M. R. Abernathy

Calibration of the Advanced LIGO detectors for the discovery of the binary black-hole merger GW150914

In Advanced LIGO, detection and astrophysical source parameter estimation of the binary black hole merger GW150914 requires a calibrated estimate of the gravitational-wave strain sensed by the detectors. Producing an estimate from each detector’s differential arm length control loop readout signals requires applying time domain filters, which are designed from a frequency domain model of the detector’s gravitational-wave response. The gravitational-wave response model is determined by the detector’s opto-mechanical response and the properties of its feedback control system. The measurements used to validate the model and characterize its uncertainty are derived primarily from a dedicated photon radiation pressure actuator, with cross-checks provided by optical and radio frequency references. We describe how the gravitational-wave readout signal is calibrated into equivalent gravitational-wave-induced strain and how the statistical uncertainties and systematic errors are assessed. Detector data collected over 38 calendar days, from September 12 to October 20, 2015, contain the event GW150914 and approximately 16 days of coincident data used to estimate the event false alarm probability. The calibration uncertainty is less than 10% in magnitude and 10° in phase across the relevant frequency band, 20 Hz to 1 kHz.

Order within disorder: the atomic structure of ion-beam sputtered amorphous tantala (a-Ta2O5)

Amorphous tantala (a-Ta2O5) is a technologically important material often used in high-performance coatings. Understanding this material at the atomic level provides a way to further improve performance. This work details extended X-ray absorption fine structure measurements of a-Ta2O5 coatings, where high-quality experimental data and theoretical fits have allowed a detailed interpretation of the nearest-neighbor distributions. It was found that the tantalum atom is surrounded by four shells of atoms in sequence; oxygen, tantalum, oxygen, and tantalum. A discussion is also included on how these models can be interpreted within the context of published crystalline Ta 2O5 and other a-T2O5 studies.

Investigation of the Young’s modulus and thermal expansion of amorphous titania-doped tantala films

The current generation of advanced gravitational wave detectors utilize titania-doped tantala/silica multilayer stacks for their mirror coatings. The properties of the low-refractive-index silica are well known; however, in the absence of detailed direct measurements, the material parameters of Youngs modulus and coefficient of thermal expansion (CTE) of the high refractive index material, titania-doped tantala, have been assumed to be equal to values measured for pure tantala coatings. In order to ascertain the true values necessary for thermal noise calculations, we have undertaken measurements of Youngs modulus and CTE through the use of nanoindentation and thermal-bending measurements. The measurements were designed to assess the effects of titania-doping concentration and post-deposition heat-treatment on the measured values in order to evaluate the possibility of optimizing material parameters to further improve thermal noise in the detector. Youngs modulus measurements on pure tantala and 25% and 55% titania-doped tantala show a wide range of values, from 132 to 177 GPa, which are dependent on both titania concentration and heat-treatment. Measurements of CTE give values of (3.9±0.1)×10⁻⁶ K⁻¹ and (4.9±0.3)×10⁻⁶ K⁻¹ for 25% and 55% titania-doped tantala, respectively, without dependence on post-deposition heat-treatment.

Measurement of mechanical loss in the Acktar Black coating of silicon wafers

Some proposed interferometric gravitational wave detectors of the next generation are designed to use silicon test masses cooled to cryogenic temperatures. The test masses will need to be partially coated with high emissivity coating to provide sufficient cooling when they absorb the laser light. The mechanical loss of the Acktar Black coating is determined based on the measurements of the Q-factors of the bending vibration modes of coated and uncoated commercial silicon wafers. The Youngs modulus of the coating material is determined using nanoindentation. We use this information to calculate thermal noise of the silicon test masses associated with a high emissivity coating on its lateral side (barrel). It is found that such a coating results in a less than 9% increase of the total strain noise of LIGO Voyager design for a future cryogenic gravitational wave detector.

Improving the mechanical quality factor of ultra-low-loss silicon resonators

In its as-fabricated state, a silicon mechanical resonator with a very high quality factor at liquid helium temperatures is found to have two energy loss mechanisms which can be removed with a 3 h anneal at 300 °C. Because of the silicon wafer processing history, these mechanisms are likely introduced during the resonator fabrication process. One energy loss mechanism contributes to the overall background damping over the entire measured temperature range, 400 mK ≤ T ≤ 300 K, at a level of Δ Q − 1 ≈ 3 × 10 − 9, and gradually reappears after aging on the order of 100 d timescales. The second energy loss mechanism is a broad peak, Δ Q − 1 ≈ 2 × 10 − 8, centered near 100 K. This peak does not re-appear upon aging and is tentatively attributed to the tetrafluoromethane reactive ion etch step, despite the fact that the silicon resonator is protected with silicon nitride and photoresist during the process.In its as-fabricated state, a silicon mechanical resonator with a very high quality factor at liquid helium temperatures is found to have two energy loss mechanisms which can be removed with a 3 h anneal at 300 °C. Because of the silicon wafer processing history, these mechanisms are likely introduced during the resonator fabrication process. One energy loss mechanism contributes to the overall background damping over the entire measured temperature range, 400 mK ≤ T ≤ 300 K, at a level of Δ Q − 1 ≈ 3 × 10 − 9, and gradually reappears after aging on the order of 100 d timescales. The second energy loss mechanism is a broad peak, Δ Q − 1 ≈ 2 × 10 − 8, centered near 100 K. This peak does not re-appear upon aging and is tentatively attributed to the tetrafluoromethane reactive ion etch step, despite the fact that the silicon resonator is protected with silicon nitride and photoresist during the process.

Investigating the atomic structure and properties ofTa 2 O 5 coatings

The atomic structure and properties of Ta2O5 coatings are studied using a combination of X-ray absorption spectroscopy, transmission electron microcopy and atomic modeling, which is then correlated to macroscopic material properties.

Investigation of the Young's modulus and thermal expansion of amorphous titania-doped tantala films

Nano-indentation and thermal bending techniques are used to measure the Youngs modulus and thermal expansion of tantala coatings as a function of heat-treatment and titania-doping.