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Abstract We have investigated crystalline AlGaAs/GaAs optical coatings with three ultra-stable cavities operating at 4 K, 16 K, 124 K and 297 K. The response of the cavities’ resonance frequencies to variations in optical power indicates non-thermal effects beyond the photo-thermo-optic effect observed in dielectric coatings. These effects are strongly dependent on the intensity of the intracavity light at 1.5 μm. When the rear side of the mirrors is illuminated with external light, we observe a prominent photo-modified birefringence for photon energies above the GaAs bandgap, which points to a possible mechanism relating our observations to the semiconductor properties of the coatings. Separately, we also present a low maintenance evolution of our 124 K silicon cavity system where the liquid nitrogen based cooling system is replaced with closed cycle cooling from a pulse-tube cryo-cooler. 

Ceramics are brittle due in large part to the limited availability of energy dissipation pathways when they are subjected to an impact load. The primary avenue for improving the material reliability and energy-absorption capability is to create new energy dissipation mechanisms that can be used to replace or minimize the kinetic energy associated with the debris shattering. In this paper, a computational framework is developed to investigate the relationship between phase composition and energy dissipation pathways in polymer derived ceramic (PDC) composites by accounting for the key processing parameters and deformation/failure mechanisms. It is found that the phase composition that promotes both the Mullins effect and the ligament bridging mechanism can significantly improve the structural integrity of the composite material. A fundamental understanding of how to redistribute the impact energy dissipation in a controllable path would hold great promise for fabricating PDC composites with tailored properties. 

Abstract Surface performance is critically influenced by topography in virtually all real-world applications. The current standard practice is to describe topography using one of a few industry-standard parameters. The most commonly reported number is$$R$$ $R$a, the average absolute deviation of the height from the mean line (at some, not necessarily known or specified, lateral length scale). However, other parameters, particularly those that are scale-dependent, influence surface and interfacial properties; for example the local surface slope is critical for visual appearance, friction, and wear. The present Surface-Topography Challenge was launched to raise awareness for the need of a multi-scale description, but also to assess the reliability of different metrology techniques. In the resulting international collaborative effort, 153 scientists and engineers from 64 research groups and companies across 20 countries characterized statistically equivalent samples from two different surfaces: a “rough” and a “smooth” surface. The results of the 2088 measurements constitute the most comprehensive surface description ever compiled. We find wide disagreement across measurements and techniques when the lateral scale of the measurement is ignored. Consensus is established through scale-dependent parameters while removing data that violates an established resolution criterion and deviates from the majority measurements at each length scale. Our findings suggest best practices for characterizing and specifying topography. The public release of the accumulated data and presented analyses enables global reuse for further scientific investigation and benchmarking. 

Polymer-derived ceramics (PDCs) which are fabricated through pyrolysis of preceramic polymers have attracted increasing attention due to their versatility in structure architecture design and property tailoring. Shaping at the polymer state using 3D printing allows the final ceramic products to exhibit arbitrary shapes and complex architectures that are otherwise impossible to achieve through traditional processing routes. The polymer-to-ceramic phase transition also provides additional space for mechanical property tailoring. A multiscale computational model is developed to explore the phase transition mechanisms and their correlations with processing parameters and mechanical response. Calculations in this work concern PMHS/DVB. Molecular dynamics simulations are carried out first to track the chemical reaction mechanisms and atomic structure evolution. The density of generated gas during pyrolysis is transferred to the finite element model (FEM) for coupled heat transfer and phase transition analysis. FEM calculations reveal the effect of pyrolysis temperature and heating rate on structure-level phase composition and elastic modulus. It is found that there is a threshold of pyrolysis temperature above which full ceramic phase is formed. Higher heating rate promotes ceramization and leads to higher elastic modulus. In addition, volume shrinkage is found to accelerate ceramic formation which slightly enhances material strength. 

Abstract The search for a dark photon holds considerable interest in the physics community. Such a force carrier would begin to illuminate the dark sector. Many experiments have searched for such a particle, but so far it has proven elusive. In recent years the concept of a low mass dark photon has gained popularity in the physics community. Of particular recent interest is the 8 Be and 4 He anomaly, which could be explained by a new fifth force carrier with a mass of 17 MeV/ c 2 . The proposed Darklight experiment would search for this potential low mass force carrier at ARIEL in the 10-20 MeV/ c 2 e + e − invariant mass range. This proceeding will focus on the experimental design and physics case of the Darklight experiment.