NSF Award
2208079
In 2015 NSF's LIGO (Laser Interferometer Gravitational-wave Observatory) launched the field of gravitational wave astronomy with the first direct detection of gravitational waves. LIGO detects gravitational waves, the ripples in spacetime, using an “L”-shaped detector, known as an interferometer, with 4 km long arms. Gravitational waves cause tiny differential stretching in the arms, which is measured by reflecting laser light off mirrors at the end of each arm and comparing the reflected beams. One of the main limits to LIGO’s sensitivity is that the mirror surfaces move as a result of thermally-induced vibrations. Known as thermal noise, these vibrations mask the gravitational wave signal. This research project will investigate a means for reducing this “coating thermal noise” by using a mirror coating formed from layers of crystalline semiconductor materials. Initial measurements indicate that this crystalline coating will lower the coating thermal noise by a factor of ten. As a result LIGO will be able to see several times farther out in the universe, with a dramatic increase in its rate of observing black holes and neutron stars. This rapidly growing catalogue of observations will inform current models of the composition, formation, and evolution of our universe. Answering the fundamental questions about the universe are ideas that excite, unite, and inspire all of humankind.<br/><br/>The focus of this research program is the continued development of the GaAs/AlGaAs crystalline coating for use in the next major upgrade of the LIGO detectors. In addition to having excellent optical properties (scatter < 10 ppm, absorption < 1 ppm), these coatings have demonstrated an extremely low elastic loss. The dominant source of coating thermal noise (CTN) for crystalline GaAs/AlGaAs is thermo-optic (TO) noise, which is the combination of thermo-elastic (TE) and thermo-refractive (TR) noises. Using TO optimization, one can adjust the coating layer thicknesses so that the TE and TR effects are cancelling. These TO-optimized coatings have demonstrated a 10× lower CTN than the current LIGO coatings. While these results are extremely encouraging, a great deal of work remains to be able to realize these gains in LIGO mirrors. The measurements, to date, have been performed on small (≤ 75 mm) samples. This project oversees the development of these crystalline coatings to 20- and eventually 30-cm diameters, which are suitable for LIGO. The PI is working with the LIGO Lab to test the surface uniformity and optical properties at increasing sizes. The PI is collaborating with the Syracuse University group on tests of possible electro-optic noise and on the development of a new arm-locking system using 2 µm lasers. The PI is developing a finite element model of the coating to accurately predict the CTN. In parallel he is working with the MIT LIGO Lab group to improve the sensitivity of their CTN experiment so that it is capable of measuring the low CTN observed in GaAs/AlGaAs crystalline coating . The PI is collaborating with colleagues at Embry-Riddle, American, and Stanford to test possible birefringence noise. Finally the PI is exploring interferometer designs that could utilize the currently available 20-cm GaAs/AlGaAs coatings, rather than waiting to deploy these coatings after the 3+ year manufacturing process for 30-cm coatings.<br/><br/>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.
