Collaborative Research: Stanford-Florida program in Support of LIGO on Coatings and Core Optics

高级LIGO探测器对黑洞合并过程中引力波的探测，开启了引力波天文学的新领域。多次提高LIGO探测器的灵敏度将增加引力波的数量和观察到的事件类型。未来的探测器，如计划于2021年发射的A+ LIGO探测器，将受到与探测器光学系统中使用的镜面涂层相关的热噪声的限制。拟议的工作是斯坦福大学的Martin费耶尔小组和佛罗里达大学的Hai-Ping Cheng小组之间的合作努力，旨在开发具有较低热噪声的反射镜涂层，以解决A+ LIGO及以后的问题。斯坦福大学引力波研究计划已经参与了二十多年的研究，通过与LIGO科学合作组织（LSC）密切合作，使引力波探测器能够进行关键研究并减轻困难的挑战。在过去，斯坦福大学对新型干涉仪组件和设计的开发、光学的详细研究、减轻光学和热噪声以及先进LIGO（aLIGO）中使用的先进地震隔离系统做出了广泛的贡献。佛罗里达的程海萍小组从事计算材料模拟。为了支持LIGO，她对非晶薄膜的原子结构进行了建模，并评估了与这些结构相关的机械损失，作为LSC开发低热噪声镜面涂层的更广泛努力的一部分。虽然先进的LIGO现在已经有足够的灵敏度来探测黑洞的合并，但它的中波段灵敏度将受到镜面涂层中机械耗散产生的热噪声的限制。斯坦福大学在开发实验方法以表征组成多层介质镜的非晶材料的光学、弹性和结构特性方面在LSC中发挥了主导作用。佛罗里达在LSC内进行当前的计算材料建模工作。拟议的计划是一个协同团队，将这些技能组合联合收割机，以解决一个关键问题，以满足A+ LIGO的设计目标，开发机械损失比目前最好的少2-4倍的反射镜。非晶材料的机械损耗取决于其原子结构中的细微的、依赖于结晶的特征。通过电子衍射和X射线散射方法获得的这些结构特征的数据在这里提出的是具有挑战性的解释，因为是分子动力学预测的结构。存在使用建模来帮助解释数据和数据来帮助约束建模的方法，这导致了这里提出的团队安排。弹性损失对材料成分和工艺条件的依赖性的结构数据和预测将成为更广泛的LSC计划的主要贡献者，为A+ LIGO开发镜子，指导其他人通过可能的合成和表征实验来解决这个问题。斯坦福大学的另一项长期努力是在亚ppm/cm水平上对低光损耗材料进行光学表征，这可以追溯到最初LIGO测试质量的二氧化硅和蓝宝石之间的选择。该小组最近开始使用为这些研究开发的干涉测量工具来表征单晶硅样品中的低温损失，以评估它们作为计划中的低温LIGO Voyager中的测试质量的适用性。

The detections of gravitational waves from coalescing black holes by the Advanced LIGO detectors has launched the field of gravitational wave astronomy. Increasing the sensitivity of the LIGO detector several times would increase the number of gravitational waves and the types of events observed. Future detectors, such as the A+ LIGO detector, planned for 2021, will be limited by thermal noise associated with the mirror coatings used in the detector optics. The proposed work is a collaborative effort between Martin Fejer's group at Stanford University and Hai-Ping Cheng's group at the University of Florida to develop mirror coatings with lower thermal noise to address this problem for A+ LIGO and beyond. The Stanford gravitational wave research program has been involved for more than two decades in research to enable gravitational wave detectors by working closely with the LIGO Science Collaboration (LSC) to do critical research and mitigate difficult challenges. In the past, Stanford has contributed broadly to the development of novel interferometer components and design, detailed studies of the optics and mitigating optical and thermal noise, and the advanced seismic isolation systems used in Advanced LIGO (aLIGO). Hai-Ping Cheng's group at Florida is involved in computational materials simulations. In support of LIGO, she has modeled atomic structure of amorphous films and evaluated mechanical losses associated with those structures as part of a broader LSC effort to develop low-thermal-noise mirror coatings. While Advanced LIGO has now operated with adequate sensitivity to detect black hole coalescences, its mid-band sensitivity will be limited by thermal noise resulting from mechanical dissipation in the mirror coatings. Stanford has had a leading role within the LSC in developing experimental methods to characterize the optical, elastic, and structural properties of the amorphous materials composing multilayer dielectric mirrors. Florida carries out the current computational materials modeling effort within LSC. The proposed program is a synergistic teaming to combine these skill sets to address a critical issue to meet the design goals of A+ LIGO, developing mirrors with 2-4 times less mechanical loss than the best currently available. The mechanical losses in amorphous materials depend on subtle, preparation-dependent features in their atomic structure. Data on these structural features obtained via the electron diffraction and X-ray scattering methods proposed here is challenging to interpret, as are molecular dynamics predictions of the structure. Methods exist to use the modeling to help interpret the data and the data to help constrain the modeling, which led to the teaming arrangement proposed here. The structural data and predictions for dependence of elastic losses on material composition and process conditions, will become a major contributor to the broader LSC program to develop mirrors for A+ LIGO, guiding the others working on this problem through the thicket of possible synthesis and characterization experiments. Another long-standing effort at Stanford has been in the optical characterization of low-optical loss materials at the sub-ppm/cm level, dating back to the selection between silica and sapphire for initial LIGO test masses. The group has recently begun using the interferometric tool developed for those studies to characterize cryogenic losses in single-crystal silicon samples to evaluate their suitability as test masses in the planned cryogenic LIGO Voyager.