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日的引力波探测是物理学和天文学的一个历史性里程碑。这次探测是对广义相对论的一次重要验证，既证实了引力波的存在，也证实了预测波形的准确性。这一事件也是天文学的一个突破，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，随之而来的是对更高灵敏度的更多观测的期望。提高灵敏度的主要障碍是LIGO反射镜涂层中的热噪声。LIGO使用干涉仪来感测引力波，干涉仪是一个具有4公里长臂的L形探测器。相同的光波从顶点沿着正交的臂发送到镜子。当反射的光束在顶点重新组合时，相位差对应于臂长差，臂长差可以部分地由引力波引起。因此，引力波的探测依赖于端镜表面的精确探测。LIGO在室温或300度（绝对零度以上）下运行。因此，镜子相对较热。这种热能被表示为镜子共振频率下的振动。这些频率远远高于LIGO用来探测引力波的频率。如果反射镜是由理想的弹性材料组成的，这些振动将被忽略，不需要担心。实际上，用于反射镜基板的特殊玻璃是一种近乎理想的弹性材料。然而，高反射镜涂层应用到基板有足够的内部摩擦，它转移了一些镜子的振动能量下降到引力波频率。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具有足够降低的热噪声的反射镜涂层，以使LIGO的灵敏度显着增加。该奖项支持通过降低涂层材料中的耗散或机械损耗来降低涂层热噪声的研究。这种耗散发生在应变的振荡导致状态转变时，例如键角旋转，其在去激发时发射光子或声子。这种两态模型被称为非对称双阱势。通过增加态的能量不对称性，从而降低跃迁几率，可以减少耗散。退火通过允许材料松弛到其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。但是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晶体的混合物，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，通过在加热的基底上沉积涂层，可以获得退火的好处。因为涂层表面分子较少受到约束，所以衬底温度比本体退火温度低得多。该小组将在非晶金属氧化物涂层中测试这一过程，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，从而在没有任何加热过程的情况下有效地退火涂层。最后，该小组将继续与斯坦福大学的研究人员合作研究导电涂层，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.