Folie 1

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Optical absorption in bulk crystalline silicon as
well as in the crystal surfaces
Alexander Khalaidovski1, Jessica Steinlechner2,
Roman Schnabel2
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 Institute for Cosmic Ray Research (ICRR) The
University of Tokyo http//www.icrr.u-tokyo.ac.jp/
KAGRA face-2-face meeting ???? August 3rd 2013
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Outline
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Motivation Einstein Telescope (ET)
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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
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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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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/2010/10/2
0/global-foundries-gtc-2010/4
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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 ...
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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
?
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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
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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
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Optical absorption measurements at the AEI
Hannover
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Photo-thermal self-phase modulation

• Thermal effect increases with
• Increasing power
• Decreasing scan frequency

Dr. Jessica Steinlechner
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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 ? gt 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.
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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
a (264 39) ppm/cm or 3430 ppm/round trip
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much higher than expected
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Measurements by the LMA group
Using beam deflection method
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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
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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