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Nano-structured Optics for GW Detectors
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  1. Max-Planck-Institut Für Gravitationsphysik (Albert-Einstein-Institut) Universität Hannover Nano-structured Optics for GW Detectors Bunkowski, O. Burmeister, D. Friedrich, K. Danzmann, and R. Schnabel in collaboration with T. Clausnitzer, S. Fahr, E.-B. Kley, and A. Tünnermann Institute of Applied Physics, Jena

  2. Outline Summary of: All-reflective Interferomtry (with multilayer coatings) Outlook to: High reflective diffractive structures (without multilayer coatings)

  3. Motivation #1 Interferometers beyond LIGOII (MW of power) Transmission  Absorption  Thermal problems All-reflective interferometers: • Allow opaque test mass • materials • New interferometer • topologies

  4. MI with all-reflective beamsplitter

  5. Grating equation: What kind of gratings? d>>l: scalar approach d~l: rigorous theories (RCWA, Modal method) d<<l: effective medium theories (ETM) Period defines # of orders & direction Coating on top Coating below

  6. Appl. Opt. (in press) 99.62% achieved (Finesse of 1580) A. Bunkowski et al, Applied Optics (in press) http://ao.osa.org/upcoming_pdf.cfm?id=66481 Cavity couplers 1st order Littrow 2nd order Littrow Input Input Input Input 1R 1R 0R 2R 0R Need high efficiency (which is hard to achieve) Only low efficiency (3 ports are more Complicated)

  7. 99.62% achieved (Finesse of 1580) A. Bunkowski et al, Applied Optics (in press) http://ao.osa.org/upcoming_pdf.cfm?id=66481 Cavity couplers 1st order Littrow 2nd order Littrow Input Input Input Input 1R 1R 0R 2R 0R Need high efficiency (which is hard to achieve) Only low efficiency (3 ports are more Complicated) A. Bunkowski et al, Applied Optics (in press) http://ao.osa.org/upcoming_pdf.cfm?id=66481

  8. f [°] f [°] 3-port couplers A. Bunkowski et al, Opt. Lett. 30, 1183 (2005) R. Schnabel et al, Opt. Lett. 31 658 (2006) A. Bunkowski et al, Opt. Lett. 29, 2342 (2004) A. Bunkowski et al, Opt. Lett. (in press) http://ol.osa.org/upcoming_pdf.cfm?id=69970

  9. Angle resolved scattering -1st +1st E-beam exposure „Variable shaped beam“ diffraction orders due to e-beam writing errors electron beam work field stitching apertures shaped beam background due to statistical roughness

  10. Grating beneath coating same diffraction efficiency systematical errors reduced statistical background reduced Small fill factor groove depth 40nm fill factor 50% T. Clausnitzer et. al., Optics Express, 13, 4370 (2005)

  11. diffraction efficiency 1.5%  0.03% systematical errors statistical background Grating beneath coating II Large fill factor groove depth 40nm fill factor 82% T. Clausnitzer et al, Optics Express, 13, 4370 (2005)

  12. Conclusion #1 Summary: • High quality all-reflective components • Demonstration of new cavity coupling concept • Reduction of scattering loss Outlook: • Further reduction of loss • Scale to large test mass • Suspended 10 m all-reflected cavity to be built in Glasgow later this year

  13. Motivation #2 Several fundamental noise sources as currently estimated for advanced LIGO: S. Penn et.al, LIGO-P050049-00-R (2005)

  14. Coating stack Substrate Beating Coating Noise • The high refractive material tantala • (Ta2O5) has been identified to be the • main source of coating thermal noise. • Some ideas so far: • Less tantala in stack • Innocenzo Pinto’s talk • Doping of tantala with TiO2 • Harry et al, Appl. Opt. 45, 1569 (2006) • Total internal reflection •  Adalberto Giazotto’s talk •  V. Braginsky and S. Vyatchanin, • Phys. Lett. A 324, 345 (2004) • Double mirrors: •  F. Ya. Khalili, Phys. Lett. A 334, 67 (2005) AR-Coating stack

  15. Coating stack Substrate Substrate Coating Thermal Noise Another approach: AR-Coating stack Grating waveguide reflector

  16. waveguide nHigh nLow (substrate) Total internal reflection Grating waveguide Structures 0R 100% reflection (Simplified ray picture) see for example: Sharon et al, J. Opt. Soc. Am. A, 14 (1997) 1T Interfere destructively 0T

  17. Narrow reflection peak Liu et al,Opt. Lett. 23, 1556 (1998)

  18. d r g s n=2.04 n=1.45 (substrate) Fill factor Design width of reflection peak [%] g [µm] d: period g: groove depth r: ridge width s: film thickness r/d: fill factor

  19. 1-R g [µm] Fill factor Design width of reflection peak [%] X

  20. Broadband mirror for 1550nm Poly-Si SiO2 Si 0dB  reference mirror with R=98.5% Mateus et al, IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 7, JULY 2004

  21. Conclusion #2 Summary: • High reflectivity is possible with single layer Outlook: • Check fabrication tolerances, finite size effects… • Design, build and test gratings • Think of better suited wave guide structures

  22. great, greater, grating