1 / 30

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

heinz
Download Presentation

Alexander Khalaidovski 1 , Jessica Steinlechner 2 , Roman Schnabel 2

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related