Enabling Astronomy with Gravitational Waves

在接下来的6年里，新一代更灵敏的引力波探测器有望开始寻找引力波的天体物理来源。人们普遍预计，这些仪器将在投入使用后的几周内探测到引力波，开启天文学的新纪元。我提出的这项研究旨在提高这些“第二代”探测器的灵敏度，对于计划中的“第三代”探测器的成功至关重要，这些探测器可能在低温下工作。引力波是时空曲率中的涨落，由质量的不对称加速产生，预计将由激发双星中子星、相互作用的黑洞和超新星等天体发出。目前的长基线引力波探测器使用激光干涉法来监测试块的相对位移，这些试块被镀膜以形成在1064 nm处高反射的反射镜。它们以钟摆的形式悬挂，相距可达几公里。据预测，引力波将在手臂长度上引起小于10^-19米的位移。这种位移非常小，以至于反射镜及其悬浮液的热致振动对探测器的灵敏度构成了一个重要的限制。特别是，与离子束溅射反射镜涂层有关的机械耗散已被认为是一个重要的噪声源，它将限制未来探测器的灵敏度。我建议开发一些方法来减少反射涂层的机械损耗，从而减少热噪声的贡献，这对这些探测器的运行至关重要。这将提高未来探测器的灵敏度，使它们能够在更大范围的宇宙中搜索引力波的来源。特别是，来自两个轨道中子星等紧密结合的双星系统的重力波信号预计将在涂层热噪声预计限制未来探测器灵敏度的频段内发射。电流检测器中使用的涂层由五氧化二钽(Tantala)和二氧化硅交替层组成。实验表明，机械损耗主要来自Tantala层，掺杂二氧化钛(TiO2)可以降低Tantala的机械损耗。我最近的研究表明，在掺杂二氧化钛的钽涂层中存在一个低温耗散峰，这是一种特殊的耗散机制，可能与钽氧键在两个稳定的能态之间振荡有关。我将进行一系列实验，以了解涂层材料中与机械损失相关的基本物理。如上所述，低温损耗测量是了解材料中损耗机制的有力方法。特别是，我计划调查是否可以通过改变掺杂浓度或涂层的热处理来改变低温耗散峰的形状、高度和温度，目的是最初在塔塔拉开发一个详细的耗散模型。我建议参与LIGO科学合作计划对可能的新涂层材料的研究，并开发模型来了解和减少它们的机械耗散。这些研究的结果应该使我能够开发出比目前可能的更低的机械损失和热噪声的涂层设计，并相应地增强先进的引力波探测器的灵敏度。这一提议对于未来低温冷却引力波探测器的成功也是至关重要的，比如目前正在进行的一项ECFP 7设计研究的主题--“爱因斯坦望远镜”。

Within the next 6 years, a new generation of more sensitive gravitational wave detectors is expected to start searching for astrophysical sources of gravitational waves. It is widely expected that these instruments will detect gravitational waves within weeks of becoming operational, beginning a new era of astronomy. The research I propose aims to increase the sensitivity these 'second generation' detectors and is critical for the success of planned 'third generation' detectors, which may operate at cryogenic temperatures. Gravitational waves are fluctuations in the curvature of space-time, produced by the asymmetric acceleration of mass and predicted to be emitted by astrophysical objects such as inspiralling binary neutron stars, interacting black holes and supernovae. Current long-baseline gravitational wave detectors use laser interferometry to monitor the relative displacements of test-masses, which are coated to form mirrors that are highly reflective at 1064 nm. These are suspended as pendulums up to several kilometres apart. Gravitational waves are predicted to induce displacements of less than ~10^-19m in the length of the arms. This displacement is so small that that thermally induced vibrations of the mirrors and their suspensions form an important limit to detector sensitivity. In particular, the mechanical dissipation associated with the ion-beam sputtered mirror coatings has been identified as an important noise source which will limit the sensitivity of future detectors. I propose to develop methods of reducing the mechanical loss, and thus the thermal noise contribution, of the reflective coatings which are crucial to the operation of these detectors. This will increase the sensitivity of future detectors, enabling them to search a larger volume of the Universe for sources of gravitational waves. In particular, gravity wave signals from coalescing compact binary systems, such as two orbiting neutron stars, are predicted to be emitted in the frequency band at which coating thermal noise is expected to limit the sensitivity of future detectors. The coatings used in current detectors consist of alternating layers of tantalum pentoxide (tantala) and silica. Experiments have shown that the mechanical dissipation arises primarily from the tantala layers and that the mechanical loss of tantala can be reduced by doping the material with titanium dioxide (titania). My recent research has shown the presence of a low-temperature dissipation peak in a tantala coating doped with titania, which is characteristic of a particular dissipation mechanism, possibly associated with the tantalum-oxygen bond oscillating between two stable energy states. I would undertake a series of experiments to gain an understanding of the fundamental physics associated with mechanical loss in the coating materials. As noted above, cryogenic loss measurements can be a powerful method of understanding the loss mechanisms in a material. In particular, I plan to investigate whether the shape, height and temperature of the low temperature dissipation peak can be altered by varying the doping concentration, or by heat treatment of the coating, with the aim of developing a detailed model of dissipation initially in tantala. I propose to participate in research planned by the LIGO Scientific Collaboration into possible new coating materials and develop models to understand and reduce their mechanical dissipation. The results of these studies should enable me to develop coating designs with lower mechanical loss, and hence thermal noise, than currently possible, with corresponding enhancement to the sensitivity of advanced gravitational wave detectors. This proposed is also critical for the success of future cryogenically cooled gravitational wave detectors such as the 'Einstein Telescope' which is the subject of a current EC FP7 design study.