for the ET mirrors ongoing research and current status Ronny Nawrodt Annual Meeting Budapest 24112010 Institut für Festkörperphysik FriedrichSchillerUniversität Jena ID: 921335
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Slide1
Substrate specifications for the ET mirrors - ongoing research and current status -
Ronny NawrodtAnnual Meeting, Budapest24/11/2010Institut für Festkörperphysik, Friedrich-Schiller-Universität JenaSonderforschungsbereich Transregio 7 „Gravitationswellenastronomie“Institute for Gravitational Research, University of GlasgowEinstein Telescope Design Study, WP2 „Suspension“
Slide2Overview of the Talkmotivationmaterial parametersoptical properties of silicon
thermal noiseavailability of bulk materials(ongoing) R&D
#2/193rd Annual Meeting, Budapest
Slide3Motivation#3/193rd Annual Meeting, Budapest
cryogenic mirrorthermal noise
optical requirements
thermal requirements
optimisation
process
based
on requirements and properties of
the materials
Slide4Starting pointET HF detector is based on well known fused silica technology
at room temperature not within the focus of this
talkET LF detector operates at
cryogenic temperatures based on crystalline substrate
materials (sapphire, silicon)initial starting
point: necessary
substrate mass due to radiation pressure noise
needed substrate mass ~200 kg#4
/193rd Annual Meeting, Budapest
Slide5Parameters needed thermal parameterswell known for different materials and impurity levels
mechanical parameterswell known for most bulk materials at 300 K
(good) values/upper limits for coatings/bonds
intensive studies currently ongoing (e.g. mechanical loss)optical
propertiesmost of them known at room
temperaturesome values available temperature dependent – but
often not in the temperature/wavelength range neededall values dependet on impurity
/doping concentration large parameter field#
5/193rd Annual Meeting, Budapest
Slide6Optical Properties of Siliconsilicon – optical material for IR applications (typ. > 2…5 µm)typical applications are in
the MID IR regionoxygen causes local absoption bands around 6 and 9 µm which
are avoided by high purity FZ silicon ( „optical
silicon“)silicon: indirect semiconductor absorption
near or below the gap
energy needs phonons strong temperature
dependencere-emission of a significant amount of absorbed radiation as
luminescence radiation around 1.1 eV not all absorbed photons create
heat calorimetriy measurements#
6/193rd Annual Meeting, Budapest
Slide7Optical Absorption of Siliconsimplified electronic band structure
direct
transition
indirect transition
phonon
contributionk
Dk = 0photons do not carry momentum
Dk = kphonon
Ephoton + Ephonon = E
photons with E < Egap=1.1eV
can
be
absorbed
by
assistance
of
phonons
#
7
/19
3rd Annual Meeting, Budapest
VB
CB
Slide8Optical Absorption of Silicon#8/193rd Annual Meeting, Budapest
300 K
1 phonon
2 phonons
3 phonons
[Keeves et al., J. Appl. Phys.]
photon
phonon
density
of
phonons
is
strongly
temperature
dependent
much
smaller
absorption
can
be
expected
at
low
temperatures
measurements
needed
Slide9Thermo-refractive coefficientimportant for cavity coupler, parameter =dn/dT
unknown at low temperatures measurements exist for n(T) down
to 30 K (only 1 reference available!) n(T0) = const.
(due to 3rd law of thermo-
dynamics) #9/19
3rd Annual Meeting, Budapest
1550 nm
behaviour
unknown
Slide10Thermo-refractive coefficientmost likely: continous decrease of n(T) down to 0 K
suggested value for dn/dT at 20 K: < 10-6 K-1
(conservative value!)extrapolation below 20 K not serious, indications predict
further decrease of dn/dT
#10/193rd Annual Meeting, Budapest
based
on measurementsn ~ 1/E
gap
Slide11Mirror Thermal Noise for ET-LFTN estimates based on 2 ETM, 2 ITM (no beam splitter)#
11/193rd Annual Meeting, Budapest
Silicon
Sapphirecalculated thermo-refractive contribution in silicon
is large due to upper limit value for dn
/dT ~ 10-6 K-1 measurements
neededbulk thermo-elastic noise starts dominating
above ~22 K
Slide12Availability of bulk materialsFused Silicalarge pieces available „simple“ technique due
to amorphous state (fused silica = glass)remelting of small pieces
to one large piece is possibleSapphirelargest
crystal grown: dia. 330 mm x 200 mmcrystal growing techniques
provide larger piecesNo demand for larger pieces
in industry or military applications
High price for large samples can be expected.
Siliconcurrently up to 16 inch diameter available for
semiconductor industryCrystal growing technique allows much larger samples
, industry pushed for 18 and 20 inch samples within
the next 5 years#
12
/19
3rd Annual Meeting, Budapest
Slide13Crystalline Silicon and Size LimitationsCzochralski grownlimit:
mechanical strength of seed crystalhigh oxygen and carbon
concentration (1018 cm-3)Float Zone
grownlimit: inductive remelting of
silicon, cost intensive technique (not needed for standard
semiconductor applications)low impurity concentration
#13/19
3rd Annual Meeting, Budapest
Slide14Influence of impurities on the mechanical lossoxygen causes dissipation peaks in
the mechanical spectrumR&D aim: set an upper
limit on impurity concentrations that are tolerable based on the
thermal noise estimates for CZ silicon#14
/193rd Annual Meeting, Budapest
Si-O-Si induced
Mechanical loss
Slide15LF interferometer – substrate material options#15/193rd Annual Meeting, Budapest
SapphireSiliconmechanical loss++
++mechanical strength+(+)*++optical material
+othermal conductivity++++
polishing-+size availability-…+
+* bond strength
not sufficient (silicate bonding)
Slide16What R&D is needed in the near future? (1)coating researchmechanical parameters (
annealing – loss – scatter)thermal parameters (thermal conductivity, thermal expansion)optical parameters (absorption
, scattering)coating technology#16/19
3rd Annual Meeting, Budapest
Slide17What R&D is needed in the near future? (2)bulk researchbonding techniques
and ist implications on thermal noisemechanical loss vs. impuritiesthermal properties vs. impurities (suspension
elements)optical properties (n(T), dn/dT, scattering, absorption)
#17/193rd Annual Meeting, Budapest
Slide18Current work on optical propertiesmeasurement of dn/dTe.g. record transmission
of Si sample during cooling #18/19
3rd Annual Meeting, Budapest
expected
transmission based on current values for
n(T)
Slide19Summary and ConclusionsET HF detector based on currently available techniquesET LF requirements can
be reached with current upper limits of unknown parameters
availability of the materials under
investigationstrong R&D needed on the material side to get „real“ values
and confirm the assumptions and refine upper
limit estimates#19/19
3rd Annual Meeting, BudapestT ~ 10 K
dia. 450-500 mm
1) thickness: 300 mm (for TN purposes) + additional mass2) thickness
: 460 mm (Tref optimisation with beam splitter needed)
Slide20Daub, Würfel, PRL 74 (1995)3rd Annual Meeting, Budapest
90 K
295 K
measured
absorption of silicon from
luminescence spectra
comparison with
transmission measurements