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，从而开启天文学的新时代。我提出的研究旨在提高这些“第二代”探测器的灵敏度，这对计划中的可能在低温下工作的“第三代”探测器的成功至关重要。引力波是时空曲率的波动，由质量的非对称加速产生，预计会由天体物理物体发射，如吸入双中子星、相互作用的黑洞和超新星。目前的长基线引力波探测器使用激光干涉测量法来监测测试质量的相对位移，测试质量被涂覆成反射镜，在1064nm处具有高反射性。它们像钟摆一样悬挂在空中，间隔达几公里。据预测，引力波会引起臂长小于~10^-19m的位移。这种位移是如此之小，以致于反射镜及其悬架的热诱发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相关的机械耗散已被确定为一个重要的噪声源，这将限制未来探测器的灵敏度。我建议开发减少机械损耗的方法，从而减少反射涂层的热噪声贡献，这对这些探测器的操作至关重要。这将提高未来探测器的灵敏度，使它们能够在宇宙的更大范围内搜索引力波的来源。特别是，来自合并紧密双星系统的引力波信号，如两颗轨道中子星，预计将在涂层热噪声预计将限制未来探测器灵敏度的频带内发射。用于电流探测器的涂层由五氧化二钽（钽）和二氧化硅交替层组成。实验表明，机械损耗主要是由钽层引起的，通过在材料中掺入二氧化钛（二氧化钛）可以降低钽的机械损耗。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的特征，可能与钽-氧键在两个稳定能态之间振荡有关。我将进行一系列的实验，以了解与涂层材料机械损耗相关的基本物理。如上所述，低温损耗测量是了解材料损耗机制的有力方法。特别是，我计划研究低温耗散峰的形状、高度和温度是否可以通过改变掺杂浓度或通过涂层的热处理来改变，目的是首先在钽中建立一个详细的耗散模型。我提议参加LIGO科学协作计划的研究，研究可能的新型涂层材料，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机械损耗和热噪声的涂层设计，并相应地提高先进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“爱因斯坦望远镜”，这是目前欧共体FP7设计研究的主题。

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.