Towards reliable quantification of mixed oxide coating for LIGO using DualEELS

[摘要] Gravitational Waves are a prediction of the General Theory of relativity, the last one to be experimentally verified by big international collaborations, using kilometre-sized interferometric detectors. Although the waves were indirectly observed in the spin-down of pulsars since the 80’s, the direct detections was a significant breakthrough, and opened new vistas for astrophysicists and relativity researchers. The huge interferometers involved in the discovery used highly developed optics for the manipulation (reflection, beam splitting, and detection of interference) of infrared laser beams. The reflectors at the end of each interferometer arm are an integral part of the detector, allowing the enlargement of the optical path of the laser beam and increasing the sensitivity. In order to further improve the sensitivity of these detectors, improvement of these reflectors’ surface and optical characteristics is of paramount importance. In the present thesis, new methods of surface characterization and quantification of the optical surfaces of these mirrors are developed and presented.Highly reflective coating materials for the interferometer mirrors are selected based on their thermal/mechanical properties. It is a well-established fact that the mechanical bulk properties are directly connected to the thermal quantum noise, which is the parameter to minimize in a reflective coating used in a gravitational wave detector. However, the parameter space to explore is vast, including atoms used, composition percentages and thermal processing of the final material, and impossible to analyze well on a trial-and-error basis. Furthermore, the amorphous nature of the coating materials defies a simple theoretical description of the bulk or microscopic properties, and calls for detailed reporting of all processing steps. The microscopic examination of processed, candidate samples can reveal discrepancies between nominal and actual composition, and also give information on the effects of the thermal annealing on the exact atom arrangement in the material.The novelty of our approach is dual: First, we use the concept of ‘standard’ samples, i.e. materials that contain the same or similar atoms as in the coatings under consideration, in similar spatial arrangements. Second, we apply the recent advances in electron microscopy optics and electron energy loss spectrometry (EELS) to obtain signature spectra in energy loss ranges higher than those used before. These two innovations in tandem allow us to study in detail one coating configuration already used in Advanced-LIGO, a working gravitational wave detector, and a new, candidate coating for a next generation detector. The results of our analysis show significant discrepancies between nominal and actual composition values, up to almost 35% in some cases. They also show composition inhomogeneity over the surface of a sample, depending on the thermal history and composition values thereof. The techniques described here can be used for two purposes: On the one hand, they can be used together with mechanical property data from the exact same samples in order to inform a phenomenological theory connecting composition and thermal history to bulk mechanical properties (and hence to thermal noise level). Furthermore, if automated enough, these techniques can be used as a ‘quality control’ method of massively produced coating for a future gravitational wave detector.

[发布机构] University:University of Glasgow;Department:School of Physics and Astronomy

[关键词] EELS, DualEELS, gravitational wave, microscopy.