RUI: Investigations of Mirror Thermal Noise for Gravitational Wave Detectors

2015年，NSF的LIGO（激光干涉引力波天文台）首次直接探测到引力波，从而开创了引力波天文学领域。LIGO探测引力波，时空中的涟漪，使用一个“L”形的探测器，被称为干涉仪，有4公里长的手臂。引力波会在臂上产生微小的差异拉伸，这是通过将激光反射到每个臂末端的镜子上并比较反射光束来测量的。LIGO灵敏度的主要限制之一是镜面由于热致振动而移动。这些振动被称为热噪声，掩盖了引力波信号。本研究项目将研究通过使用由晶体半导体材料层形成的镜面涂层来降低这种“涂层热噪声”的方法。初步测量表明，这种结晶涂层将降低涂层热噪声的一个因素的十。因此，LIGO将能够在宇宙中看到更远的几倍，其观测黑洞和中子星的比率将大幅增加。这一快速增长的观测目录将为我们宇宙的组成、形成和演化的当前模型提供信息。探索宇宙的基本问题是激发、团结和激励全人类的想法。该研究计划的重点是继续开发用于LIGO探测器下一次重大升级的GaAs/AlGaAs晶体涂层。 除了具有优异的光学性能（散射10 ppm，吸收1 ppm）外，这些涂层还表现出极低的弹性损耗。晶体GaAs/AlGaAs的涂层热噪声（CTN）的主要来源是热光噪声（TO），它是热弹性噪声（TE）和热折射噪声（TR）的组合。使用TO优化，可以调整涂层厚度，使得TE和TR效应抵消。这些TO优化的涂层已经证明CTN比目前的LIGO涂层低10倍。虽然这些结果非常令人鼓舞，但要在LIGO反射镜中实现这些增益，还有大量的工作要做。迄今为止，测量是在小样本（≤ 75 mm）上进行的。该项目监督这些晶体涂层的发展，直径为20厘米，最终为30厘米，适用于LIGO。PI正在与LIGO实验室合作，以测试越来越大尺寸的表面均匀性和光学特性。PI正在与锡拉丘兹大学的小组合作，测试可能的电光噪声，并开发一种使用2微米激光的新臂锁定系统。PI正在开发涂层的有限元模型，以准确预测CTN。与此同时，他正在与麻省理工学院LIGO实验室小组合作，提高CTN实验的灵敏度，以便能够测量GaAs/AlGaAs晶体涂层中观察到的低CTN。PI正在与美国安柏瑞德大学和斯坦福大学的同事合作，测试可能的双折射噪声。最后，PI正在探索可以利用目前可用的20 cm GaAs/AlGaAs涂层的干涉仪设计，而不是等待在30 cm涂层的3年以上制造过程后部署这些涂层。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.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.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.