Highly tunable low-threshold optical parametric oscillation in radially poled whispering gallery resonators

Whispering gallery resonators (WGR's), based on total internal reflection, possess high quality factors in a broad spectral range. Thus, nonlinear optical processes in such cavities are ideally suited for the generation of broadband or tunable electromagnetic radiation. Experimentally and theoretically, we investigate the tunability of optical parametric oscillation in a radially structured WGR made of lithium niobate. With a 1.04 /mum pump wave, the signal and idler waves are tuned from 1.78 to 2.5 \mum - including the point of degeneracy - by varying the temperature between 20 and 62 {\deg}C. A weak off-centering of the radial domain structure extends considerably the tuning capabilities. The oscillation threshold lies in the mW-power range.Comment: 4 pages, 5 figure

Can students be encouraged to read? Experimental evidence from a large lecture

One of the structural problems of introductory lectures is that students’ learning progress is primarily assessed by taking a final exam. Weekly preparation and reading are driven only by self-motivation. Can a student’s decision to complete her weekly assignments be influenced by a simple reminder? In a pre-registered experimental design, we test if personalised reminders from the instructor delivered via text messages contribute to learning outcomes. We assess formative learning via regular quizzes at the beginning of each class, and summative learning via grades in a final exam. We do not find statistically significant differences in learning outcomes, and discuss how design features potentially drive this result. In the conclusion, we stress the importance of experimental design in assessing innovative and new learning techniques

High power and spectral purity continuous-wave photonic THz source tunable from 1 to 4.5 THz for nonlinear molecular spectroscopy

We report a diffraction-limited photonic terahertz (THz) source with linewidth \u3c 10 MHz that can be used for nonlinear THz studies in the continuous wave (CW) regime with uninterrupted tunability in a broad range of THz frequencies. THz output is produced in orientation-patterned (OP) gallium arsenide (GaAs) via intracavity frequency mixing between the two closely spaced resonating signal and idler waves of an optical parametric oscillator (OPO) operating near lambda = 2 mu m. The doubly resonant type II OPO is based on a periodically poled lithium niobate (PPLN) pumped by a single-frequency Yb:YAG disc laser at 1030 nm. We take advantage of the enhancement of both optical fields inside a high-finesse OPO cavity: with 10 W of 1030 nm pump, 100 W of intracavity power near 2 mu m was attained with GaAs inside cavity. This allows dramatic improvement in terms of generated THz power, as compared to the state-of-the art CW methods. We achieved \u3e 25 mu W of single-frequency tunable CW THz output power scalable to \u3e 1 mW with proper choice of pump laser wavelength

Production of Radiobromide: new Nickel Selenide target and optimized separation by dry distillation

Introduction Radioisotopes of bromine are of special interest for nuclear medical applications. The positron emitting isotopes 75Br (T½ = 1.6 h; β+ = 75.5 %) and 76Br (T½ = 16.2 h; β+ = 57 %) have suitable decay properties for molecular imaging with PET, while the Auger electron emitters 77Br (T½ = 57.0 h) and 80mBr (T½ = 4.4 h) as well as the β−-emitter 82Br (T½ = 35.3 h) are useful for internal radiotherapy. 77Br is additionally suited for SPECT. The isotopes 75Br, 76Br and 77Br are usually produced at a cyclotron either by 3He and α-particle induced reactions on natural arsenic or by proton and deuteron induced reactions on enriched selenium isotopes [1]. As target mate-rials for the latter two reactions, earlier ele-mental selenium [2] and selenides of Cu, Ag, Mn, Mo, Cr, Ti, Pb and Sn were investigated [cf. 3–7]. Besides several wet chemical separation techniques the dry distillation of bromine from the irradiated targets was investigated, too [cf. 2, 4, 5]. However, the method needs further development. Nickel selenide was investigated as a promising target to withstand high beam currents, and the dry distillation technique for the isolation of n.c.a. radiobromine from the target was optimized. Material and Methods Crystalline Nickel-(II) selenide (0.3–0.5 g) was melted into a 0.5 mm deep cavity of a 1 mm thick Ni plate covered with a Ni grid. NiSe has a melting point of 959 °C. For development of targeting and the chemical separation, natural target material was used. Irradiations of NiSe were usually performed with protons of 17 MeV using a slanting water cooled target holder at the cyclotron BC1710 [8]. For radiochemical studies a beam current of 3 µA and a beam time of about 1 h were appropriate. To separate the produced no-carrier-added (n.c.a.) radiobromine from the target material a dry distillation method was chosen. The apparatus was developed on the basis of a dry distillation method for iodine [cf. 9,10] and optimized to obtain the bromine as n.c.a. [*Br]bromide in a small volume of sodium hydroxide solution. Changing different components of the apparatus, the dead volume could be minimized and an almost constant argon flow as carrier medium was realized. Various capillaries of platinum, stainless steel and quartz glass with different diameters and lengths were tested to trap the radiobromine. Results and Conclusion Nickel selenide proved successful as target material for the production of radiobromine by proton irradiation with 17 MeV protons. The target was tested so far only at beam currents up to 10 µA, but further investigations are ongoing. The optimized dry distillation procedure allows trapping of 80–90 % of the produced radiobromine in a capillary. For this purpose quartz glass capillaries proved to be most suitable. After rinsing the capillary with 0.1 M NaOH solution the activity can be nearly completely obtained in less than 100 µL solution as [*Br]bromide immediately useable for radiosynthesis. So, the overall separation yield was estimated to 81 ± 5 %. The radionuclidic composition and activity of the separated radiobromide was measured by γ-ray spectrometry. Due to the use of natural selenium the determination of the isotopic purity was not meaningful, but it could be shown that the radiobromine was free from other radioisotopes co-produced in the target material and the backing. The radiochemical purity as well as the specific activity were determined by radio ionchromatography. Further experiments using NiSe produced from nickel and enriched selenium are to be per-formed. The isotopic purity of the produced respective radiobromide, the production yield at high beam currents and the reusability of the target material have to be studied

Quantum Criticality of an Ising-like Spin-1/2 Antiferromagnetic Chain in Transverse Magnetic Field

We report on magnetization, sound velocity, and magnetocaloric-effect measurements of the Ising-like spin-1/2 antiferromagnetic chain system BaCo$_2$V$_2$O$_8$ as a function of temperature down to 1.3 K and applied transverse magnetic field up to 60 T. While across the N\'{e}el temperature of $T_N\sim5$ K anomalies in magnetization and sound velocity confirm the antiferromagnetic ordering transition, at the lowest temperature the field-dependent measurements reveal a sharp softening of sound velocity $v(B)$ and a clear minimum of temperature $T(B)$ at $B^{c,3D}_\perp=21.4$ T, indicating the suppression of the antiferromagnetic order. At higher fields, the $T(B)$ curve shows a broad minimum at $B^c_\perp = 40$ T, accompanied by a broad minimum in the sound velocity and a saturation-like magnetization. These features signal a quantum phase transition which is further characterized by the divergent behavior of the Gr\"{u}neisen parameter $\Gamma_B \propto (B-B^{c}_\perp)^{-1}$. By contrast, around the critical field, the Gr\"{u}neisen parameter converges as temperature decreases, pointing to a quantum critical point of the one-dimensional transverse-field Ising model.Comment: Phys. Rev. Lett., to appea

Cross section measurements on 61Cu for proton beam monitoring above 20 MeV

Introduction All experimental studies involving charged particle induced nuclear reactions require a precise knowledge of monitor reactions. A number of well described proton induced monitor reactions exist in the lower energy range [1], which is covered by most medical cyclotrons. Concerning proton energies above 20 MeV, however, the accuracy of the monitor reactions declines as cross section data becomes scarcer. Furthermore, the growing interest in precise determination of projectile energies by comparing of ratios of monitor reaction cross sections demands new measurements and evaluations of known data for high threshold monitor radionuclides. In this work cross section measurements on the formation of 61Cu were done and energy de-pendent radionuclide ratios were calculated. Material and Methods For investigation of the natCu(p,x)61Cu reaction copper foils of natural isotopic composition (Goodfellow Ltd.) were irradiated. The targets were of 10 and 20 μm thickness, having a diameter of 15 mm. Proton bombardments up to 45 MeV incident energy were done in the stacked-foil arrangement at the accelerator JULIC of the Nuclear Physics Institute (IKP) of the Forschungszentrum Jülich. In addition to an internal irradiation possibility the cyclotron is equipped with an external target station which was used for most experiments. It can adapt standard and slanting solid target holders and is equipped with a water cooled four sector collimator and additional helium cooling of the entry foil. Several irradiations were executed. In each stack, besides copper samples, aluminium absorbers and additional nickel monitor foils were also placed, the latter for the determination of the respective beam current. The produced radioactivity of 61Cu was analysed non-destructively using HPGe γ-ray detectors (EG&G Ortec). Results and Conclusion Reaction cross sections of the natCu(p,x)61Cu process up to 45 MeV were measured and com-pared with existing data from the literature (FIG. 2). Except for the data of Williams et al. our results are in good agreement, showing a maxi-mum of about 165 mbarn at 37.5 MeV proton energy. The overall uncertainty of the new cross section data is between 8 and 10 %. In FIG. 3, the excitation functions of the relevant monitor reactions on Cu are shown. In combination with the excitation function of the natCu(p,xn)62Zn reaction, isotope ratios were calculated which can be used for determination of the proton energy within a target stack in the energy range of 22–40 MeV as described by Piel et al. [3]. FIGURE 4 shows the cross section ratio in dependence of the proton energy. Above this energy, 65Zn could be used to generate isotope ratios for energy determination, although the long half-life (T½ = 244.3 d) of that radionuclide may be a problem. Additional cross section measurements are planned in order to further strengthen the data base of this potential monitor reaction. The results of this work shall be evaluated in the framework of an ongoing Coordinated Research Project of the IAEA

Architecture of the yeast elongator complex

The highly conserved eukaryotic Elongator complex performs specific chemical modifications on wobble base uridines of tRNAs, which are essential for proteome stability and homeostasis. The complex is formed by six individual subunits (Elp1-6) that are all equally important for its tRNA modification activity. However, its overall architecture and the detailed reaction mechanism remain elusive. Here, we report the structures of the fully assembled yeast Elongator and the Elp123 sub-complex solved by an integrative structure determination approach showing that two copies of the Elp1, Elp2, and Elp3 subunits form a two-lobed scaffold, which binds Elp456 asymmetrically. Our topological models are consistent with previous studies on individual subunits and further validated by complementary biochemical analyses. Our study provides a structural framework on how the tRNA modification activity is carried out by Elongator

Modeling plasticity and dysplasia of pancreatic ductal organoids derived from human pluripotent stem cells

Personalized in vitro models for dysplasia and carcinogenesis in the pancreas have been constrained by insufficient differentiation of human pluripotent stem cells (hPSCs) into the exocrine pancreatic lineage. Here, we differentiate hPSCs into pancreatic duct-like organoids (PDLOs) with morphological, transcriptional, proteomic, and functional characteristics of human pancreatic ducts, further maturing upon transplantation into mice. PDLOs are generated from hPSCs inducibly expressing oncogenic GNAS, KRAS, or KRAS with genetic covariance of lost CDKN2A and from induced hPSCs derived from a McCune-Albright patient. Each oncogene causes a specific growth, structural, and molecular phenotype in vitro. While transplanted PDLOs with oncogenic KRAS alone form heterogenous dysplastic lesions or cancer, KRAS with CDKN2A loss develop dedifferentiated pancreatic ductal adenocarcinomas. In contrast, transplanted PDLOs with mutant GNAS lead to intraductal papillary mucinous neoplasia-like structures. Conclusively, PDLOs enable in vitro and in vivo studies of pancreatic plasticity, dysplasia, and cancer formation from a genetically defined background

Comparison of Network Intrusion Detection Performance Using Feature Representation

P. 463-475Intrusion detection is essential for the security of the components of any network. For that reason, several strategies can be used in Intrusion Detection Systems (IDS) to identify the increasing attempts to gain unauthorized access with malicious purposes including those base on machine learning. Anomaly detection has been applied successfully to numerous domains and might help to identify unknown attacks. However, there are existing issues such as high error rates or large dimensionality of data that make its deployment di cult in real-life scenarios. Representation learning allows to estimate new latent features of data in a low-dimensionality space. In this work, anomaly detection is performed using a previous feature learning stage in order to compare these methods for the detection of intrusions in network tra c. For that purpose, four di erent anomaly detection algorithms are applied to recent network datasets using two di erent feature learning methods such as principal component analysis and autoencoders. Several evaluation metrics such as accuracy, F1 score or ROC curves are used for comparing their performance. The experimental results show an improvement for two of the anomaly detection methods using autoencoder and no signi cant variations for the linear feature transformationS