2023, Knauer, A., Kolbe, T., Hagedorn, S., Hoepfner, J., Guttmann, M., Cho, H.K., Rass, J., Ruschel, J., Einfeldt, S., Kneissl, M., Weyers, M.

High temperature annealed AlN/sapphire templates exhibit a reduced in-plane lattice constant compared to conventional non-annealed AlN/sapphire grown by metalorganic vapor phase epitaxy (MOVPE). This leads to additional lattice mismatch between the template and the AlGaN-based ultraviolet-C light emitting diode (UVC LED) heterostructure grown on these templates. This mismatch introduces additional compressive strain in AlGaN quantum wells resulting in enhanced transverse electric polarization of the quantum well emission at wavelengths below 235 nm compared to layer structures deposited on conventional MOVPE-grown AlN templates, which exhibit mainly transverse magnetic polarized emission. In addition, high temperature annealed AlN/sapphire templates also feature reduced defect densities leading to reduced non-radiative recombination. Based on these two factors, i.e., better outcoupling efficiency of the transverse electric polarized light and an enhanced internal quantum efficiency, the performance characteristic of far-UVC LEDs emitting at 231 nm was further improved with a cw optical output power of 3.5 mW at 150 mA.

2020, Trager-Cowan, C., Alasmari, A., Avis, W., Bruckbauer, J., Edwards, P.R., Hourahine, B., Kraeusel, S., Kusch, G., Jablon, B.M., Johnston, R., Martin, R.W., Mcdermott, R., Naresh-Kumar, G., Nouf-Allehiani, M., Pascal, E., Thomson, D., Vespucci, S., Mingard, K., Parbrook, P.J., Smith, M.D., Enslin, J., Mehnke, F., Kneissl, M., Kuhn, C., Wernicke, T., Knauer, A., Hagedorn, S., Walde, S., Weyers, M., Coulon, P.-M., Shields, P.A., Zhang, Y., Jiu, L., Gong, Y., Smith, R.M., Wang, T., Winkelmann, A.

In this article we describe the scanning electron microscopy (SEM) techniques of electron channelling contrast imaging and electron backscatter diffraction. These techniques provide information on crystal structure, crystal misorientation, grain boundaries, strain and structural defects on length scales from tens of nanometres to tens of micrometres. Here we report on the imaging and analysis of dislocations and sub-grains in nitride semiconductor thin films (GaN and AlN) and tungsten carbide-cobalt (WC-Co) hard metals. Our aim is to illustrate the capability of these techniques for investigating structural defects in the SEM and the benefits of combining these diffraction-based imaging techniques.

2022, Porteanu, Horia-Eugen, Kaempf, Rudolf, Flisgen, Thomas, Heinrich, Wolfgang

The microwave properties of a number of polymers common in industry are investigated. A cylindrical resonator in the TM012 mode is used. The cavity perturbation method and detailed COMSOL simulations are applied for extracting the complex permittivity as a function of temperature. The results are useful for the design of plastic processing tools by heating with electromagnetic fields. The intrinsic parameters of absorption are derived based on two exponential decays: polarization and Arrhenius dependence of the decay times on temperature.

2020, Assmann, R. W., Weikum, M. K., Akhter, T., Alesini, D., Alexandrova, A. S., Anania, M. P., Andreev, N. E., Andriyash, I., Artioli, M., Aschikhin, A., Audet, T., Jafarinia, F. J., Jakobsson, O., Jaroszynski, D. A., Jaster-Merz, S., Joshi, C., Kaluza, M., Kando, M., Karger, O. S., Karsch, S., Khazanov, E., Bacci, A., Khikhlukha, D., Kirchen, M., Kirwan, G., Kitégi, C., Knetsch, A., Kocon, D., Koester, P., Kononenko, O. S., Korn, G., Kostyukov, I., Barna, I. F., Kruchinin, K. O., Labate, L., Le Blanc, C., Lechner, C., Lee, P., Leemans, W., Lehrach, A., Li, X., Li, Y., Libov, V., Bartocci, S., Lifschitz, A., Lindstrøm, C. A., Litvinenko, V., Lu, W., Lundh, O., Maier, A. R., Malka, V., Manahan, G. G., Mangles, S. P. D., Marcelli, A., Bayramian, A., Marchetti, B., Marcouillé, O., Marocchino, A., Marteau, F., Martinez de la Ossa, A., Martins, J. L., Mason, P. D., Massimo, F., Mathieu, F., Maynard, G., Beaton, A., Mazzotta, Z., Mironov, S., Molodozhentsev, A. Y., Morante, S., Mosnier, A., Mostacci, A., Müller, A. -S., Murphy, C. D., Najmudin, Z., Nghiem, P. A. P., Beck, A., Nguyen, F., Niknejadi, P., Nutter, A., Osterhoff, J., Oumbarek Espinos, D., Paillard, J. -L., Papadopoulos, D. N., Patrizi, B., Pattathil, R., Pellegrino, L., Bellaveglia, M., Petralia, A., Petrillo, V., Piersanti, L., Pocsai, M. A., Poder, K., Pompili, R., Pribyl, L., Pugacheva, D., Reagan, B. A., Resta-Lopez, J., Beluze, A., Ricci, R., Romeo, S., Rossetti Conti, M., Rossi, A. R., Rossmanith, R., Rotundo, U., Roussel, E., Sabbatini, L., Santangelo, P., Sarri, G., Bernhard, A., Schaper, L., Scherkl, P., Schramm, U., Schroeder, C. B., Scifo, J., Serafini, L., Sharma, G., Sheng, Z. M., Shpakov, V., Siders, C. W., Biagioni, A., Silva, L. O., Silva, T., Simon, C., Simon-Boisson, C., Sinha, U., Sistrunk, E., Specka, A., Spinka, T. M., Stecchi, A., Stella, A., Bielawski, S., Stellato, F., Streeter, M. J. V., Sutherland, A., Svystun, E. N., Symes, D., Szwaj, C., Tauscher, G. E., Terzani, D., Toci, G., Tomassini, P., Bisesto, F. G., Torres, R., Ullmann, D., Vaccarezza, C., Valléau, M., Vannini, M., Vannozzi, A., Vescovi, S., Vieira, J. M., Villa, F., Wahlström, C. -G., Bonatto, A., Walczak, R., Walker, P. A., Wang, K., Welsch, A., Welsch, C. P., Weng, S. M., Wiggins, S. M., Wolfenden, J., Xia, G., Yabashi, M., Boulton, L., Zhang, H., Zhao, Y., Zhu, J., Zigler, A., Brandi, F., Brinkmann, R., Briquez, F., Brottier, F., Bründermann, E., Büscher, M., Buonomo, B., Bussmann, M. H., Bussolino, G., Campana, P., Cantarella, S., Cassou, K., Chancé, A., Chen, M., Chiadroni, E., Cianchi, A., Cioeta, F., Clarke, J. A., Cole, J. M., Costa, G., Couprie, M. -E., Cowley, J., Croia, M., Cros, B., Crump, P. A., D’Arcy, R., Dattoli, G., Del Dotto, A., Delerue, N., Del Franco, M., Delinikolas, P., De Nicola, S., Dias, J. M., Di Giovenale, D., Diomede, M., Di Pasquale, E., Di Pirro, G., Di Raddo, G., Dorda, U., Erlandson, A. C., Ertel, K., Esposito, A., Falcoz, F., Falone, A., Fedele, R., Ferran Pousa, A., Ferrario, M., Filippi, F., Fils, J., Fiore, G., Fiorito, R., Fonseca, R. A., Franzini, G., Galimberti, M., Gallo, A., Galvin, T. C., Ghaith, A., Ghigo, A., Giove, D., Giribono, A., Gizzi, L. A., Grüner, F. J., Habib, A. F., Haefner, C., Heinemann, T., Helm, A., Hidding, B., Holzer, B. J., Hooker, S. M., Hosokai, T., Hübner, M., Ibison, M., Incremona, S., Irman, A., Iungo, F.

This report presents the conceptual design of a new European research infrastructure EuPRAXIA. The concept has been established over the last four years in a unique collaboration of 41 laboratories within a Horizon 2020 design study funded by the European Union. EuPRAXIA is the first European project that develops a dedicated particle accelerator research infrastructure based on novel plasma acceleration concepts and laser technology. It focuses on the development of electron accelerators and underlying technologies, their user communities, and the exploitation of existing accelerator infrastructures in Europe. EuPRAXIA has involved, amongst others, the international laser community and industry to build links and bridges with accelerator science — through realising synergies, identifying disruptive ideas, innovating, and fostering knowledge exchange. The Eu-PRAXIA project aims at the construction of an innovative electron accelerator using laser- and electron-beam-driven plasma wakefield acceleration that offers a significant reduction in size and possible savings in cost over current state-of-the-art radiofrequency-based accelerators. The foreseen electron energy range of one to five gigaelectronvolts (GeV) and its performance goals will enable versatile applications in various domains, e.g. as a compact free-electron laser (FEL), compact sources for medical imaging and positron generation, table-top test beams for particle detectors, as well as deeply penetrating X-ray and gamma-ray sources for material testing. EuPRAXIA is designed to be the required stepping stone to possible future plasma-based facilities, such as linear colliders at the high-energy physics (HEP) energy frontier. Consistent with a high-confidence approach, the project includes measures to retire risk by establishing scaled technology demonstrators. This report includes preliminary models for project implementation, cost and schedule that would allow operation of the full Eu-PRAXIA facility within 8—10 years.

2021, Hjort, Filip, Enslin, Johannes, Cobet, Munise, Bergmann, Michael A., Gustavsson, Johan, Kolbe, Tim, Knauer, Arne, Nippert, Felix, Häusler, Ines, Wagner, Markus R., Wernicke, Tim, Kneissl, Michael, Haglund, Åsa

Ultraviolet light is essential for disinfection, fluorescence excitation, curing, and medical treatment. An ultraviolet light source with the small footprint and excellent optical characteristics of vertical-cavity surface-emitting lasers (VCSELs) may enable new applications in all these areas. Until now, there have only been a few demonstrations of ultraviolet-emitting VCSELs, mainly optically pumped, and all with low Al-content AlGaN cavities and emission near the bandgap of GaN (360 nm). Here, we demonstrate an optically pumped VCSEL emitting in the UVB spectrum (280-320 nm) at room temperature, having an Al0.60Ga0.40N cavity between two dielectric distributed Bragg reflectors. The double dielectric distributed Bragg reflector design was realized by substrate removal using electrochemical etching. Our method is further extendable to even shorter wavelengths, which would establish a technology that enables VCSEL emission from UVA (320-400 nm) to UVC (<280 nm). © 2020 American Chemical Society. All rights reserved.

2023, Tyborski, Christoph, Hassan, Muhammad T., Flisgen, Thomas, Schiemangk, Max, Wicht, Andreas

We determine Faraday rotations and measure the optical reflection and transmission from magneto-optical Cd1−xMnxTe crystals with various stoichiometric ratios. For wavelengths between 675 and 1025 nm, we derive Verdet constants, optical loss coefficients, and the complex indices of reflection that are relevant measures to find suitable stoichiometric ratios of Cd1−xMnxTe for the realization of miniaturized optical isolators. By reflection and transmission measurements, we determine the stoichiometric ratios of several different Cd1−xMnxTe crystals and discuss the observed dependence of the optical properties on the stoichiometric ratio with respect to their use in optical isolators. Finally, we show the relevant figure of merit, i.e., the ratio of Verdet constants and optical loss coefficients for Cd1−xMnxTe crystals with Mn contents ranging from x = 0.14 to x = 0.50.

2020, Gwaro, Jared O., Brenner, Carsten, Theurer, L.S., Maiwald, M., Sumpf, Bernd, Hofmann, Martin R.

We analyse the use of a tunable dual wavelength Y-branch DBR laser diode for THz applications. The laser generates electrically tunable THz difference frequencies in the range between 100 and 300 GHz. The optical beats are tuned via current injection into a micro-resistor heater integrated on top of one of the distributed Bragg reflector (DBR) section of the diode. The laser is integrated in a homodyne THz system employing fiber coupled ion-implanted LT-GaAs log spiral antennas. The applicability of the developed system in THz spectroscopy is demonstrated by evaluating the spectral resonances of a THz filter as well as in THz metrology in thickness determination of a polyethylene sample.

2021, Torun, Cem Güney, Schneider, Philipp-Immanuel, Hammerschmidt, Martin, Burger, Sven, Munns, Joseph H. D., Schröder, Tim

Nanostructures can be used for boosting the light outcoupling of color centers in diamond; however, the fiber coupling performance of these nanostructures is rarely investigated. Here, we use a finite element method for computing the emission from color centers in inverted nanocones and the overlap of this emission with the propagation mode in a single-mode fiber. Using different figures of merit, the inverted nanocone parameters are optimized to obtain maximal fiber coupling efficiency, free-space collection efficiency, or rate enhancement. The optimized inverted nanocone designs show promising results with 66% fiber coupling or 83% free-space coupling efficiency at the tin-vacancy center zero-phonon line wavelength of 619 nm. Moreover, when evaluated for broadband performance, the optimized designs show 55% and 76% for fiber coupling and free-space efficiencies, respectively, for collecting the full tin-vacancy emission spectrum at room temperature. An analysis of fabrication insensitivity indicates that these nanostructures are robust against imperfections. For maximum emission rate into a fiber mode, a design with a Purcell factor of 2.34 is identified. Finally, possible improvements offered by a hybrid inverted nanocone, formed by patterning into two different materials, are investigated and increase the achievable fiber coupling efficiency to 71%. © 2021 Author(s).

2020, Brunner, F., Cancellara, L., Hagedorn, S., Albrecht, M., Weyers, M.

The effect of high-temperature annealing (HTA) at 1700 °C on AlN films grown on 4H-SiC substrates by metalorganic vapor phase epitaxy has been studied. It is shown that the structural quality of the AlN layers improves significantly after HTA similar to what has been demonstrated for AlN grown on sapphire. Dislocation densities reduce by one order of magnitude resulting in 8 × 108 cm-2 for a-type and 1 × 108 cm-2 for c-type dislocations. The high-temperature treatment removes pits from the surface by dissolving nanotubes and dislocations in the material. XRD measurements prove that the residual strain in AlN/4H-SiC is further relaxed after annealing. AlN films grown at higher temperature resulting in a lower as-grown defect density show only a marginal reduction in dislocation density after annealing. Secondary ion mass spectrometry investigation of impurity concentrations reveals an increase of Si after HTA probably due to in-diffusion from the SiC substrate. However, C concentration reduces considerably with HTA that points to an efficient carbon removal process (i.e., CO formation). © 2020 Author(s).

2020, Tsao, Yi-Fan, Würfl, Joachim, Hsu, Heng-Tung

In this paper, we propose a new configuration for improving the isolation bandwidth of MMIC single-pole-double-throw (SPDT) passive high-electron-mobility transistor (HEMT) switches operating at millimeter frequency range. While the conventional configuration adopted open-stub loading for compensation of the off-state capacitance, radial stubs were introduced in our approach to improve the operational bandwidth of the SPDT switch. Implemented in 0.15 m GaAs pHEMT technology, the proposed configuration exhibited a measured insertion loss of less than 2.5 dB with better than 30 dB isolation level over the frequency range from 33 GHz to 44 GHz. In terms of the bandwidth of operation, the proposed configuration achieved a fractional bandwidth of 28.5% compared to that of 12.3% for the conventional approach. Such superior bandwidth performance is mainly attributed to the less frequency dependent nature of the radial stubs.