Study of low-loss materials for mirrors used in the next-generation gravitational wave detectors

研究低损耗样品的机械损耗一直是引力波探测领域的一个重要但困难的课题。反射镜内模的热运动被认为是限制引力波探测器灵敏度的基本噪声源之一。热运动的振幅与镜面机械损耗的平方根成正比。因此，研究反射镜材料的本征损耗对于估计引力波探测器的灵敏度极限，并通过寻找损耗更低的材料来提高其灵敏度极限具有重要意义。然而，低损耗样品的损耗由于支持测量而造成损耗，因此难以测量。我们开发了一种测量圆柱形样品机械损耗的方法。通过使用我们的节点支持技术，可以排除由于支持造成的损失。通过系统地分析测量到的损耗，我们分别评估了材料的本征损耗和表面粗糙度造成的损耗。然后，我们测量了晶体各向异性样品，硅和蓝宝石的机械损失，使用节点支持。在室温下，硅样品的质量因数达到1.0 × 10^8。蓝宝石样品的质量因子较低，最大为6.4 x 10^7，分析证实其主要是表面损失。此外，我们系统地测量和比较了不同种类的大块熔融二氧化硅的机械损耗。它们的质量因子范围从7 × 10^5到4 × 10^7，后者是在熔融硅镜面衬底中报道的最高的。我们观察到q的频率依赖性。我们也发现了退火后q的增加。

Investigating the mechanical loss of low-loss samples has been an important, but difficult, subject in the field of detecting gravitational-waves. The thermal motion of the internal mode of the mirrors is considered to be one of the fundamental noise sources which limits the sensitivities of gravitational-wave detectors. The amplitude of the thermal motion is proportional to the square root of the mechanical loss of the mirror. Therefore, it is significant to investigate the intrinsic loss of the materials for the mirror to estimate the sensitivity limit of gravitational-wave detectors, and to improve it by finding lower loss materials. However, the loss of low-loss samples is difficult to measure because of the loss due to the support for the measurement.We have developed a method to measure the mechanical loss of cylindrical samples. By using our nodal support technique, the loss due to the support can be excluded. We evaluated separately the intrinsic loss of the material and the loss due to the surface roughness by systematically analyzing the measured losses.Then, we measured the mechanical loss of crystalline anisotropic samples, silicon and sapphire, using a nodal support. Measured quality factor of the silicon sample reached 1.0 x 10^8 at room temperature. The sapphire sample showed lower quality factors, 6.4 x 10^7 at most, which were analytically confirmed to be dominated by surface loss.Also, we systematically measured and compared the mechanical-losses of various kinds of bulk fused silica. Their quality factors widely ranged from 7 x 10^5 to 4 x 10^7, the latter being the highest reported among fused silica mirror substrates. We observed the frequency dependence of Qs. We also found increase in Qs by an annealing.