Collaborative Research: LSC Center for Coatings Research

LIGO科学合作（LSC）涂层研究中心（CCR）由美国国家科学基金会（NSF）资助，戈登和贝蒂·摩尔基金会（Gordon and Betty Moore Foundation）计划共同资助，旨在通过解决限制其性能的主要噪声源，即干涉仪反射镜中的热噪声，来扩展下一代引力波探测器的范围。这种噪声减少了来自天文来源的可观测引力波信号的数量。它是由LIGO探测器光学中反射镜振动模式的热激发引起的。随着反射镜的机械质量因子(Q)的增加，这些激励的影响减小。由于反射镜的Q值受到其表面反射涂层的限制，因此降低噪声需要开发更好的涂层。CCR联合了来自美国10个机构的团队，致力于非晶材料的计算建模、涂层的沉积以及其原子结构和宏观特性的表征。这些组件通常由三个不同的社区执行，它们彼此相对隔离地工作。CCR的力量及其加速发现的承诺源于这三个专注于统一研究目标的社区的密切整合。涂层热噪声也是精密定时、量子信息、低噪声干涉测量和精密测量（如寻找引力平方反比定律的偏差）等领域的限制因素。根据该计划开发的涂层在机械和光学性能方面取得了进步，也将适用于这些社区。在更广泛的教育影响方面，本科院校与精英研究生课程的结合为所有教育水平的学生提供了研究机会，并为希望在物理科学领域从事职业的学生建立了明确的晋升途径。此外，CCR还坚定致力于提高妇女和代表性不足的少数民族的地位；它还将通过所有主要的公共和社会媒体渠道，继续其参与者在教育问题和公众宣传方面的活动。计划于2021年投入使用的A+ LIGO探测器将通过压缩光注入来减少量子噪声，使热噪声在探测器的中波段占主导地位。减少这种噪声源需要减少测试质量上反射镜涂层的机械耗散。该项目的目标是开发符合A+ LIGO机械和光学要求的镜面涂层。实现这一目标需要解决一个长期存在于非晶态材料物理学中的问题：非晶态氧化物的低能量激发的性质和控制。在更长的时间尺度上，为低温的LIGO旅行者探测器开发镜面涂层，扩大了包括非晶或晶体半导体的可能性。主要的研究重点是寻找可以沉积非晶金属氧化物涂层作为“超稳定玻璃”的条件，这种玻璃具有低密度的结构基元，形成两能级系统，这是弹性耗散的来源。第二个研究方向是寻求在高温退火过程中稳定涂层以防止结晶的方法，这是另一种已知的减少室温弹性损失的方法。对于LIGO Voyager应用的长期研究，该小组将研究非氧化物涂层，如单晶半导体AlGaAs和AlGaP，它们在小几何形状下显示出较低的机械损失，但其规模扩大到LIGO光学器件的尺寸是具有挑战性的；还有非晶硅和氮化硅，它们的力学性能很好，但光学性能还有待提高。

The LIGO Scientific Collaboration (LSC) Center for Coatings Research (CCR), funded by the NSF and with planned co-funding by the Gordon and Betty Moore Foundation, seeks to extend the reach of the next generation of gravitational wave detectors by addressing the dominant noise source limiting their performance, thermal noise in the interferometer mirrors. This noise reduces the number of observable gravitational wave signals from astronomical sources. It arises from thermal excitation of the vibrational modes of the mirrors in the LIGO detector optics. The effect of these excitations is reduced as the mechanical quality factor (Q) of the mirrors is increased. Since the Q of the mirrors is limited by the reflective coatings deposited on their surfaces, lower noise requires development of better coatings. The CCR combines groups from 10 institutions in the US working on computational modeling of amorphous materials, deposition of coatings, and characterization of their atomic structure and macroscopic properties. These components are often performed by three diverse communities that work in relative isolation from each other. The strength of the CCR and its promise of accelerating discoveries arises from close integration of these three communities focused on a unified research goal. Coating thermal noise is also a limiting factor in the fields of precision timing, quantum information, low noise interferometry, and precision measurements like the search for deviations in the gravitational inverse-square law. Coatings improving on the state of the art in mechanical and optical properties developed under this program would be applicable to these communities as well. On broader impacts in education, the mixture of undergraduate institutions with elite graduate programs provides research opportunities at all education levels and establishes clear avenues of advancement for students who wish to pursue a career within the physical sciences. In addition, the CCR has a strong commitment to the advancement of women and underrepresented minorities; and will also continue its participants' activities in issues of education and public outreach through all major channels of public and social media.The A+ LIGO detector, planned for 2021, will reduce quantum noise with squeezed light injection, leaving thermal noise dominant in the mid-band of the detector. Reducing this noise source requires reducing the mechanical dissipation in the mirror coatings on the test masses. The goal of this project is to develop mirror coatings consistent with A+ LIGO's mechanical and optical requirements. Meeting this goal requires solution of a longstanding problem in the physics of amorphous materials: the nature and control of the low-energy excitations in amorphous oxides. On a longer time scale, developing mirror coatings for the cryogenic LIGO Voyager detector broadens the possibilities to include amorphous or crystalline semiconductors. The primary research focus is to find conditions under which amorphous metal-oxide coatings can be deposited as "ultrastable glasses", which have a low density of the structural motifs that form two-level systems, the source of elastic dissipation. A second research track seeks methods to stabilize coatings against crystallization during high temperature annealing, another method known to reduce room temperature elastic losses. For the longer term research towards LIGO Voyager applications, the group will investigate non-oxide coatings such as single-crystal semiconductors AlGaAs and AlGaP, which have shown low mechanical loss for small geometries but whose scale up to the size of LIGO optics is challenging; and also amorphous silicon and silicon nitride, which have attractive mechanical properties but whose optical properties must be improved.