  References flats known by “three flat test” measurement Advanced Virgo : Flatness Metrology

21 Cascina – October 19, 2011 ASPERA Forum Laurent Pinard  Measurement on larger diameter (320 mm) with stitching interferometry  Mirror on motorized sample holder  Measurement of Sub-pupils, total wavefront reconstructed mathematically Mirror X Y Advanced Virgo : Flatness Metrology

22 Cascina – October 19, 2011 ASPERA Forum Laurent Pinard Flat surface Advanced Ligo Optic 0.62 nm RMS, 6 nm PV (LMA) 0.57 nm RMS, 4,7 nm PV (Tinsley) Advanced Virgo : Flatness Metrology

23 Cascina – October 19, 2011 ASPERA Forum Laurent Pinard  Wavefront measurement has to be improved to measure sub-nm surface flatness on curved surface and large diameter (stitching)  Reference sphere necessary  Vibration isolation improvement  Protection from turbulences  Improve the way to support the mirror : more stable, no deformation Advanced Virgo : Flatness Metrology

24 Cascina – October 19, 2011 ASPERA Forum Laurent Pinard  Identical interferometer cavities : Necessary to coat two large substrates at the same time  Coating uniformity needed on 80 cm diameter Twin mirrors IBS Virgo coating machine Advanced Virgo : Coating Uniformity 35 cm

25 Cascina – October 19, 2011 ASPERA Forum Laurent Pinard  Masks between targets and substrates  Mask shape calculated with a home-made software  Two different masks for H and L layers, several iterations Advanced Virgo : Coating Uniformity

26 Cascina – October 19, 2011 ASPERA Forum Laurent Pinard Advanced Virgo : Coating Uniformity Ti: Ta 2 O 5 SiO 2 0.2% rms on  160 mm 0.08% rms on  160 mm

27 Cascina – October 19, 2011 ASPERA Forum Laurent Pinard Advanced Virgo : Coatings Uniformity Mirror n°1 Mirror n°2 1.6 nm rms on  160 mm  Mirrors performances matched : same T, same wavefront  Flatness consistent with thickness profile of monolayers  Not enough for Advanced detector (0.5 nm RMS)

28 Cascina – October 19, 2011 ASPERA Forum Laurent Pinard Advanced Virgo : Coatings Uniformity Recent result (July 2011) an a 34 cm LIGO mirror (single rotation) 0.56 nm RMS on  160 mm Next development : planetary motion to coat two substrates at the same time

29 Cascina – October 19, 2011 ASPERA Forum Laurent Pinard Scenario for the 3 rd generation of mirrors  Einstein Telescope (ET) design study : final document May 2011  Strategy for the next generation of mirrors ET – High FrequencyET – Low Frequency (cryogenic) Fused Silica, 1064 nm 62 cm diameter, 30 cm thick 200 kg Polishing spec.: same as Advanced Virgo Same coating as AdV Silicon : new material, 1550 nm > 45 cm diameter, #50 cm thick, 211 kg Polishing spec.: same as Advanced Virgo Same coating as AdV BUT study of the perf. at 1.55 µm and 10°K necessary

30 Cascina – October 19, 2011 ASPERA Forum Laurent Pinard Scenario for the 3 rd generation of mirrors Cryogenic setup developed to measure at 10°K the coating quality factor (thermal noise) and the substrate/coating absorption at 1.55 µm