RUI: Investigations of Mirror Coatings for A+ and Third Generation Gravitational Wave Detectors

Hobart And William Smith Colleges, Geneva NY

Investigators

Abstract

This award supports research in gravitational wave detector instrumentation and it addresses the priority areas of NSF's "Windows on the Universe" Big Idea. The Laser Interferometer Gravitational-wave Observatory (LIGO) project has opened new windows to observe and understand the universe. The first direct detection of gravitational waves was also the first observation of the inspiral and merger of binary black holes. While black hole mergers are one of the most energetic events in the universe, they were not visible by electromagnetic telescopes. Now, after viewing several such observations, LIGO is developing a catalogue of the population of binary black hole systems. In addition, LIGO's observation of a binary neutron star merger inaugerated the field of multi-messenger astronomy. These simultaneous observations of gravitational waves and light allow for deeper insights into neutron star structure, and they allow us to test fundamental concepts like the speed of gravitational waves and the Hubble constant. The major challenges in this field are the noise sources that limit sensitivity, most notably thermal noise in LIGO's mirror coatings. LIGO is an interferometer, an L-shaped detector with 4 km long arms, which detects gravitational waves by observing tiny differential stretching in the arms. Identical light waves, emitted at the vertex, pass down each arm to a mirror and are reflected back. Any phase difference in the recombining beams corresponds to a difference in arm length. Thus detecting gravitational waves depends on the precision detection of the surface of the end mirrors, but for LIGO that precision is a daunting one billionth of an atom width. The mirrors, at room temperature (300 K), vibrate due to thermal energy at the mirror's resonant frequencies, which are much higher than the gravitational waves frequencies that LIGO can detect. If the mirrors were composed of an ideal elastic material, these vibrations could be ignored and of no concern. Indeed the fused silica glass used for the mirror substrates is a nearly ideal elastic material. However the highly reflective mirror coatings have internal friction that shifts some of the vibrational energy down to gravitational wave frequencies. That motion, which masks the gravitational wave signal, is mirror coating thermal noise (CTN). This research project is designed to understand and reduce CTN in order to improve LIGO's sensitivity. This project aims to reduce coating thermal noise by lowering the dissipation, or mechanical loss, in the coating materials. This dissipation occurs when a fluctuation in thermal or strain energy causes a dissipative state transition. This dissipative process is commonly modeled as an asymmetric double-well potential. The dissipation is reduced by increasing the energy asymmetry in the states, which lowers the transition probability. The team will investigate crystalline coatings, specifically AlGaAs, which has excellent optical properties. The AlGaAs elastic loss is very low for small samples, but further study is needed to understand the loss for large coatings. The team will also investigate stabilized amorphous dielectric coatings. Amorphous dielectric coatings produced by ion beam sputtering can have excellent optical properties, but they typically have high elastic loss. Annealing lowers the dissipation by allowing the material to relax into its lowest energy state. But annealing is limited by low crystallization temperature for these materials. Stabilized amorphous coatings are mixtures of dielectrics in which material mixture frustrates crystallization and allows a higher annealing temperature and lower elastic loss. The team will collaborate on experiments to test if the effects of annealing can be achieved by heating the substrate during deposition. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.