Sapphire Could Become Interferometry's Crowning Jewel

Kathleen G. Tatterson

NEDLANDS, Australia -- The gemology community considers sapphire to be one of the most precious stones on Earth, and researchers believe that the blue crystals' use as beamsplitters in laser interferometer gravitational wave detectors could be just as valuable.
Current gravitational wave detectors use fused silica beamsplitters in their Fabry-Perot-Michelson interferometers. Scientists at the University of Western Australia and the University of Paris South are investigating using large, high-quality sapphire crystals in gravitational wave projects.
The research team chose to work with a sapphire sample provided by Crystal Systems of Salem, Mass., because of its high mechanical Q, high thermal conductivity, low acoustic loss and a high stress-to-strain ratio, according to David Blair, a University of Western Australia scientist. "Because of sapphire's extreme hardness and high sound speed (10 kms), it has intrinsically small thermal vibrations, which means that it can allow extremely accurate position measurements less than one billionth of the size of an atom," Blair said.

Crystal bell?
Also, sapphire has the lowest acoustic loss of any material. "It rings like a bell without losing energy," he said. Finally, the crystal has high thermal conductivity, so it doesn't deform when laser coatings on the surface absorb laser light. The team, led by L. Ju, reports in Applied Physical Letters that the use of sapphire beamsplitters in laser interferometer gravitational wave detectors results in lower noise optical losses and birefringence in these devices.
Projects such as the Laser Interferometer Gravitational Wave Observatory (LIGO) joint venture of the California Institute of Technology and Massachusetts Institute of Technology (MIT) could benefit from the improved performance. When fully developed, the technology would improve LIGO's sensitivity by up to 16 times, Blair said. Gravitational wave detectors such as LIGO can "see" cataclysmic events in the universe and explore the nature of gravity.
However, the team still has some concerns about the use of sapphire in laser interferometers. There are still uncertainties about whether larger sapphire samples required for large-scale applications can maintain the optical performance observed in the smaller-scale experiments.