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
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Absorption
bulk crystalline
silicon– and
in the crystal
surfaces
KAGRAinface-2-face
meeting
富山大学
– August
3rd 2013
Outline
Aleksandr Khalaidovski
Absorption in bulk crystalline silicon and in the crystal surfaces
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Motivation – Einstein Telescope (ET)
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Absorption in bulk crystalline silicon and in the crystal surfaces
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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
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Properties of crystalline silicon
 High Q-factor at both room temperature and cryogenic temperatures
Credits: Ronny Nawrodt
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Absorption in bulk crystalline silicon and in the crystal surfaces
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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
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Absorption in bulk crystalline silicon and in the crystal surfaces
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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
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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
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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
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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
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Optical absorption measurements
at the AEI Hannover
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Photo-thermal self-phase modulation
Lcavity
substrate
Lcavity
Thermal effect
increases with
• Increasing
power
Dr. Jessica
Steinlechner
• Decreasing
scan frequency
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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
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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)
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More about the method
(Journal: Applied Optics)
(Journal: Applied Optics)
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Silicon absorption at 1550 nm
measurement at a fixed optical power
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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 %.
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Measurement results are …
α = (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
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… much higher than expected
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Measurements by the LMA group
Using beam deflection method
[J. Degallaix, 4th ET symposium, Dec. 2012]
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Silicon absorption at 1550 nm
power-dependent measurements
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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
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Results
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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.
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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.
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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
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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)
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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
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Discussion II
(a) Our data
(b) LMA data with added offset of 250 ppm/cm
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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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