This award supports research to overcome noise sources that are expected to limit the sensitivity of Advanced LIGO. The high-index of refraction coating material to be used initially in Advanced LIGO is the main source of thermal noise. Past research to determined the source of excess loss in a low-index coating material allowed demonstration of how it may be reduced or eliminated. These studies of low-index material provided important indicators to guide the search for the necessary high-index coating but with low mechanical loss. LIGO scientists have studied and reduced the multitude of sources of noise that directly couple into the interferometer, thereby exposing indirect, bilinear noise processes, such as up-conversion of noise produced outside LIGO's band of sensitive frequencies into that band. Bicoherence provides a measure of such bilinear coupling while being insensitive to Gaussian noise. Development of a data monitor for bicoherence will continue under this award. Once identified, this noise can be monitored and if possible, eliminated. The bicoherence calculation engine could be adapted eventually to search for gravitational waves, since inspiral and burst signals both have nonzero bicoherence.
Advanced LIGO, which is scheduled to be fully operational by 2015, has been designed with a sensitivity to allow it to make regular observations of gravitational waves. Improvement in Advanced LIGO sensitivity through improved optical coatings and amelioration of up-conversion noise will allow achievement of the greatest discovery potential. The PI plans to involve several undergraduates in his research. This matches well with Hobart and William Smith 9HWS0 Colleges' ongoing effort to expand their science program. The opportunity to work in research programs like LIGO at HWS is helping to increase the number of physics majors and to change their attitude about a career in physics.
Gravitational Wave Astronomy is one of the most exciting frontiers in physics research today. With the first direct detection of gravitational waves predicted to occur in the next 3-4 years when the current Advanced detectors reach full sensitivity, the physics community will have finally verified the last major prediction of Einstein’s General Theory of Relativity, and, in so doing, inaugurated a new era in astronomy. Gravitational wave observatories will finally allow us to directly "see" the dynamics of black holes, to observe binary neutron star mergers, and possibly even witness the earliest moments of the Big Bang. The NSF-funded, Laser Interferometer Gravitational-wave Observatories (LIGO) are a pair of 4-km long interferometers that are designed to observe gravitational waves by measuring the relative stretching of space-time along the two orthogonal arms of the detector. The LIGO detectors have achieved an unprecedented relative arm length sensitivity of better than 10-18 m, or 1/1000th of the diameter of a proton. This precision means that LIGO has the capability of observing the merger of a binary neutron star system out to a distance of 20 Mpc (6.5 million light-years). Unfortunately, given the density of neutron stars and black holes, frequent observations of gravitational wave events should only be possible if LIGO increases its sensitivity by 10x. So, with more than a decade of basic research and development to inform the new design, LIGO has been upgrades and Advanced LIGO has now been switched on. In a few years, when the new detector has reached full sensitivity, the era of gravitational wave astronomy will finally begin. Gains in the detector sensitivity are achieved through reductions in the sources of noise. Arguably the most challenging noise source, and the one that current limits Advanced LIGO in its central frequency region, is thermal noise in the mirror coatings. The primary goal of the PI's research project is to understand the source of coating thermal noise and to develop improved coatings for the Advanced and Third generation detectors. Coating thermal noise arises from dissipative (inelastic) processes in the coating materials that shift some of the energy from the thermally-driven resonances into the frequency band of the detector. The mirrors operate at room temperature (300 K), so they have the vibrational energy equivalent to that temperature. If the mirror materials were purely elastic, this vibrational motion would be confined to the resonant frequencies of the mirror. Mirror motion at a single, well-defined frequency could be ignored and would not impede the detection of gravitational waves. However, inelasticity (dissipation) in the coating material causes the mirror to have some response off-resonance, incuding frequencies in the gravitational wave detection band. This motion of the mirror face cannot be distinguished from a displacement of the mirror due to a gravitational wave. The strategy in reducing coating thermal noise is to develop mirror coatings with lower dissipation (closer to a purely elastic response). In the development of Advanced LIGO, the PI used a similar research strategy to reduce the dissipation in the mirror substrate material. Now the 15 cm thick fused silica substrate contributes less thermal noise than does the 7 micron thick mirror coating. Optically, the best mirror coatings are formed from multiple layers of dielectric material that alternate between high and low index of refraction. These coatings are applied using an ion-beam sputtering process to produce a uniform, amorphous layer. The randomly ordered structure reduces scattering, but it makes the mechanical properties more difficult to model. What has been determined empirically is that, as one might expect, the coating dissipation depends on material composition. But in addition, if the coatings are annealed (raised to a higher temperature and then allowed to cool slowly) the molecules reorient into a more uniform, though still amorphous, distribution and the dissipation is reduced. Unfortunately in the annealing process if the differential thermal stresses become too great the coating can rupture. Or if the temperature is too high the coating may crystallize. A major effort of the PI’s research is to stabilize these materials so that they can withstand a high temperature anneal and achieve reduced dissipation. The original LIGO mirror coatings were annealed to only 300 C. The PI has now measured silica-stabilized hafnia annealed to 600 C, silica-stabilized zirconia annealed to 700 C, and zirconia-stabilized tantala annealed to 800 C. This final coating has recently completed measurements of its dissipation, its molecular structure, and its elastic modulus. Analysis of the results is ongoing. The PI is also engaged in the development of coatings for a third generation cryogenic detector. The PI is currently analyzing how the dissipation in an amorphous silicon coating is reduced by annealing. And the PI is exploring crystalline coatings formed from AlGaAs. This material has low dissipation but the coating is limited by production size and flaws in substrate adhesion.
