Optical Interference Coatings: from advanced to future GW detectors L. Balzarini, E. Berthélémy, G. Cagnoli, J. Degallaix, V. Dolique, R. Flaminio. D. Forest, M. Granata, C. Michel, N. Morgado, L. Pinard, B. Sassolas, N. Straniero, J. Teillon 2 nd ELiTES Meeting Tokyo – 05 December 2013

Outline The R&D on 2 nd generation is providing an extremely valuable experience for the future GW detectors optics, either amorphous or crystalline Specific aspects considered: –Materials Mechanical losses Optical absorption –Deposition related parameters Scattering Thickness uniformity Thickness control Wave front control and correction Conclusions 205 Dec 2013G.Cagnoli – 2 nd ELiTES Meeting - Tokyo

Mechanical losses of coatings Any reduction is valuable Annealing temperature increase –Nano-layers (Pinto’s talk) –Replacing Ti:Ta 2 O 5 with new materials No major technological problems expected Change of wavelength opens the choice to new materials still Possibility to measure thermal noise directly 305 Dec 2013G.Cagnoli – 2 nd ELiTES Meeting - Tokyo AMORPHOUS Silica S. Penn GWADW 2012 Undoped tantala LMA – 19 th IVC 2013 Ti:Ta2O5 loss is 2e-4 (with Y=140 GPa)

Big gain … and big pain! Sapphire (KAGRA) and Silicon (ET) are the substrates Materials and techniques are many –Compatible with low loss optics? –Compatible with 60 cm diameter optics? Materials Science and Technology are together on a long road 405 Dec 2013G.Cagnoli – 2 nd ELiTES Meeting - Tokyo CRYSTALLINE GWADW 2012 Mechanical losses of coatings S. Penn LVC 2013 K. Craig LVC 2013 GaP single layer

Optical absorption Long experience of the photodeflection technique at LMA with 3 metrology benches based on the mirage effect –Excellent sensibility (down to 0.3 ppm on silica) –Measure surface absorption or volume absorption –Can derive 3D maps –Insensitive to sample dimension 505 Dec 2013G.Cagnoli – 2 nd ELiTES Meeting - Tokyo

Advanced Virgo experience Used to characterize and validate the substrates 605 Dec 2013G.Cagnoli – 2 nd ELiTES Meeting - Tokyo Top view  = 1064 nm P = 30W  = 633 nm P = 3 mW

Some results Absorption Map on aLIGO ETM HR coating (0.36 +/- 0.06) ppm Absorption map 150 mm of a Sapphire substrate (2004) 705 Dec 2013G.Cagnoli – 2 nd ELiTES Meeting - Tokyo Coating absorption Bulk absorption

 = 1550 nm P = 30W  = 1310 nm P = 3 mW For cryogenic detectors 805 Dec 2013G.Cagnoli – 2 nd ELiTES Meeting - Tokyo J. Degallaix et al., Absorption on silicon Labs: –LMA –Jena –AEI –Glasgow Coatings: –~5 ppm GaAs/AlGaAs (G. Cole) –Not reproduced in other labs: lot to learn… Cryostat where Q measurements are 300 K

Scattering It’s related to the substrate cleanness before the coating deposition A cleaning procedure based on the ultrasound bath has been implemented regularly But cannot remove any chemical contamination of the surface. Facility for large surface analysis ? 905 Dec 2013G.Cagnoli – 2 nd ELiTES Meeting - Tokyo

Scattering performance 1005 Dec 2013G.Cagnoli – 2 nd ELiTES Meeting - Tokyo Scattering Map on aLIGO ITM HR coating # 3 ppm on 160 mm Maps of integrated scattering power validate the cleaning procedure CASI at LMA

Scattering on future detectors optics Squeezed light –Scattering is the dominant loss in the filter cavities Crystalline coatings –Planar growth: amplification of substrate imperfections due to the step edge potential? –Non-thermal diffusivity assistance? –G. Cole has 0.13 nm rms on his coatings 1105 Dec 2013G.Cagnoli – 2 nd ELiTES Meeting - Tokyo

Coating uniformity From simple to planetary motion –Better uniformity + 2-mirrors deposition 1205 Dec 2013G.Cagnoli – 2 nd ELiTES Meeting - Tokyo 2 Advanced LIGO ETM Mirrors after HR coating on the planetary system Planetary motion Coating type: aLIGO HRITM

Coating uniformity performance There are some effects not yet under control that are relevant at the 0.05% uniformity level –There is an effort specifically directed to improve further the uniformity control –Important for future detectors (LG33 mode) 1305 Dec 2013G.Cagnoli – 2 nd ELiTES Meeting - Tokyo Coating type: aLIGO HRETM Materials: Ti:Ta2O5 / SiO2 Optimized layer thickness ~3 nm ETM wavefront 0.74 nm RMS 160 mm All the Zernike coefficients <0.5 nm except the Sph. Aber. Losses in cavity: 12 ppm

The effect of the planetary motion 1405 Dec 2013G.Cagnoli – 2 nd ELiTES Meeting - Tokyo Caltech – GariLynn Billingsley Trajectory of a point at 50 mm from centre Spiral = consequence of the combination of the planetary motion and the mask Amplitude max ~1.5 nm on the 40 mm radius circumference The part of the trajectory that is almost radial makes the total amount of deposited material sensitive to the gradient of sputtered material in this direction

Coating thickness control The ability to control the layer thickness at present is good enough for the future HR coatings –Control based on quartz balance For the AR coatings, layer thickness is extremely important (constructive vs destructive interference) –Solution 1: post deposition etching –Solution 2: corrective layer (done at LMA) 200 ppm after 1 correction –Ph.D. thesis at LMA: Study of an in-situ optical thickness measurement system for coating deposition 1505 Dec 2013G.Cagnoli – 2 nd ELiTES Meeting - Tokyo

Wave front interferometer New ZYGO interferometer coupled to a 18” (45 cm) beam expander, wavelength shifting technique at 1064 nm (first system in the world at this wavelength), performances to measure the Advanced Virgo/LIGO optics (0.5 nm RMS) Able to measure substrate with parallel sides : measurement of the 2 surfaces and the bulk homogeneity (ITF algorithm) 1605 Dec 2013G.Cagnoli – 2 nd ELiTES Meeting - Tokyo About to be installed at LMA Ready to be used by January 2014

Wave front distortion by the substrate transmission Effect on sideband amplitude and coupling coefficient to the F-P cavities It might be compensated by the corrective coating 1705 Dec 2013G.Cagnoli – 2 nd ELiTES Meeting - Tokyo Caltech – GariLynn Billingsley

Wave front control Corrective coatings to compensate the inhomogeneity of the substrate 1805 Dec 2013G.Cagnoli – 2 nd ELiTES Meeting - Tokyo BEFORE CORRECTION AFTER CORRECTION

The stress effect on the wave front distortion The stress has an effect greater or at least equal to the coating uniformity on the wave front In the coating profile design the stress effect has to be considered Coating stress has to be perfectly controlled 1905 Dec 2013G.Cagnoli – 2 nd ELiTES Meeting - Tokyo Before annealing After annealing

Wave front distortion in crystalline optics Stress induced birefringence –Suspensions: astigmatic effect? –Coating Intrinsic birefringence 2005 Dec 2013G.Cagnoli – 2 nd ELiTES Meeting - Tokyo Birefringence map on 280 mm on a Virgo Input Mirror substrate Birefringence map on a large sapphire substrate

Conclusions nd generation mirrors are currently under production Results: –Thickness uniformity and control, scattering, optical absorption and mechanical losses are OK –Spirals: predicted, they do not limit the first detection –Milky layer appeared on a ITM run: origin unknown 2117 Oct 2013G.Cagnoli - Séminaires au FEMTO-ST

Conclusions - 2 Lessons learned –The mirror quality is “detector-dependent”. A key point is to have production, characterization and optical simulation on the same lab –Spirals: there is already a simple way to reduce them 3 to 5 times but it has consequences on coating uniformity –Minimizing the risk: surface characterization on the substrates before coating single piece production mentality 2205 Dec 2013G.Cagnoli – 2 nd ELiTES Meeting - Tokyo

Conclusions - 3 For the future –Room T: any gain on mechanical losses is important; technology improvement already plotted –Low T: enormous scientific effort; even larger technological effort 2305 Dec 2013G.Cagnoli – 2 nd ELiTES Meeting - Tokyo RIBER 32 Purchased by LMA to investigate the many technological problems related to the production of crystalline coatings of optical quality on large substrates

BNS detection range 2405 Dec 2013G.Cagnoli – 2 nd ELiTES Meeting - Tokyo GRB050509B Einstein Telescope aLIGO AdV KAGRA 1 ST GENERATION