RUI: Proposal to Investigate Coating and Substrate Thermal Noise for Advanced and Next Generation Gravitational Wave Detectors

RUI：研究先进和下一代引力波探测器的涂层和基底热噪声的提案

• 批准号: 1611821
• 负责人: Steven Penn
• 金额: $ 24万
• 依托单位: Hobart and William Smith Colleges
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2016
• 资助国家: 美国
• 起止时间: 2016-07-01 至 2019-10-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1611821&HistoricalAwards=false
• 关键词: RUI; Proposal; Investigate; Coating; Substrate

The detection of gravitational waves on 14 September 2015 was an historic milestone in physics and astronomy. The detection was an important validation for General Relativity in both its confirmation of the existence of gravitational waves and in the accuracy of the predicted waveforms. The event was also a breakthrough in astronomy with the first direct detection of a black hole binary system. The era of gravitational wave astronomy has begun and with it comes increased expectations for more observations at greater sensitivity. The main obstacle to improved sensitivity is thermal noise in LIGO's mirror coatings. LIGO senses gravitational waves using an interferometer, an L-shaped detector with 4 km long arms. Identical light waves are sent from the vertex down orthogonal arms to a mirror. When the reflected beams recombine at the vertex the difference in phase corresponds to the arm length difference that can arise, in part, from gravitational waves. Thus the detection of gravitational waves depends on the precision detection of the surface of the end mirrors. LIGO operates at room temperature or 300; above absolute zero. Therefore the mirrors are relatively hot. That thermal energy is expressed as vibrations at the mirror's resonant frequencies. Those frequencies are much higher than the frequencies at which LIGO is designed to detect gravitational waves. If the mirrors were composed of ideal elastic materials, these vibrations would be ignored and of no concern. Indeed the special glass used for the mirror substrates is a nearly ideal elastic material. However the highly reflective mirror coating applied to the substrate has enough internal friction that it shifts some of the mirror's vibrational energy down to gravitational wave frequencies. That motion masks the gravitational wave signal and is termed mirror coating thermal noise. The goal of this research project is to produce a mirror coating with suffi;ciently reduced thermal noise in order to enable a significant increase in LIGO's sensitivity. This award supports research to reduce coating thermal noise by lowering the dissipation, or mechanical loss, in the coating materials. This dissipation occurs when an oscillation in strain causes a state transition, such as a bond angle rotation, that emits a photon or phonon at de-excitation. This two state model is known as an asymmetric double-well potential. The dissipation is reduced by increasing the energy asymmetry in the states and thus lowering the transition probability. Annealing lowers dissipation by allowing the material to relax into its lowest energy state. It also reduces density fluctuations thereby raising the transition energy. But annealing is limited by low crystallization temperatures. Amorphous coatings are mixtures of high-index metal-oxide dielectrics in which the crystallization temperature is shifted above the effective annealing temperature. Recent advanced in work on amorphous silicon coatings show that the benefits of annealing can be obtained by depositing the coating on a heated substrate. Because the coating surface molecules are less constrained, the substrate temperatures are much less than the bulk annealing temperatures. The group will test this process in amorphous metal-oxide coatings and will investigate whether ion-assisted beam deposition might provide suffi;cient energy to the surface layer to effectively anneal the coating without any heating process. Finally, the group will continue work with Stanford's researchers on conductive coatings to combat charging noise.

2015 年 9 月 14 日探测到引力波是物理学和天文学的历史性里程碑。这次探测是对广义相对论的重要验证，无论是确认引力波的存在还是预测波形的准确性。该事件也是天文学的一项突破，首次直接探测到黑洞双星系统。引力波天文学的时代已经开始，随之而来的是人们对以更高灵敏度进行更多观测的期望越来越高。提高灵敏度的主要障碍是 LIGO 镜面涂层中的热噪声。 LIGO 使用干涉仪（一个具有 4 公里长臂的 L 形探测器）来感知引力波。相同的光波从顶点沿着正交臂发送到镜子。当反射光束在顶点重新组合时，相位差对应于部分由引力波引起的臂长差。因此，引力波的检测取决于端镜表面的精确检测。 LIGO 在室温或 300 下运行；高于绝对零。因此镜子相对较热。该热能表示为镜子共振频率的振动。这些频率远高于 LIGO 设计用来探测引力波的频率。如果镜子由理想的弹性材料组成，这些振动将被忽略并且无需担心。事实上，用于镜子基板的特殊玻璃是一种近乎理想的弹性材料。然而，涂覆在基底上的高反射镜涂层具有足够的内摩擦力，可以将镜子的一些振动能量降低到引力波频率。这种运动掩盖了引力波信号，被称为镜面涂层热噪声。该研究项目的目标是生产一种能够充分降低热噪声的镜面涂层，从而显着提高 LIGO 的灵敏度。该奖项支持通过降低涂层材料的耗散或机械损失来减少涂层热噪声的研究。当应变振荡引起状态转变（例如键角旋转）时，就会发生这种耗散，从而在去激发时发射光子或声子。这种两种状态模型被称为不对称双阱势。通过增加状态中的能量不对称性来减少耗散，从而降低跃迁概率。退火通过使材料松弛到最低能量状态来降低耗散。它还减少了密度波动，从而提高了跃迁能量。但退火受到低结晶温度的限制。非晶涂层是高折射率金属氧化物电介质的混合物，其中结晶温度高于有效退火温度。非晶硅涂层研究的最新进展表明，通过将涂层沉积在加热的基材上可以获得退火的好处。由于涂层表面分子受到的约束较少，因此基材温度远低于整体退火温度。该小组将在非晶金属氧化物涂层中测试这一工艺，并将研究离子辅助束沉积是否可以为表面层提供足够的能量，从而在不进行任何加热过程的情况下有效地对涂层进行退火。最后，该小组将继续与斯坦福大学的研究人员合作开发导电涂层，以对抗充电噪音。

项目成果

Mechanical ringdown studies of large-area substrate-transferred GaAs/AlGaAs crystalline coatings

大面积基底转移 GaAs/AlGaAs 晶体涂层的机械衰荡研究

• DOI: 10.1364/josab.36.000c15
• 发表时间: 2019
• 期刊: Journal of the Optical Society of America B
• 作者: Penn, Steven D.;Kinley-Hanlon, Maya M.;MacMillan, Ian A. O.;Heu, Paula;Follman, David;Deutsch, Christoph;Cole, Garrett D.;Harry, Gregory M.
• 通讯作者: Harry, Gregory M.

High Precision Detection of Change in Intermediate Range Order of Amorphous Zirconia-Doped Tantala Thin Films Due to Annealing

高精度检测非晶氧化锆掺杂钽薄膜因退火引起的中程序变化

• DOI: 10.1103/physrevlett.123.045501
• 发表时间: 2019
• 期刊: Physical Review Letters
• 影响因子: 8.6
• 作者: Prasai, K.;Jiang, J.;Mishkin, A.;Shyam, B.;Angelova, S.;Birney, R.;Drabold, D. A.;Fazio, M.;Gustafson, E. K.;Harry, G.
• 通讯作者: Harry, G.