Optical Interference Coatings from advanced to future GW

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 ELi. TES 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 05 Dec 2013 G. Cagnoli – 2 nd ELi. TES Meeting - Tokyo 2

Mechanical losses of coatings AMORPHOUS • Any reduction is valuable • Annealing temperature increase Silica S. Penn GWADW 2012 – Nano-layers (Pinto’s talk) – Replacing Ti: Ta 2 O 5 with new materials Ti: Ta 2 O 5 loss is 2 e-4 (with Y=140 GPa) • No major technological problems expected • Change of wavelength opens the choice to new materials still • Possibility to measure thermal noise directly 05 Dec 2013 G. Cagnoli – 2 nd ELi. TES Meeting - Tokyo Undoped tantala LMA – 19 th IVC 2013 3

Mechanical losses of coatings CRYSTALLINE • Big gain … and big pain! S. Penn LVC 2013 • Sapphire (KAGRA) and Silicon (ET) are the substrates • Materials and techniques are many – Compatible with low loss optics? – Compatible with 60 cm diameter optics? GWADW 2012 K. Craig LVC 2013 Ga. P single layer • Materials Science and Technology are together on a long road 05 Dec 2013 G. Cagnoli – 2 nd ELi. TES Meeting - Tokyo 4

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 3 D maps Insensitive to sample dimension 05 Dec 2013 G. Cagnoli – 2 nd ELi. TES Meeting - Tokyo 5

Advanced Virgo experience • Used to characterize and validate the substrates Top view = 633 nm P = 3 m. W = 1064 nm P = 30 W 05 Dec 2013 G. Cagnoli – 2 nd ELi. TES Meeting - Tokyo 6

Some results • Absorption Map on a. LIGO ETM HR coating • Absorption map 150 mm of a Sapphire substrate (2004) • (0. 36 +/- 0. 06) ppm Coating absorption 05 Dec 2013 Bulk absorption G. Cagnoli – 2 nd ELi. TES Meeting - Tokyo 7

For cryogenic detectors • Absorption on silicon J. Degallaix et al. , @ 300 K • Labs: – – LMA Jena AEI Glasgow • Coatings: = 1550 nm P = 30 W = 1310 nm P = 3 m. W – ~5 ppm Ga. As/Al. Ga. As (G. Cole) – Not reproduced in other labs: lot to learn… 05 Dec 2013 G. Cagnoli – 2 nd ELi. TES Meeting - Tokyo Cryostat where Q measurements are made 8

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 ? 05 Dec 2013 G. Cagnoli – 2 nd ELi. TES Meeting - Tokyo 9

Scattering performance • Maps of integrated scattering power validate the cleaning procedure CASI at LMA Scattering Map on a. LIGO ITM HR coating # 3 ppm on 160 mm 05 Dec 2013 G. Cagnoli – 2 nd ELi. TES Meeting - Tokyo 10

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 05 Dec 2013 G. Cagnoli – 2 nd ELi. TES Meeting - Tokyo 11

Coating uniformity 2 Advanced LIGO ETM Mirrors after HR coating on the planetary system • From simple to planetary motion – Better uniformity + 2 -mirrors deposition Coating type: a. LIGO HRITM Planetary motion 05 Dec 2013 G. Cagnoli – 2 nd ELi. TES Meeting - Tokyo 12

Coating uniformity performance ETM wavefront 0. 74 nm RMS ~3 nm 160 mm All the Zernike coefficients <0. 5 nm except the Sph. Aber. Coating type: a. LIGO HRETM Materials: Ti: Ta 2 O 5 / Si. O 2 Optimized layer thickness Losses in cavity: 12 ppm • 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 (LG 33 mode) 05 Dec 2013 G. Cagnoli – 2 nd ELi. TES Meeting - Tokyo 13

The effect of the planetary motion • Spiral = consequence of the Caltech – Gari. Lynn Billingsley 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 Trajectory of a point at 50 mm from centre 05 Dec 2013 G. Cagnoli – 2 nd ELi. TES Meeting - Tokyo 14

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 05 Dec 2013 G. Cagnoli – 2 nd ELi. TES Meeting - Tokyo 15

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) About to be installed at LMA Ready to be used by January 2014 05 Dec 2013 G. Cagnoli – 2 nd ELi. TES Meeting - Tokyo 16

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 Caltech – Gari. Lynn Billingsley 05 Dec 2013 G. Cagnoli – 2 nd ELi. TES Meeting - Tokyo 17

Wave front control • Corrective coatings to compensate the inhomogeneity of the substrate BEFORE CORRECTION 05 Dec 2013 G. Cagnoli – 2 nd ELi. TES Meeting - Tokyo AFTER CORRECTION 18

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 Before annealing After annealing • In the coating profile design the stress effect has to be considered • Coating stress has to be perfectly controlled 05 Dec 2013 G. Cagnoli – 2 nd ELi. TES Meeting - Tokyo 19

Wave front distortion in crystalline optics • Stress induced birefringence Birefringence map on 280 mm on a Virgo Input Mirror substrate – Suspensions: astigmatic effect? – Coating • Intrinsic birefringence Birefringence map on a large sapphire substrate 05 Dec 2013 G. Cagnoli – 2 nd ELi. TES Meeting - Tokyo 20

Conclusions - 1 • 2 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 17 Oct 2013 G. Cagnoli - Séminaires au FEMTO-ST 21

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 05 Dec 2013 G. Cagnoli – 2 nd ELi. TES Meeting - Tokyo 22

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 RIBER 32 Purchased by LMA to investigate the many technological problems related to the production of crystalline coatings of optical quality on large substrates 05 Dec 2013 G. Cagnoli – 2 nd ELi. TES Meeting - Tokyo 23

BNS detection range GRB 050509 B 1 ST GENERATION a. LIGO Ad. V KAGRA Einstein Telescope 05 Dec 2013 G. Cagnoli – 2 nd ELi. TES Meeting - Tokyo 24