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Alexander Khalaidovski 1 , Jessica Steinlechner 2 , Roman Schnabel 2

Optical absorption in bulk crystalline silicon as well as in the crystal surfaces. Alexander Khalaidovski 1 , Jessica Steinlechner 2 , Roman Schnabel 2. 1: Institute for Cosmic Ray Research (ICRR) The University of Tokyo http://www.icrr.u-tokyo.ac.jp/. 2: Albert Einstein Institute

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Alexander Khalaidovski 1 , Jessica Steinlechner 2 , Roman Schnabel 2

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  1. Optical absorption in bulk crystalline silicon as well as in the crystal surfaces Alexander Khalaidovski1, Jessica Steinlechner2, Roman Schnabel2 1: Institute for Cosmic Ray Research (ICRR) The University of Tokyo http://www.icrr.u-tokyo.ac.jp/ 2: Albert Einstein Institute Max Planck Institute for Gravitational Physics Institute for Gravitational Physics of the Leibniz University Hannover http://www.qi.aei-hannover.de KAGRA face-2-face meeting – 富山大学 – August 3rd 2013

  2. Outline

  3. Motivation – Einstein Telescope (ET)

  4. Motivation – ET Low Frequency Interferometer  Low frequency interferometer: cryogenic temperature (10 K)  Conventional fused silica optics no longer usable  Use crystalline silicon

  5. Properties of crystalline silicon  High Q-factor at both room temperature and cryogenic temperatures Credits: Ronny Nawrodt

  6. Properties of crystalline silicon  High Q-factor at both room temperature and cryogenic temperatures  Available in large diameters (currently about 450mm – 500mm) Source: http://www.bit-tech.net/hardware/2010/10/20/global-foundries-gtc-2010/4

  7. Properties of crystalline silicon  High Q-factor at both room temperature and cryogenic temperatures  Available in large diameters (currently about 450mm – 500mm)  Completely opaque at 1064 nm, but ...

  8. Properties of crystalline silicon  High Q-factor at both room temperature and cryogenic temperatures  Available in large diameters (currently about 450mm – 500mm)  Completely opaque at 1064 nm, but ...  ... expected to have very low optical absorption at 1550 nm ?

  9. Properties of crystalline silicon  High Q-factor at both room temperature and cryogenic temperatures  Available in large diameters (currently about 450mm – 500mm)  Completely opaque at 1064 nm, but ...  ... expected to have very low optical absorption at 1550 nm  currently chosen as candidate material for ET-LF test masses

  10. Properties of crystalline silicon  High Q-factor at both room temperature and cryogenic temperatures  Available in large diameters (currently about 450mm – 500mm)  Completely opaque at 1064 nm, but ...  ... expected to have very low optical absorption at 1550 nm  currently chosen as candidate material for ET-LF test masses  we need to confirm low optical absorption at RT and CT

  11. Optical absorption measurements at the AEI Hannover

  12. Photo-thermal self-phase modulation • Thermal effect increases with • Increasing power • Decreasing scan frequency Dr. Jessica Steinlechner

  13. Photo-thermal self-phase modulation  Absorption leads to a heating of the analyzed substrate and thus (for a sum of the thermo-refractive index dn/dT and the thermal expansion coefficient  > 0 ) to a thermally induced optical expansion.  When the substrate is placed in an optical cavity and the cavity length is scanned, this thermal expansion affects the detected cavity resonance peaks in a different way for an increase and a decrease of the cavity length.  An external increase of the cavity length and the thermally-induced expansion act in the same direction, resulting in a faster scan over the resonance and thus in a narrowing of the resonance peak.  In contrast, an external cavity length decrease and the thermally-induced expansion partly compensate. As a result, the scan over the resonance is effectively slower, leading to a broader resonance peak.

  14. Photo-thermal self-phase modulation Advantages  Suitable to measure absorption in bulk and coatings  High sensitivity (sub-ppm), small error bars  Does not require high laser power Drawbacks  Thermal effect visible not at all laser powers  Requires a cavity setup around the sample (can be the sample itself with dielectric coatings)

  15. More about the method (Journal: Applied Optics) (Journal: Applied Optics)

  16. Silicon absorption at 1550 nm - measurement at a fixed optical power

  17. Measurement setup Monolithic Si cavity  Length 65mm, diameter 100 mm.  Curved end surfaces, ROC = 1m.  Specific resistivity 11 kcm (boron)  Coatings: SiO2/Ta2O5. R = 99.96 %.

  18. Measurement results are … Measurement Number Result of a single Measurement Mean value + error bar α = (264 ± 39) ppm/cm or 3430 ppm/round trip

  19. … much higher than expected

  20. Measurements by the LMA group [J. Degallaix, 4th ET symposium, Dec. 2012] Using beam deflection method

  21. Silicon absorption at 1550 nm - power-dependent measurements

  22. Facts about the measurement  Same monolithic cavity as in previous setup  Intra-cavity peak intensity: 0.4 W/cm² - 21 kW/cm²  Impedance-mismatch measurement

  23. Results

  24. Discussion I) Non-linear dependence of absorption on optical intensity  Results by Degallaix et al. qualitatively confirmed  Reason: probably two-photon absorption, quantitative analysis in progress II) Our results are still much higher than the for other groups  Main differences: - material purities (difference not too large) - measurement approach. Our approach is sensitive to absorption in both the bulk crystal and the surfaces.

  25. Possible reason  Surface layer of amorphous silicon  Literature absorption values: ca. 100/cm – 2000/cm  High a-Si absorption verified in a different experiment measuring Si/SiO2 dielectric coatings.

  26. Possible implications  Absorption contribution of about 800 ppm per surface transmission  1600 ppm for transmission through input test mass (ITM)  Absorbed laser power needs to be extracted through the suspensions

  27. Outlook  Planned measurements: - Analysis of samples of different length - Analysis of samples of different purity, Czochralski and Float Zone  Analysis of the surfaces in view of a possible layer of amorphous material  Comparison with other groups, exchange of samples  Measurements at cryogenic temperatures (Jena)

  28. Conclusions  High absorption was found in Si-samples at the AEI  Such a high absorption contribution is neither expected from the bulk crystal, nor could it be confirmed by beam deflection measurements  The absorption probably originates in the crystal surfaces, possibly due to a layer of amorphous material generated during polishing  Further measurements are required to clearly separate the bulk and surface contributions and to evaluate a possible impact on ET Thank you very much

  29. Discussion II (a) Our data (b) LMA data with added offset of 250 ppm/cm

  30. Absorption measurement approaches • Power-Measurement • Power detection before and behind substrate (photo diode, power meter,…) • Simplest absorption measurement method • Not very sensitive • Beam-deflection measurement • Pump beam heats substrate • Probe beam is deflected by thermal lens • Deflection measurement on quadrant photo diode • Possible limit: available laser power

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