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A + timeline, plans, and requirements

This project focuses on the design sensitivity of A+LIGO, which requires mirror coatings with improved mechanical properties and the ability to maintain challenging optical specifications. This study aims to reduce thermal noise and meet the necessary requirements for successful operation.

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A + timeline, plans, and requirements

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  1. A+ timeline, plans, and requirements M.M. Fejer Stanford University fejer@Stanford.edu

  2. Problem Statement • Mid-band design sensitivity of A+LIGO: • requires 2 x less thermal noise than a-LIGO • Requires mirror coatings with improved mechanical properties • and maintain current challenging optical specs • Boundary conditions: • room temperature • 1 μm operation • silica substrate • 34 cm diameter

  3. Time Scale • Assuming 1-year pathfinder after science solution: May 2020 • accept status and proceed with pathfinder or later “Phase-B” installation requiring re-proposal to NSF • Moore foundation/NSF initiated CCR: Nov 2017 for 3 years A+LIGO aVIRGO+ Preliminary KAGRA+ LIGO-P1200087

  4. Thermal Noise in IF Mirrors f0 • Oversimple: kTof energy per mechanical mode, viscous damping • moves front of mirror w.r.t. center of mass • For coating dominated noise x Δx(f)2 low Q signal band  1/Q high Q f / f0 coating elastic loss coating thickness aLIGO: beam radius A+LIGO: reduce  d by 4x Y Levin Phys. Rev. D57 659(1998)

  5. Basic Coating Concepts • Dielectric mirror • alternating high/low index ¼ wavelength-thick layers • large index contrast  fewer required layers: • Key optical properties • absorption < 0.5 ppm, scatter – ppm’s • industry standard: ion-beam sputtering • R.T. deposition followed by 300 C – 500 C annealing • scaling to >30 cm nontrivial • with ~1 nm RMS figure: LMA, Lyon • Current LIGO mirrors: • Ti(20%):Ta2O5 : n = 2.07,  = 3 x 10-4 SiO2: n = 1.45,  = 4 x 10-5 F.O.M (approx.) ~ ( nH– nL) /

  6. Nature is Perverse Low Elastic Loss a-Si a-Si3N4 Low Elastic Loss a-Si a-Si3N4 Low Optical Loss Ta2O5 Nb2O5 … Low Optical Loss Ta2O5 Nb2O5 … ?

  7. What We Knew at t = 0

  8. General Observations About Coating Elastic Loss • Volume rather than interface losses dominate in tantala/silica mirror • current values: Ti:tantala ~5x lossier than silica • Typical behavior vs temperature and acoustic frequency • amorphous materials have loss peak at low temperatures D. Crooks, Class. Quantum Grav. 23 (2006) 4953–4965 large variety of glasses have similar cryo behavior bulk SiO2 aLIGO Ti:Ta2O5 SiO2 R.T. dip I W Martin et al, Class. Quant. Grav.27 225020, (2010) K.A. Topp, Z. PhysikB Condensed Matter101235–45 (1996)

  9. Doping and Annealing Alter Dissipation • Loss modified by dopants • TiO2 doping • reduces losses in Ta2O5 • Annealing modifies loss spectra increasing Ti Ti:Ta2O5 P. Murray et al, U. Glasgow LIGO-G1500874 annealing temp. Ta2O5 annealing modifies behavior usually improve loss at R.T. while worsening loss at cryo annealing temperature limited by crystallization suppressing crystallization important [I W Martin et al, Class. Quant. Grav. 27 225020, 2010]

  10. Low-frequency losses in amorphous dielectrics • Conventionally associated with low energy excitations (LEEs) • conceptualized as two-level systems (TLS) Oversimple picture: bond flopping V E2 crystal quartz Distribution of TLS in silica due to disordered structure Δ E1 fused silica figures from B.S. Lunin monograph

  11. Theoretical Guidance: Molecular Dynamics • Molecular dynamics calculations for amorphous materials • provide insight into dissipation mechanisms • Some observations: simple bond-flopping inadequate picture fails • TLS involves dozens of atoms in nm-scale configurations “medium-range” order important J. Jiang LIGO G1800533 cause cryogenic losses cause 300 K losses J. Trinastic, R. Hamdan, C. Billman, H. Cheng, Phys. Rev. B93 , 014105 (2016)

  12. Ultrastable Glass: Toy Model • Vapor-deposited glasses can reach “ultrastable” state • isolated low-energy state • High surface mobility required • deposit at ~0.8 Tglass • Low deposition rates liquid liquid vs vapor deposition) supercooled liquid Tglass glassy regime Ts~0.8 Tglass reach more stable glass from vapor than liquid S. Singh, Nature Mater. 12, 139 (2013)

  13. Ultra-stable Glasses: amorphous silicon (a-Si) • a-Si experiment: steep improvement for deposition at Ts~ 400 C: ϕ ~ 10-6 (!) • much lower loss than deposit at 300 C and anneal at 400 C • critical Ts/Tglass~ 0.75 vs predictedTs~ 0.8 Tglass • First example of inorganic ultra-stable glass • potential for Voyager mirror coating annealed 350C Formation of ultrastable glass favored by: Deposition at Ts~ 0.8 Tglass Low deposition rates Ion-beam assisted deposition (?) Applicable to amorphous oxides? Reduces RT loss as well as cryogenic? deposition temperatures, Ts X. Liu, F. Hellman, et al, PRL 113, 025503 (2014)

  14. Approaches to A+ Coatings at t=0 • Empirical • seek ultrastable oxide glass • high temperature deposition • ion-beam assist • low rate deposition • frustrate crystallization for higher annealing temperatures • chemically: suitable dopants • geometrically: nanolayers • try random things • Atomic structure characterization • investigate connection of structure (changes) to loss (changes) • X-ray and TEM methods • novel acquisition methods for scattering data : GIPDF, FEM • novel inversion methods to recover structure • Theoretical: molecular dynamics, DFT, … • predict loss properties: two-level systems • predict suggestive properties: glass temperature, fragility, …

  15. Who’s Working on It in LSC+? CCR GEO U.S. INT’L NRL LL/MIT CSLA/Sannio Strathclyde Hamburg Berkeley Col. State Montreal Tsinghua Deposition Caltech NRL MIT Syracuse HWS SU Mechanical Loss Glasgow Tsinghua Florida SU Atomic Structure Model Glasgow Atomic Char. SU Fullerton Col State Whitman Glasgow Strathclyde Optical Char. CSLA/Sannio Tsinghua Strathclyde Hamburg Glasgow Fullerton American SU Macroscopic Model MIT

  16. Examples of things we’ve learned so far

  17. Ultrastable Glass: Experiment • Exhaustive investigation of tantala and doped tantala • deposition up to 500 C: no RT improvement over annealing at same T • ion-beam assist: Ar, Xe, various energies and fluxes: nope • Alumina (NRL) • high T deposition better than annealing • suggests first ultrastable oxide • room temperature data needed • ties up with theory Vajente, C.Q.G.35, 075001 (2018) L. Yang, P1900123 10-3 RT ϕ Ts = 300 C Tanneal = 600 C 10-4 Ts = 500 C M. Abernathy NRL, LIGO G1800418 100 K 10 K 1 K

  18. Ultrastable Glass: MD Theory Consistent with Expt Glass temperatures with caloric curves Glass “fragility” from density of states Ta2O5 Tglass Al2O3 K. Prasai 0.8*Tglass: Ta2O5: 1190 C Al2O3: 591 C consistent with expt 0.8*Tglass: Ta2O5: least fragile Al2O3: ~most fragile consistent with expt Ta2O5: unlikely to form ideal glass Al2O3: 0.8 Tglass ~ 591 C, close to ~600 C expt A year in the lab can save you a week of computation!

  19. Crystallization Suppression by Dopants Higher annealing temperatures for oxide glasses push toward more uniform glass, even if not ultra-stable Frustrate crystallization with suitable dopant Zr:Ta2O5 Zirconia (34%)-Tantala: Tanneal = 800 C 𝜙 = 1.4 x 10-4 at 300 K (~15% Zr, S. Reid et al), potentially useful tantalizing (not yet reproducible) 𝜙 = 0.7 x 10-4 importance of deposition rate/energy not yet clear SiO2:Ta2O5 Zr:Ta2O5 Steinlechner LIGO-G1800585b R Robin’s thesis (U of Glasgow), G Vajente (Caltech), G1800783 Le Yang, G1900528

  20. Chemical Suppression of Crystallization • Ta2O5dopant search to suppress crystallization • Dopants kept at 20-25% range: • Cracking frequently occurs at high temperatures • balance thermal expansion with hot substrate deposition? • Ternaries being explored M. Fazio, G1900527

  21. Geometrical Crystallization Suppression: Nanolayers • Frustrate crystallization using “nanolayers” [S. Chao, LIGO-G1900289 ] • [Chao, Pinto, DeSalvo] • intersperse thin stable (SiO2) layers in high index material • 1.8 nm TiO2/ 3.6 nm SiO2nanolayers: 75 sublayers • Tanneal < 800 C, crystallization suppressed: 𝜙 ≈ 1.2 x 10-4 • volume/interface scatter an issue? n ~ 1.77 Suppresses low-T loss peak of silica – reducing low-energy TLS S Chao et al., P1900090

  22. Silicon Nitride • LPCVD deposition of SiN0.40H0.79 • R.T.: ϕ~1 x 10-4 • no low temperature loss peak • Optical absorption of SiN/SiO2 HR: ~50 ppm • annealing studies underway • requires multi-material coating • with Ta2O5/SiO2 • IBS deposition at Strathclyde SiO2 SiNH/SiO2 bilayers SiNH Pan et al NTHU, P1800164-v3 MMC I.Martin LIGO-G1801548 High optical loss Low elastic loss Low optical loss High elastic loss a-Si SiNH SiO2:HfO2 High-T Al2O3? Ti:Ta2O5 SiO2 other oxides

  23. Structural Characterization (Zr:Ta2O5) • Model system with strong annealing dependence • Invert X-ray scattering data with RMC and DFT

  24. Structural Characterization (Zr:Ta2O5) • Model system with strong annealing dependence • Structural motifs altered by annealing • correlate with RT vs cryo losses K. Prasai

  25. Structural Characterization (Zr:Ta2O5) • Void analysis: function of coating density, correlated with loss • Analysis on intermediate range order • significant for understanding RT loss J. Jiang, G1900557 A. Mishkin, G1900556 • Continuing to develop TLS calculations — pathway to predicted loss

  26. Summary • Building blocks + staffing established over first 18 months • deposition tools • CSU, UCB, UH, Strathclyde, Sannio, Montreal, LL • characterization tools • high throughput RT mechanical loss: Caltech, Syracuse • additional cryo loss tools: SU, U Glasgow • computational tools: SU, UF • scattering data -> structures • structure-property relations • fabrication concepts • high-T deposition • doping suppression of crystallization • nano-layer suppression of crystallization • Further connections emerging between theory and exp’t • Design concepts tested • multi-material coatings • Using theory to direct experimental choices

  27. What Next? • Two limiting strategies: • spaghetti on the wall • + try a lot of things, might get lucky • - after 12 months you don’t know anything if it fails • follow the science • + you’ll know more in 12 months either way • - progress is more deliberate charting path between these extremes goodness Research on a Development Schedule Be lucky is not a plan! Work continuing on cryogenic mirrors (at lower level) to avoid repeating this situation for 2.5 and 3G synergy + luck synergy -- luck good enough good enough Funding agencies should be convinced not to cut research funds for 2.5G and 3G coatings during A+ construction (as they did for A+ coatings during a-LIGO construction) tmax/2 0 0 tmax/2 tmax tmax

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