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

该奖项支持引力波探测器仪器的研究，并解决了NSF“宇宙之窗”大创意的优先领域。激光干涉引力波天文台（LIGO）项目为观察和理解宇宙打开了新的窗口。第一次直接探测到引力波也是第一次观测到双黑洞的螺旋和合并。虽然黑洞合并是宇宙中最具能量的事件之一，但它们无法通过电磁望远镜观测到。现在，在观察了几次这样的观测之后，LIGO正在开发一个二元黑洞系统的目录。此外，LIGO观测到的中子星星并合现象，也为多信使天文学的研究开辟了新的领域。这些同时观测引力波和光的方法可以让我们更深入地了解中子星星的结构，也可以让我们测试引力波的速度和哈勃常数等基本概念。该领域的主要挑战是限制灵敏度的噪声源，最明显的是LIGO反射镜涂层中的热噪声。LIGO是一个干涉仪，一个具有4公里长的臂的L形探测器，通过观察臂中微小的微分拉伸来探测引力波。从顶点发出的相同光波，沿着每条手臂传递到镜子上，然后被反射回来。重组光束中的任何相位差对应于臂长的差。因此，探测引力波取决于对末端反射镜表面的精确探测，但对于LIGO来说，这种精度是原子宽度的十亿分之一。在室温（300 K）下，镜子由于镜子共振频率的热能而振动，这比LIGO可以探测到的引力波频率高得多。如果镜子是由理想的弹性材料组成的，这些振动可以忽略不计，也就没有什么关系了。实际上，用于反射镜基板的熔融石英玻璃是接近理想的弹性材料。然而，高反射率的镜面涂层具有内部摩擦力，可以将一些振动能量转移到引力波频率。这种运动掩盖了引力波信号，是镜面涂层热噪声（CTN）。本研究项目旨在了解和减少CTN，以提高LIGO的灵敏度。该项目旨在通过降低涂层材料中的耗散或机械损耗来降低涂层热噪声。这种耗散发生在热能或应变能的波动引起耗散状态转变时。这种耗散过程通常被建模为非对称双阱势。通过增加态的能量不对称性来减少耗散，这降低了跃迁概率。该团队将研究晶体涂层，特别是具有优异光学性能的AlGaAs。对于小样品，AlGaAs的弹性损耗非常低，但需要进一步研究以了解大涂层的损耗。该团队还将研究稳定的非晶介电涂层。通过离子束溅射产生的非晶介电涂层可以具有优异的光学性质，但是它们通常具有高弹性损耗。退火通过允许材料松弛到其最低能量状态来降低耗散。但由于这些材料的晶化温度较低，退火受到限制。稳定的无定形涂层是金属的混合物，其中材料混合物阻碍结晶并允许更高的退火温度和更低的弹性损失。该团队将合作进行实验，以测试是否可以通过在沉积过程中加热衬底来实现退火的效果。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。

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.