Absorption in bulk crystalline silicon and in the crystal surfaces Aleksandr Khalaidovski 1 Alexander Khalaidovski 1, Jessica Steinlechner 2, Roman Schnabel 2 KAGRA face-2-face meeting – 富山大学 – August 3rd : Albert Einstein Institute Max Planck Institute for Gravitational Physics Institute for Gravitational Physics of the Leibniz University Hannover Optical absorption in bulk crystalline silicon as well as in the crystal surfaces 1: Institute for Cosmic Ray Research (ICRR) The University of Tokyo

Absorption in bulk crystalline silicon and in the crystal surfaces Aleksandr Khalaidovski 2 Outline

Absorption in bulk crystalline silicon and in the crystal surfaces Aleksandr Khalaidovski 3 Motivation – Einstein Telescope (ET)

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

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

Absorption in bulk crystalline silicon and in the crystal surfaces Aleksandr Khalaidovski 6 Properties of crystalline silicon Source:  High Q-factor at both room temperature and cryogenic temperatures  Available in large diameters (currently about 450mm – 500mm)

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

Absorption in bulk crystalline silicon and in the crystal surfaces Aleksandr Khalaidovski 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

Absorption in bulk crystalline silicon and in the crystal surfaces Aleksandr Khalaidovski 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

Absorption in bulk crystalline silicon and in the crystal surfaces Aleksandr Khalaidovski 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

Absorption in bulk crystalline silicon and in the crystal surfaces Aleksandr Khalaidovski 11 Optical absorption measurements at the AEI Hannover

Absorption in bulk crystalline silicon and in the crystal surfaces Aleksandr Khalaidovski 12 Photo-thermal self-phase modulation Thermal effect increases with Increasing power Decreasing scan frequency Dr. Jessica Steinlechner

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

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

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

Absorption in bulk crystalline silicon and in the crystal surfaces Aleksandr Khalaidovski 16 Silicon absorption at 1550 nm - measurement at a fixed optical power

Absorption in bulk crystalline silicon and in the crystal surfaces Aleksandr Khalaidovski 17 Measurement setup  Length 65mm, diameter 100 mm.  Curved end surfaces, ROC = 1m.  Specific resistivity 11 k  cm (boron) Monolithic Si cavity  Coatings: SiO 2 /Ta 2 O 5. R = %.

Absorption in bulk crystalline silicon and in the crystal surfaces Aleksandr Khalaidovski 18 Measurement results are … Measurement Number Result of a single MeasurementMean value + error bar α = (264 ± 39) ppm/cm or 3430 ppm/round trip

Absorption in bulk crystalline silicon and in the crystal surfaces Aleksandr Khalaidovski 19 … much higher than expected

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

Absorption in bulk crystalline silicon and in the crystal surfaces Aleksandr Khalaidovski 21 Silicon absorption at 1550 nm - power-dependent measurements

Absorption in bulk crystalline silicon and in the crystal surfaces Aleksandr Khalaidovski 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

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

Absorption in bulk crystalline silicon and in the crystal surfaces Aleksandr Khalaidovski 24 Discussion  Results by Degallaix et al. qualitatively confirmed  Reason: probably two-photon absorption, quantitative analysis in progress I) Non-linear dependence of absorption on optical intensity 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.

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

Absorption in bulk crystalline silicon and in the crystal surfaces Aleksandr Khalaidovski 26 Possible implications  Absorption contribution of about 800 ppm per surface transmission  Absorbed laser power needs to be extracted through the suspensions  1600 ppm for transmission through input test mass (ITM)

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

Absorption in bulk crystalline silicon and in the crystal surfaces Aleksandr Khalaidovski 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

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

Absorption in bulk crystalline silicon and in the crystal surfaces Aleksandr Khalaidovski 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