Absorption in bulk crystalline silicon and in the crystal surfacesAleksandr Khalaidovski 1

Alexander Khalaidovski1, Jessica Steinlechner2, Roman Schnabel2

KAGRA face-2-face meeting – 富山大学 – August 3rd 2013

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

Optical absorption in bulk crystalline silicon as well as in the crystal surfaces

1: Institute for Cosmic Ray Research (ICRR)

The University of Tokyo

http://www.icrr.u-tokyo.ac.jp/

Absorption in bulk crystalline silicon and in the crystal surfacesAleksandr Khalaidovski 2

Outline

Absorption in bulk crystalline silicon and in the crystal surfacesAleksandr Khalaidovski 3

Motivation – Einstein Telescope (ET)

Absorption in bulk crystalline silicon and in the crystal surfacesAleksandr 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 surfacesAleksandr 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 surfacesAleksandr Khalaidovski 6

Properties of crystalline silicon

Source: http://www.bit-tech.net/hardware/2010/10/20/global-foundries-gtc-2010/4

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 surfacesAleksandr 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 surfacesAleksandr 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 surfacesAleksandr 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 surfacesAleksandr 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 surfacesAleksandr Khalaidovski 11

Optical absorption measurementsat the AEI Hannover

Absorption in bulk crystalline silicon and in the crystal surfacesAleksandr Khalaidovski 12

Photo-thermal self-phase modulation

cavityL cavityLsubstrateL

Thermal effect increases with

• Increasing power

• Decreasing scan frequency

Dr. Jessica Steinlechner

Absorption in bulk crystalline silicon and in the crystal surfacesAleksandr 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 surfacesAleksandr 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 surfacesAleksandr Khalaidovski 15

More about the method

(Journal: Applied Optics)

(Journal: Applied Optics)

Absorption in bulk crystalline silicon and in the crystal surfacesAleksandr Khalaidovski 16

Silicon absorption at 1550 nm-

measurement at a fixed optical power

Absorption in bulk crystalline silicon and in the crystal surfacesAleksandr Khalaidovski 17

Measurement setup

Length 65mm, diameter 100 mm.

Curved end surfaces, ROC = 1m.

Specific resistivity 11 kcm (boron)

Monolithic Si cavity

Coatings: SiO2/Ta2O5. R = 99.96 %.

Absorption in bulk crystalline silicon and in the crystal surfacesAleksandr Khalaidovski 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

Absorption in bulk crystalline silicon and in the crystal surfacesAleksandr Khalaidovski 19

… much higher than expected

Absorption in bulk crystalline silicon and in the crystal surfacesAleksandr 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 surfacesAleksandr Khalaidovski 21

Silicon absorption at 1550 nm-

power-dependent measurements

Absorption in bulk crystalline silicon and in the crystal surfacesAleksandr 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 surfacesAleksandr Khalaidovski 23

Results

Absorption in bulk crystalline silicon and in the crystal surfacesAleksandr 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 surfacesAleksandr 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 surfacesAleksandr 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 surfacesAleksandr 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 surfacesAleksandr 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 surfacesAleksandr 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 surfacesAleksandr 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