57 research outputs found
Mechanical loss in state-of-the-art amorphous optical coatings
We present the results of mechanical characterizations of many different
high-quality optical coatings made of ion-beam-sputtered titania-doped tantala
and silica, developed originally for interferometric gravitational-wave
detectors. Our data show that in multi-layer stacks (like high-reflection Bragg
mirrors, for example) the measured coating dissipation is systematically higher
than the expectation and is correlated with the stress condition in the sample.
This has a particular relevance for the noise budget of current advanced
gravitational-wave interferometers, and, more generally, for any experiment
involving thermal-noise limited optical cavities.Comment: 31 pages, 14 figure
Correlated evolution of structure and mechanical loss of a sputtered silica film
Energy dissipation in amorphous coatings severely affects high-precision
optical and quantum transducers. In order to isolate the source of coating
loss, we performed an extensive study of Raman scattering and mechanical loss
of a thermally-treated sputtered silica coating. Our results show that loss is
correlated with the population of three-membered rings of Si-O tetrahedral
units, and support the evidence that thermal treatment reduces the density of
metastable states separated by a characteristic energy of about 0.5 eV, in
favour of an increase of the states separated by smaller activation energies.
Finally, we conclude that three-fold rings are involved in the relaxation
mechanisms only if they belong to more complex chain-like structures of 10 to
100 tetrahedra.Comment: 5 pages, 3 figure
Effect of heating treatment and mixture on optical properties of coating materials used in gravitational-wave detectors
The interferometer mirrors of Gravitational-Wave Detectors (GWD) are Bragg reflectors made of alternate
amorphous silica (SiO2) and titania-doped tantala (TiO2:Ta2O5) layers as low- and high-refractive index
material, respectively. A thermal treatment is usually performed to reduce both mechanical losses and NIR
optical absorptions of the coatings. We present a spectroscopic ellipsometry (SE) investigation of the effect
of annealing and Ti:Ta mixing on Ta2O5 coatings deposited under conditions similar to those adopted for
building up mirrors of GWDs. The broad-band analysis covers both the NIR and the fundamental absorption
threshold region. The data show an evident annealing-induced reduction of the fundamental optical absorption
broadening. Modelling the data through the Cody-Lorentz formula confirms that NIR absorption are below
the SE sensitivity and shows a notable annealing-induced reduction of so-called Urbach tails. Titania-doping
of tantala slightly reduces the Urbach energy. After the heating treatment the resulting Urbach energy of
the doped material is lower than the one of annealed pure tantala. The observed reduction of Urbach tails is
important because it parallels the reduction of so-called internal friction observed in mechanical measurements.
So that SE emerges as a convenient tool for an agile diagnostic of both optical and mechanical quality of
amorphous oxide coatings
Observation of a Correlation Between Internal friction and Urbach Energy in Amorphous Oxides Thin Films
We have investigated by spectroscopic ellipsometry (SE, 190-1700 nm) the optical properties of uniform, amorphous thin films of Ta2O5 and Nb2O5 as deposited and after annealing, and after so-called "doping" with Ti atoms which leads to mixed oxides. Ta2O5 and Ti:Ta2O5 are currently used as high-index components in Bragg reflectors for Gravitational Wave Detectors. Parallel to the optical investigation, we measured the mechanical energy dissipation of the same coatings, through the so-called "loss angle" \u3d5\u2009=\u2009Q-1, which quantifies the energy loss in materials. By applying the well-known Cody-Lorentz model in the analysis of SE data we have been able to derive accurate information on the fundamental absorption edge through important parameters related to the electronic density of states, such as the optical gap (Eg) and the energy width of the exponential Urbach tail (the Urbach energy EU). We have found that EU is neatly reduced by suitable annealing as is also perceptible from direct inspection of SE data. Ti-doping also points to a minor decrease of EU. The reduction of EU parallels a lowering of the mechanical losses quantified by the loss angle \u3d5. The correlation highlights that both the electronic states responsible of Urbach tail and the internal friction are sensitive to a self-correlation of defects on a medium-range scale, which is promoted by annealing and in our case, to a lesser extent, by doping. These observations may contribute to a better understanding of the relationship between structural and mechanical properties in amorphous oxides
Measurements of mechanical thermal noise and energy dissipation in optical dielectric coatings
In recent years an increasing number of devices and experiments are shown to
be limited by mechanical thermal noise. In particular sub-Hertz laser frequency
stabilization and gravitational wave detectors, that are able to measure
fluctuations of 1E-18 m/rtHz or less, are being limited by thermal noise in the
dielectric coatings deposited on mirrors. In this paper we present a new
measurement of thermal noise in low absorption dielectric coatings deposited on
micro-cantilevers and we compare it with the results obtained from the
mechanical loss measurements. The coating thermal noise is measured on the
widest range of frequencies with the highest signal to noise ratio ever
achieved. In addition we present a novel technique to deduce the coating
mechanical losses from the measurement of the mechanical quality factor which
does not rely on the knowledge of the coating and substrate Young moduli. The
dielectric coatings are deposited by ion beam sputtering. The results presented
here give a frequency independent loss angle of (4.70 0.2)x1E-4 with a
Young's modulus of 118 GPa for annealed tantala from 10 Hz to 20 kHz. For
as-deposited silica, a weak frequency dependence (~ f^{-0.025}) is observed in
this frequency range, with a Young's modulus of 70 GPa and an internal damping
of (6.0 0.3)x1E-4 at 16 kHz, but this value decreases by one order of
magnitude after annealing and the frequency dependence disappears.Comment: Accepted for publication in Phys. Rev.
TiO2 doping effect on reflective coating mechanical loss for gravitational wave detection at low temperature
We measured the mechanical loss of a dielectric multilayer reflective coating
(ion-beam-sputtered SiO2 and Ta2O5) with and without TiO2 on sapphire disks
between 6 and 77 K. The measured loss angle exhibited a temperature dependence,
and the local maximum was found at approximately 20 K. This maximum was
7.0*10^(-4) (with TiO2) and 7.7*10^(-4) (without TiO2), although the previous
measurement for the coating on sapphire disks showed almost no temperature
dependence (Phys. Rev. D 74 022002 (2006)). We evaluated the coating thermal
noise in KAGRA and discussed future investigation strategies
Pendulum Mode Thermal Noise in Advanced Interferometers: A comparison of Fused Silica Fibers and Ribbons in the Presence of Surface Loss
The use of fused-silica ribbons as suspensions in gravitational wave
interferometers can result in significant improvements in pendulum mode thermal
noise. Surface loss sets a lower bound to the level of noise achievable, at
what level depends on the dissipation depth and other physical parameters. For
LIGO II, the high breaking strength of pristine fused silica filaments, the
correct choice of ribbon aspect ratio (to minimize thermoelastic damping), and
low dissipation depth combined with the other achievable parameters can reduce
the pendulum mode thermal noise in a ribbon suspension well below the radiation
pressure noise. Despite producing higher levels of pendulum mode thermal noise,
cylindrical fiber suspensions provide an acceptable alternative for LIGO II,
should unforeseen problems with ribbon suspensions arise.Comment: Submitted to Physics Letters A (Dec. 14, 1999). Resubmitted to
Physics Letters A (Apr. 3, 2000) after internal (LSC) review process. PACS -
04.80.Nn, 95.55.Ym, 05.40.C
A new method of probing mechanical losses of coatings at cryogenic temperatures
A new method of probing mechanical losses and comparing the corresponding
deposition processes of metallic and dielectric coatings in 1-100 MHz frequency
range and cryogenic temperatures is presented. The method is based on the use
of extremely high-quality quartz acoustic cavities whose internal losses are
orders of magnitude lower than any available coatings nowadays. The approach is
demonstrated for Chromium, Chromium/Gold and a multilayer tantala/silica
coatings. The coating has been found to
exhibit a loss angle lower than near 30 {\rm MHz} at 4 {\rm
K}. The results are compared to the previous measurements
A Systematic Error in the Internal Friction Measurement of Coatings for Gravitational Waves Detectors
Low internal friction coatings are key components of advanced technologies
such as optical atomic clocks and high-finesse optical cavity and often lie at
the forefront of the most advanced experiments in Physics. Notably, increasing
the sensitivity of gravitational-wave detectors depends in a very large part on
developing new coatings, which entails developing more suitable methods and
models to investigate their loss angle. In fact, the most sensitive region of
the detection band in such detectors is limited by the coating thermal noise,
which is related to the loss angle of the coating. Until now, models which
describe only ideal physical properties have been adopted, wondering about the
use of one or more loss angles to describe the mechanical properties of
coatings. Here we show the presence of a systematic error ascribed to
inhomogeneity of the sample at its edges in measuring the coating loss angle.
We present a model for disk-shaped resonators, largely used in loss angle
measurements, and we compare the theory with measurements showing how this
systematic error impacts on the accuracy with which the loss model parameters
are known
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