Folie 1 - University of Tokyo

Folie 1 - University of Tokyo

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/ Aleksandr Khalaidovski 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 1 Absorption in bulk crystalline silicon and in the crystal surfaces KAGRA face-2-face meeting August 3rd 2013

Outline Aleksandr Khalaidovski Absorption in bulk crystalline silicon and in the crystal surfaces 2 Motivation Einstein Telescope (ET) Aleksandr Khalaidovski Absorption in bulk crystalline silicon and in the crystal surfaces

3 Motivation ET Low Frequency Interferometer Low frequency interferometer: cryogenic temperature (10 K) Conventional fused silica optics no longer usable Use crystalline silicon Aleksandr Khalaidovski Absorption in bulk crystalline silicon and in the crystal surfaces 4 Properties of crystalline silicon

High Q-factor at both room temperature and cryogenic temperatures Credits: Ronny Nawrodt Aleksandr Khalaidovski Absorption in bulk crystalline silicon and in the crystal surfaces 5 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 Aleksandr Khalaidovski Absorption in bulk crystalline silicon and in the crystal surfaces 6 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 ... Aleksandr Khalaidovski

Absorption in bulk crystalline silicon and in the crystal surfaces 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 ... ... expected to have very low optical absorption at 1550 nm ? Aleksandr Khalaidovski

Absorption in bulk crystalline silicon and in the crystal surfaces 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 currently chosen as candidate material for ET-LF test masses Aleksandr Khalaidovski Absorption in bulk crystalline silicon and in the crystal surfaces

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 we need to confirm low optical absorption at RT and CT Aleksandr Khalaidovski Absorption in bulk crystalline silicon and in the crystal surfaces

10 Optical absorption measurements at the AEI Hannover Aleksandr Khalaidovski Absorption in bulk crystalline silicon and in the crystal surfaces 11 Photo-thermal self-phase modulation

Lcavity substrate Lcavity Thermal effect increases with Increasing power Dr. Jessica Steinlechner Decreasing

scan frequency Aleksandr Khalaidovski Absorption in bulk crystalline silicon and in the crystal surfaces 12 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. Aleksandr Khalaidovski Absorption in bulk crystalline silicon and in the crystal surfaces 13 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) Aleksandr Khalaidovski Absorption in bulk crystalline silicon and in the crystal surfaces

14 More about the method (Journal: Applied Optics) (Journal: Applied Optics) Aleksandr Khalaidovski Absorption in bulk crystalline silicon and in the crystal surfaces 15

Silicon absorption at 1550 nm measurement at a fixed optical power Aleksandr Khalaidovski Absorption in bulk crystalline silicon and in the crystal surfaces 16 Measurement setup Monolithic Si cavity Length 65mm, diameter 100 mm. Curved end surfaces, ROC = 1m. Specific resistivity 11 kcm (boron)

Coatings: SiO2/Ta2O5. R = 99.96 %. Aleksandr Khalaidovski Absorption in bulk crystalline silicon and in the crystal surfaces 17 Measurement results are = (264 39) ppm/cm or 3430 ppm/round trip Measurement Number Result of a single Measurement

Aleksandr Khalaidovski Mean value + error bar Absorption in bulk crystalline silicon and in the crystal surfaces 18 much higher than expected Aleksandr Khalaidovski Absorption in bulk crystalline silicon and in the crystal surfaces

19 Measurements by the LMA group Using beam deflection method Aleksandr Khalaidovski [J. Degallaix, 4th ET symposium, Dec. 2012] Absorption in bulk crystalline silicon and in the crystal surfaces 20 Silicon absorption at 1550 nm

power-dependent measurements Aleksandr Khalaidovski Absorption in bulk crystalline silicon and in the crystal surfaces 21 Facts about the measurement Same monolithic cavity as in previous setup Intra-cavity peak intensity: 0.4 W/cm - 21 kW/cm Impedance-mismatch measurement Aleksandr Khalaidovski

Absorption in bulk crystalline silicon and in the crystal surfaces 22 Results Aleksandr Khalaidovski Absorption in bulk crystalline silicon and in the crystal surfaces 23 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. Aleksandr Khalaidovski Absorption in bulk crystalline silicon and in the crystal surfaces

24 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. Aleksandr Khalaidovski Absorption in bulk crystalline silicon and in the crystal surfaces 25

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 Aleksandr Khalaidovski Absorption in bulk crystalline silicon and in the crystal surfaces 26 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) Aleksandr Khalaidovski Absorption in bulk crystalline silicon and in the crystal surfaces 27 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 Aleksandr Khalaidovski Absorption in bulk crystalline silicon and in the crystal surfaces 28 Discussion II

(a) Our data (b) LMA data with added offset of 250 ppm/cm Aleksandr Khalaidovski Absorption in bulk crystalline silicon and in the crystal surfaces 29 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 Aleksandr Khalaidovski

Absorption in bulk crystalline silicon and in the crystal surfaces 30

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