RUI: Proposal to Investigate Coating and Substrate Thermal Noise for Advanced and Next Generation Gravitational Wave Detectors

Hobart And William Smith Colleges, Geneva NY

Investigators

Abstract

The detection of gravitational waves on 14 September 2015 was an historic milestone in physics and astronomy. The detection was an important validation for General Relativity in both its confirmation of the existence of gravitational waves and in the accuracy of the predicted waveforms. The event was also a breakthrough in astronomy with the first direct detection of a black hole binary system. The era of gravitational wave astronomy has begun and with it comes increased expectations for more observations at greater sensitivity. The main obstacle to improved sensitivity is thermal noise in LIGO's mirror coatings. LIGO senses gravitational waves using an interferometer, an L-shaped detector with 4 km long arms. Identical light waves are sent from the vertex down orthogonal arms to a mirror. When the reflected beams recombine at the vertex the difference in phase corresponds to the arm length difference that can arise, in part, from gravitational waves. Thus the detection of gravitational waves depends on the precision detection of the surface of the end mirrors. LIGO operates at room temperature or 300 above absolute zero. Therefore the mirrors are relatively hot. That thermal energy is expressed as vibrations at the mirror's resonant frequencies. Those frequencies are much higher than the frequencies at which LIGO is designed to detect gravitational waves. If the mirrors were composed of ideal elastic materials, these vibrations would be ignored and of no concern. Indeed the special glass used for the mirror substrates is a nearly ideal elastic material. However the highly reflective mirror coating applied to the substrate has enough internal friction that it shifts some of the mirror's vibrational energy down to gravitational wave frequencies. That motion masks the gravitational wave signal and is termed mirror coating thermal noise. The goal of this research project is to produce a mirror coating with sufficiently reduced thermal noise in order to enable a significant increase in LIGO's sensitivity. This award supports research to reduce coating thermal noise by lowering the dissipation, or mechanical loss, in the coating materials. This dissipation occurs when an oscillation in strain causes a state transition, such as a bond angle rotation, that emits a photon or phonon at de-excitation. This two state model is known as an asymmetric double-well potential. The dissipation is reduced by increasing the energy asymmetry in the states and thus lowering the transition probability. Annealing lowers dissipation by allowing the material to relax into its lowest energy state. It also reduces density fluctuations thereby raising the transition energy. But annealing is limited by low crystallization temperatures. Amorphous coatings are mixtures of high-index metal-oxide dielectrics in which the crystallization temperature is shifted above the effective annealing temperature. Recent advanced in work on amorphous silicon coatings show that the benefits of annealing can be obtained by depositing the coating on a heated substrate. Because the coating surface molecules are less constrained, the substrate temperatures are much less than the bulk annealing temperatures. The group will test this process in amorphous metal-oxide coatings and will investigate whether ion-assisted beam deposition might provide sufficient energy to the surface layer to effectively anneal the coating without any heating process. Finally, the group will continue work with Stanford's researchers on conductive coatings to combat charging noise.