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Photonic waveguides are prime candidates for integrated and parallel photonic interconnects. Such interconnects correspond to large-scale vector matrix products, which are at the heart of neural network computation. However, parallel interconnect circuits realized in two dimensions, for example by lithography, are strongly limited in size due to disadvantageous scaling. We use three dimensional (3D) printed photonic waveguides to overcome this limitation. 3D optical-couplers with fractal topology efficiently connect large numbers of input and output channels, and we show that the substrate's footprint area scales linearly. Going beyond simple couplers, we introduce functional circuits for discrete spatial filters identical to those used in deep convolutional neural networks., Comment: 7 pages, 6 figures

We report on a project undertaken in Summer 2017 by pre-university student interns at Haus der Astronomie: point-particle simulations of galaxy collisions with the aim of reproducing observational data from such collisions. We succeeded in providing a visually similar representation of both NGC 5426/7 and the "Antennae" galaxies (NGC 4038/9), and were able to make deductions about the relative positions and orbits of these galaxies. The project is an example for how participants with little more than high-school level previous knowledge can successfully tackle, and understand, advanced topics from current astrophysical research. This report was written by the two participants (M. B.-C. and M. T.), on whose experiences it is based, in collaboration with their supervisor at Haus der Astronomie (M. P.)., Comment: 16 pages, 8 figures. Project presented at the WE Heraeus Summer School "Astronomy From Four Perspectives: The Dark Universe" in Heidelberg on September 1, 2017, and at the German national teacher training in astronomy at the University of Jena on July 3, 2018

Fused silica glass is the preferred material in all applications which require long-term chemical and mechanical stability as well as excellent optical properties. The manufacturing of complex hollow microstructures within transparent fused silica glass is of high interest for, among others, the miniaturization of chemical synthesis towards more versatile, configurable and environmentally friendly flow-through chemistry as well as high-quality optical waveguides or capillaries. However, microstructuring of such complex three-dimensional structures has proven evasive due to the high thermal and chemical stability as well as mechanical hardness of glass. Here we present an approach for the generation of hollow microstructures in fused silica glass with unseen precision and freedom of three-dimensional designs. The process combines the concept of sacrificial template replication with a room-temperature molding process for fused silica glass. The fabricated glass chips are versatile tools for, among other, the advance of miniaturization in chemical synthesis on chip., Comment: 11 pages, 4 figures

Dilational materials are stable three-dimensional isotropic auxetics with an ultimate Poisson's ratio of -1. We design, evaluate, fabricate, and characterize crystalline metamaterials approaching this ideal. To reveal all modes, we calculate the phonon band structures. On this basis, using cubic symmetry, we can unambiguously retrieve all different non-zero elements of the rank-4 effective metamaterial elasticity tensor, from which all effective elastic metamaterial properties follow. While the elastic properties and the phase velocity remain anisotropic, the effective Poisson's ratio indeed becomes isotropic and approaches -1 in the limit of small internal connections. This finding is also supported by independent static continuum-mechanics calculations. In static experiments on macroscopic polymer structures fabricated by three-dimensional printing, we measure Poisson's ratios as low as -0.8 in good agreement with theory. Microscopic samples are also presented., Comment: 8 figures

Conceptually, all conceivable three-dimensional mechanical materials can be built from pentamode materials. Pentamodes also enable to implement three-dimensional transformation acoustics - the analogue of transformation optics. However, pentamodes have not been realized experimentally to the best of our knowledge. Here, we investigate inasmuch the pentamode theoretical ideal suggested by Milton and Cherkaev in 1995 can be approximated by a metamaterial with current state-of-the-art lithography. Using numerical calculations calibrated by our fabricated three-dimensional microstructures, we find that the figure of merit, i.e., the ratio of bulk modulus to shear modulus, can realistically be made as large as about 1,000., Comment: 4 pages and 5 figures

We present the blueprint for a novel negative-index metamaterial. This structure is fabricated via three-dimensional two-photon direct laser writing and silver shadow evaporation. The comparison of measured linear optical spectra with theory shows good agreement and reveals a negative real part of the refractive index at around 3.85 micrometer wavelength - despite the fact that the metamaterial structure is bi-anisotropic due to the lack of inversion symmetry along its surface normal., Comment: 8 pages, 3 figures