Metrology of optical telescope components

Precision astronomic structures such as telescopes often require structural materials that possess ultra low coefficient of thermal expansion (CTE) and coefficient of moisture expansion (CME) so that strict dimensional stability requirements can be met. Composite materials such as carbon fiber reinforced epoxy (CFRE) composites can meet those requirements because of their nearly-zero CTE and potentially low CME. To employ these composite materials for telescope structural design, it is necessary to first develop practical and economical methods to determine their CTE and CME. Although many previous studies have discussed CTE and CME measurements of various materials, none of them suitably serve the purpose of measuring environmentally-induced deformations of in situ telescope structural support members in terms of feasibility, repeatability and economy. Two metrology techniques, suitable for measuring small deformations of large in situ telescope structural support members, have been developed for determining the CTE and CME of carbon fiber reinforced epoxy (CFRE) telescope components. Both are relative (rather than absolute) techniques, measuring the axial deformation of the telescope component with respect to a known reference standard. Two techniques are described: the single-mirror optical lever (SMOL) and the double-mirror optical lever (DMOL). In the first method, the temperature of the test component is varied while the reference standard is maintained at constant temperature, while in the second method the temperatures of the test component and the reference standard are varied together by changing the ambient temperature surrounding both. Quantitative CTE and CME results are reported for CFRE rods. Also, error analysis including random errors and systematic errors are discussed for each measurement. The magnitude of the error reflects the accuracy and reliability of the techniques. The DMOL technique is shown to be a significant improvement upon the first and also more practical and economical than comparable techniques reported in the literature. We believe this DMOL method is superior to metrology methods for large telescope components published to date

Experimental observation of Weyl points

In 1929, Hermann Weyl derived the massless solutions from the Dirac equation - the relativistic wave equation for electrons. Neutrinos were thought, for decades, to be Weyl fermions until the discovery of the neutrino mass. Moreover, it has been suggested that low energy excitations in condensed matter can be the solutions to the Weyl Hamiltonian. Recently, photons have also been proposed to emerge as Weyl particles inside photonic crystals. In all cases, two linear dispersion bands in the three-dimensional (3D) momentum space intersect at a single degenerate point - the Weyl point. Remarkably, these Weyl points are monopoles of Berry flux with topological charges defined by the Chern numbers. These topological invariants enable materials containing Weyl points to exhibit a wide variety of novel phenomena including surface Fermi arcs, chiral anomaly, negative magnetoresistance, nonlocal transport, quantum anomalous Hall effect, unconventional superconductivity[15] and others [16, 17]. Nevertheless, Weyl points are yet to be experimentally observed in nature. In this work, we report on precisely such an observation in an inversion-breaking 3D double-gyroid photonic crystal without breaking time-reversal symmetry.Comment: 4 pages, 3 figure

An Overview of the Capital Raising Activities Among Proptech Firms

This article presents an overview of the capital raising activities among property/real estate technology (i.e. Proptech) firms. This overview highlights the strengths and weaknesses of the Proptech sector in recent years. This article provides a detailed summary and description of how the capital raising activities distribute across different Proptech categories and geographic locations in different market conditions. The authors find that the capital-raising activities in Proptech have cooled down despite the rising real estate prices last year. The authors hope that this review can present a more comprehensive picture of the Proptech development and attract more researchers to investigate the costs and benefits of Proptech to the real estate markets. This research contributes to the understanding of Proptech sector more comprehensively by utilizing a unique hand-collected dataset. The results present a different perspective on the recent trends of Proptech firms as they feature both the promising trends and concerning issues within the field

Stability studies of ZnO and AlN thin film acoustic wave devices in acid and alkali harsh environments

Surface acoustic wave (SAW) devices based on piezoelectric thin-films such as ZnO and AlN are widely used in sensing, microfluidics and lab-on-a-chip applications. However, for many of these applications, the SAW devices will inevitably be used in acid or alkali harsh environments, which may cause their early failures. In this work, we investigated the behavior and degradation mechanisms of thin film based SAW devices in acid and alkali harsh environments. Results show that under the acid and alkali attacks, chemical reaction and corrosion of ZnO devices are very fast (usually within 45 s). During the corrosion, the crystalline orientation of the ZnO film is not changed, but its grain defects are significantly increased and the grain sizes are decreased. The velocity of ZnO-based SAW devices is decreased due to the formation of porous structures induced by the chemical reactions. Whereas an AlN thin-film based SAW device does not perform well in acid–alkali conditions, it might be able to maintain a normal performance without obvious degradation for more than ten hours in acid or alkali solutions. This work could provide guidance for the applications of both ZnO or AlN-based SAW devices in acid/alkali harsh environments