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- ItemEmittance Reduction of RF Photoinjector Generated Electron Beams by Transverse Laser Beam Shaping(Bristol : IOP Publ., 2019) Gross, M.; Qian, H.J.; Boonpornprasert, P.; Chen, Y.; Good, J.D.; Huck, H.; Isaev, I.; Koschitzki, C.; Krasilnikov, M.; Lal, S.; Li, X.; Lishilin, O.; Loisch, G.; Melkumyan, D.; Mohanty, S.K.; Niemczyk, R.; Oppelt, A.; Shaker, H.; Shu, G.; Stephan, F.; Vashchenko, G.; Will, I.Laser pulse shaping is one of the key elements to generate low emittance electron beams with RF photoinjectors. Ultimately high performance can be achieved with ellipsoidal laser pulses, but 3-dimensional shaping is challenging. High beam quality can also be reached by simple transverse pulse shaping, which has demonstrated improved beam emittance compared to a transversely uniform laser in the 'pancake' photoemission regime. In this contribution we present the truncation of a Gaussian laser at a radius of approximately one sigma in the intermediate (electron bunch length directly after emission about the same as radius) photoemission regime with high acceleration gradients (up to 60 MV/m). This type of electron bunch is used e.g. at the European XFEL and FLASH free electron lasers at DESY, Hamburg site and is being investigated in detail at the Photoinjector Test facility at DESY in Zeuthen (PITZ). Here we present ray-tracing simulations and experimental data of a laser beamline upgrade enabling variable transverse truncation. Initial projected emittance measurements taken with help of this setup are shown, as well as supporting beam dynamics simulations. Additional simulations show the potential for substantial reduction of slice emittance at PITZ. © Published under licence by IOP Publishing Ltd.
- ItemCharacterization of self-modulated electron bunches in an argon plasma(Bristol : IOP Publ., 2018) Gross, M.; Lishilin, O.; Loisch, G.; Boonpornprasert, P.; Chen, Y.; Engel, J.; Good, J.; Huck, H.; Isaev, I.; Krasilnikov, M.; Li, X.; Niemczyk, R.; Oppelt, A.; Qian, H.; Renier, Y.; Stephan, F.; Zhao, Q.; Brinkmann, R.; Martinez de la Ossa, A.; Osterhoff, J.; Grüner, F.J.; Mehrling, T.; Schroeder, C.B.; Will, I.The self-modulation instability is fundamental for the plasma wakefield acceleration experiment of the AWAKE (Advanced Wakefield Experiment) collaboration at CERN where this effect is used to generate proton bunches for the resonant excitation of high acceleration fields. Utilizing the availability of flexible electron beam shaping together with excellent diagnostics including an RF deflector, a supporting experiment was set up at the electron accelerator PITZ (Photo Injector Test facility at DESY, Zeuthen site), given that the underlying physics is the same. After demonstrating the effect [1] the next goal is to investigate in detail the self-modulation of long (with respect to the plasma wavelength) electron beams. In this contribution we describe parameter studies on self-modulation of a long electron bunch in an argon plasma. The plasma was generated with a discharge cell with densities in the 1013 cm-3 to 1015 cm-3 range. The plasma density was deduced from the plasma wavelength as indicated by the self-modulation period. Parameter scans were conducted with variable plasma density and electron bunch focusing.
- ItemElectronic structure and morphology of dark oxides on zinc generated by electrochemical treatment(London [u.a.] : Royal Society of Chemistry, 2013) Chen, Y.; Schneider, P.; Liu, B.-J.; Borodin, S.; Ren, B.; Erbe, A.Oxides formed by electrochemical treatment of metals frequently have properties and structures very different from the respective bulk oxides. Here, electronic structure and chemical composition were investigated for the oxide formed on polycrystalline zinc after electrochemical oxidation, and after subsequent reduction, in a Na2CO3 electrolyte. Photoluminescence and spectroscopic ellipsometry show the presence of states deep in the ZnO band gap in the oxidized sample, which consists of a highly disordered oxide. These states determine the absorption of light in the visible spectral range. After reduction, the characteristics of the ZnO electronic structure have disappeared, leaving a defect-dominated material with a band gap of ∼1.8 eV. Complementary detailed analysis of the morphology of the resulting surfaces shows hexagon-shaped metallic Zn-"nanoplates" to be formed in the reduction step. The optical appearance of the surfaces is dark, because of their efficient extinction of light over a large part of the visible spectrum. The optical appearance is a result of changed surface morphology and electronic structure of the oxide film. Such materials may possess interesting applications in photocatalysis or photoelectrochemistry.
- ItemHighly-efficient extraction of entangled photons from quantum dots using a broadband optical antenna(London : Nature Publishing Group, 2018) Chen, Y.; Zopf, M.; Keil, R.; Ding, F.; Schmidt, O.G.Many quantum photonic technologies require the efficient generation of entangled pairs of photons, but to date there have been few ways to produce them reliably. Sources based on parametric down conversion operate at very low efficiency per pulse due to the probabilistic generation process. Semiconductor quantum dots can emit single pairs of entangled photons deterministically but they fall short due to the extremely low-extraction efficiency. Strategies for extracting single photons from quantum dots, such as embedding them in narrowband optical cavities, are difficult to translate to entangled photons. Here, we build a broadband optical antenna with an extraction efficiency of 65% ± 4% and demonstrate a highly-efficient entangled-photon source by collecting strongly entangled photons (fidelity of 0.9) at a pair efficiency of 0.372 ± 0.002 per pulse. The high brightness achieved by our source represents a step forward in the development of optical quantum technologies.
- ItemSolid-state ensemble of highly entangled photon sources at rubidium atomic transitions(London : Nature Publishing Group, 2017) Keil, R.; Zopf, M.; Chen, Y.; Höfer, B.; Zhang, J.; Ding, F.; Schmidt, O.G.Semiconductor InAs/GaAs quantum dots grown by the Stranski-Krastanov method are among the leading candidates for the deterministic generation of polarization-entangled photon pairs. Despite remarkable progress in the past 20 years, many challenges still remain for this material, such as the extremely low yield, the low degree of entanglement and the large wavelength distribution. Here, we show that with an emerging family of GaAs/AlGaAs quantum dots grown by droplet etching and nanohole infilling, it is possible to obtain a large ensemble of polarization-entangled photon emitters on a wafer without any post-growth tuning. Under pulsed resonant two-photon excitation, all measured quantum dots emit single pairs of entangled photons with ultra-high purity, high degree of entanglement and ultra-narrow wavelength distribution at rubidium transitions. Therefore, this material system is an attractive candidate for the realization of a solid-state quantum repeater - among many other key enabling quantum photonic elements.
- ItemCellular Deformations Induced by Conical Silicon Nanowire Arrays Facilitate Gene Delivery(Weinheim : Wiley-VCH, 2019) Chen, Y.; Aslanoglou, S.; Gervinskas, G.; Abdelmaksoud, H.; Voelcker, N.H.; Elnathan, R.Engineered cell–nanostructured interfaces generated by vertically aligned silicon nanowire (SiNW) arrays have become a promising platform for orchestrating cell behavior, function, and fate. However, the underlying mechanism in SiNW-mediated intracellular access and delivery is still poorly understood. This study demonstrates the development of a gene delivery platform based on conical SiNW arrays for mechanical cell transfection, assisted by centrifugal force, for both adherent and nonadherent cells in vitro. Cells form focal adhesions on SiNWs within 6 h, and maintain high viability and motility. Such a functional and dynamic cell–SiNW interface features conformational changes in the plasma membrane and in some cases the nucleus, promoting both direct penetration and endocytosis; this synergistically facilitates SiNW-mediated delivery of nucleic acids into immortalized cell lines, and into difficult-to-transfect primary immune T cells without pre-activation. Moreover, transfected cells retrieved from SiNWs retain the capacity to proliferate—crucial to future biomedical applications. The results indicate that SiNW-mediated intracellular delivery holds great promise for developing increasingly sophisticated investigative and therapeutic tools. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
- ItemKey concepts behind forming-free resistive switching incorporated with rectifying transport properties(London : Nature Publishing Group, 2013) Shuai, Y.; Ou, X.; Luo, W.; Mücklich, A.; Bürger, D.; Zhou, S.; Wu, C.; Chen, Y.; Zhang, W.; Helm, M.; Mikolajick, T.; Schmidt, O.G.; Schmidt, H.This work reports the effect of Ti diffusion on the bipolar resistive switching in Au/BiFeO 3/Pt/Ti capacitor-like structures. Polycrystalline BiFeO 3 thin films are deposited by pulsed laser deposition at different temperatures on Pt/Ti/SiO 2/Si substrates. From the energy filtered transmission electron microscopy and Rutherford backscattering spectrometry it is observed that Ti diffusion occurs if the deposition temperature is above 600 C. The current-voltage (I-V) curves indicate that resistive switching can only be achieved in Au/BiFeO 3/Pt/Ti capacitor-like structures where this Ti diffusion occurs. The effect of Ti diffusion is confirmed by the BiFeO 3 thin films deposited on Pt/sapphire and Pt/Ti/sapphire substrates. The resistive switching needs no electroforming process, and is incorporated with rectifying properties which is potentially useful to suppress the sneak current in a crossbar architecture. Those specific features open a promising alternative concept for nonvolatile memory devices as well as for other memristive devices like synapses in neuromorphic circuits.