Spintronic terahertz emitters based on ferro- and ferrimagnetic thin film systems

Elektromagnetische Strahlung im Terahertzfrequenzbereich von 0.1 bis 30 THz bietet zahlreiche Anwendungsmöglichkeiten, beispielsweise in Spektroskopie- und Bildgebungsverfahren, sowohl im Bereich der Grundlagenforschung, als auch für industrielle Prozesse. Da der Terahertzspektralbereich jedoch zwischen den mit etablierten elektronischen und optischen Emittern gut zugänglichen Mikrowellen- und Infrarotspektralbereichen liegt, sind Emittersysteme immer noch teuer und limitiert in Bezug auf Leistung und Bandbreite. Ein neues vielversprechendes Konzept für die Terahertzerzeugung stellen die sogenannten spintronischen Emittersysteme dar. Diese bestehen aus Multilagensystemen ferromagnetischer (FM) und nichtmagnetischer (NM) Metallschichten mit Schichtdicken im Bereich weniger Nanometer. Die Anregung einer FM/NM-Bilage mit einem femtosekundenlangen optischen Laserpuls führt zur Ausbildung eines spinpolarisierten Ladungsstromes Js von der FM in die NM Schicht, der auf der Anregung spinpolarisierter Elektronen der FM Schicht über das Ferminiveau beruht. In der NM Schicht wird Js auf Grund des inversen Spin-Hall-Effekts in einen transversalen Ladungsstrompuls Jc konvertiert, der zur Emission elektromagnetischer Strahlung im Terahertzfrequenzbereich führt. Die Abstrahlcharakteristik der Emitter kann durch Variation der verwendeten Materialien, Schichtdicken, oder durch die Verwendung komplexerer Multilagensysteme optimiert werden. Die vorliegende Arbeit präsentiert Studien zu spintronischen Terahertzemittersystemen basierend auf verschiedenen magnetischen dünnen Filmen kombiniert mit Pt und W Schichten. Die Abhandlung kann in zwei Teilbereiche untergliedert werden. Das Ziel der ersten drei Studien war die Untersuchung des Einflusses der magnetischen Eigenschaften unterschiedlicher FM und insbesondere auch ferrimagnetischer (FI) Legierungsschichten auf die Terahertzemission. Hierfür wurden Bilagensysteme bestehend aus NM Pt Schichten in Kombination mit FM CoFe, sowie FI TbFe und GdFe Legierungsschichten mit variierendem Eisengehalt (0 ≤ x ≤ 1) hergestellt. Die laserangeregte Terahertzemission wurde in Abhängigkeit des angelegten Magnetfelds, der Fluenz des Anregungslasers und der Temperatur gemessen. Die Ergebnisse wurden unter Einbeziehung detaillierter Untersuchungen der strukturellen, magnetischen, elektrischen und optischen Eigenschaften der Proben erklärt. Der zweite Teilbereich der Arbeit befasst sich mit der Entwicklung komplexerer Multilagenterahertzemittersysteme, welche das Schalten der Amplitude zwischen Zuständen hoher und niedriger Terahertzemission ermöglichen und zudem Potential für eine Steigerung der Terahertzamplitude bieten. Hierfür wurde, basierend auf den Terahertzemissioncharakteristiken des zuvor untersuchten FI Pt/GdFe Systems, ein Emitter entwickelt, der das Schalten der Terahertzamplitude durch Veränderung der Temperatur ermöglicht. In einer weiteren Studie wurde zudem die Anwendung eines Spin-Valve-Systems als magnetisch schaltbares Emittersystem demonstriert. Dieses ermöglicht ein reversibles Schalten der Emissionsamplitude mit kleinen angelegten magnetischen Feldern in der Größenordnung weniger Millitesla.Electromagnetic radiation in the terahertz (THz) frequency range from 0.1 to 30 THz can be highly useful for spectroscopy and imaging experiments in fundamental scientific research as well as for industrial applications. However, as THz regime bridges the gap between electronic and optical frequencies, emitter systems are still expensive and limited in power and bandwidth. A novel approach to overcome these challenges is given by the so-called spintronic terahertz emitters, which are based on ferromagnetic (FM) and non-magnetic metal (NM) layers with thicknesses of a few nanometers. Excitation of a FM/NM bilayer with a femtosecond optical laser pump pulse leads to the formation of an ultrafast spin current Js from the FM toward the NM layer, which is caused by the excitation of spin-polarized electrons of the FM layer above the Fermi level. In the NM layer, Js is converted into a transverse charge current pulse Jc due to the inverse spin Hall effect, which leads to the emission of electromagnetic radiation in the THz frequency regime. The emission properties of the emitters can be optimized by utilizing different materials, layer thicknesses, or more complex multilayer structures. The present work shows studies of spintronic THz emitter systems that are based on different magnetic thin films combined with Pt and W layers. The experimental studies can be divided into two parts. The main goal of the first part was to investigate how the magnetic properties of different FM and in particular also ferrimagnetic (FI) materials are reflected in the THz emission properties of a spintronic emitter system. Therefore, thin bilayers consisting of FM CoFe, or FI TbFe or GdFe alloy thin films with varying Fe content (0 ≤ x ≤ 1), combined with Pt layers have been prepared. The laser-excited spintronic THz emission has been investigated in dependence on the applied magnetic field, the temperature, and the pump fluence of the excitation laser. The results have been explained with regard to detailed characterizations of the structural, magnetic, electrical, and optical properties of the samples. The second goal of this work was set on the development of more functional multilayer emitter systems that allow for the control of the THz emission amplitude between a high- and a low-amplitude state and also might open the way for higher THz emission amplitudes. Based on the results of the previously investigated FI Pt/GdFe bilayer emitter system, a new concept of a THz emitter that can be switched by a temperature change from a high- to a low-amplitude state has been developed. Additionally, the use of a spin-valve system as a spintronic emitter system that allows for the switching of the emission amplitude by small applied magnetic fields in the range of a few millitesla has been demonstrated

Influence of magnetic domain walls on all-optical magnetic toggle switching in a ferrimagnetic GdFe film

We present a microscopic magnetic domain imaging study of single-shot all-optical magnetic toggle switching of a ferrimagnetic Gd(26)Fe(74) film with out-of-plane easy axis of magnetization by X-ray magnetic circular dichroism photoelectron emission microscopy. Individual linearly polarized laser pulses of 800 nm wavelength and 100 fs duration above a certain threshold fluence reverse the sample magnetization, independent of the magnetization direction, the so-called toggle switching. Local deviations from this deterministic behavior close to magnetic domain walls are studied in detail. Reasons for nondeterministic toggle switching are related to extrinsic effects, caused by pulse-to-pulse variations of the exciting laser system, and to intrinsic effects related to the magnetic domain structure of the sample. The latter are, on the one hand, caused by magnetic domain wall elasticity, which leads to a reduction of the domain-wall length at features with sharp tips. These features appear after the optical switching at positions where the line of constant threshold fluence in the Gaussian footprint of the laser pulse comes close to an already existing domain wall. On the other hand, we identify the presence of laser-induced domain-wall motion in the toggle-switching event as a further cause for local deviations from purely deterministic toggle switching

High‐performance broadband faraday rotation spectroscopy of 2D materials and thin magnetic films

A Faraday rotation spectroscopy (FRS) technique is presented for measurements on the micrometer scale. Spectral acquisition speeds of about two orders of magnitude faster than state-of-the-art modulation spectroscopy setups are demonstrated. The experimental method is based on charge-coupled-device detection, avoiding speed-limiting components, such as polarization modulators with lock-in amplifiers. At the same time, FRS spectra are obtained with a sensitivity of 20 µrad ((Formula presented.)) over a broad spectral range (525–800 nm), which is on par with state-of-the-art polarization-modulation techniques. The new measurement and analysis technique also automatically cancels unwanted Faraday rotation backgrounds. Using the setup, Faraday rotation spectroscopy of excitons is performed in a hexagonal boron nitride-encapsulated atomically thin semiconductor WS 2 under magnetic fields of up to 1.4 T at room temperature and liquid helium temperature. An exciton g-factor of −4.4 ± 0.3 is determined at room temperature, and −4.2 ± 0.2 at liquid helium temperature. In addition, FRS and hysteresis loop measurements are performed on a 20 nm thick film of an amorphous magnetic Tb 20Fe 80 alloy

The North Atlantic Waveguide and Downstream Impact Experiment

The North Atlantic Waveguide and Downstream Impact Experiment (NAWDEX) explored the impact of diabatic processes on disturbances of the jet stream and their influence on downstream high-impact weather through the deployment of four research aircraft, each with a sophisticated set of remote sensing and in situ instruments, and coordinated with a suite of ground-based measurements. A total of 49 research flights were performed, including, for the first time, coordinated flights of the four aircraft: the German High Altitude and Long Range Research Aircraft (HALO), the Deutsches Zentrum für Luft- und Raumfahrt (DLR) Dassault Falcon 20, the French Service des Avions Français Instrumentés pour la Recherche en Environnement (SAFIRE) Falcon 20, and the British Facility for Airborne Atmospheric Measurements (FAAM) BAe 146. The observation period from 17 September to 22 October 2016 with frequently occurring extratropical and tropical cyclones was ideal for investigating midlatitude weather over the North Atlantic. NAWDEX featured three sequences of upstream triggers of waveguide disturbances, as well as their dynamic interaction with the jet stream, subsequent development, and eventual downstream weather impact on Europe. Examples are presented to highlight the wealth of phenomena that were sampled, the comprehensive coverage, and the multifaceted nature of the measurements. This unique dataset forms the basis for future case studies and detailed evaluations of weather and climate predictions to improve our understanding of diabatic influences on Rossby waves and the downstream impacts of weather systems affecting Europe

The Science Performance of JWST as Characterized in Commissioning

This paper characterizes the actual science performance of the James Webb Space Telescope (JWST), as determined from the six month commissioning period. We summarize the performance of the spacecraft, telescope, science instruments, and ground system, with an emphasis on differences from pre-launch expectations. Commissioning has made clear that JWST is fully capable of achieving the discoveries for which it was built. Moreover, almost across the board, the science performance of JWST is better than expected; in most cases, JWST will go deeper faster than expected. The telescope and instrument suite have demonstrated the sensitivity, stability, image quality, and spectral range that are necessary to transform our understanding of the cosmos through observations spanning from near-earth asteroids to the most distant galaxies.Comment: 5th version as accepted to PASP; 31 pages, 18 figures; https://iopscience.iop.org/article/10.1088/1538-3873/acb29

The James Webb Space Telescope Mission

Twenty-six years ago a small committee report, building on earlier studies, expounded a compelling and poetic vision for the future of astronomy, calling for an infrared-optimized space telescope with an aperture of at least $4m$. With the support of their governments in the US, Europe, and Canada, 20,000 people realized that vision as the $6.5m$ James Webb Space Telescope. A generation of astronomers will celebrate their accomplishments for the life of the mission, potentially as long as 20 years, and beyond. This report and the scientific discoveries that follow are extended thank-you notes to the 20,000 team members. The telescope is working perfectly, with much better image quality than expected. In this and accompanying papers, we give a brief history, describe the observatory, outline its objectives and current observing program, and discuss the inventions and people who made it possible. We cite detailed reports on the design and the measured performance on orbit.Comment: Accepted by PASP for the special issue on The James Webb Space Telescope Overview, 29 pages, 4 figure

Signaturen von Infraschall im atmosphärischen Luftleuchten

Messungen von Infraschall durch Satelliten bieten die Möglichkeit, global eine Vielzahl unterschiedlicher Infraschallquellen zu untersuchen. Dies erweitert unter Umständen auch das Wissen über die bisher noch wenig erforschten verschiedenen Entstehungsmechanismen von Infraschallwellen. Im Rahmen der geplanten InfEx- (Infrasound Explorer) Mission des Deutschen Zentrums für Luft und Raumfahrt sollen unter anderem erste Messungen von Infraschall in atmosphärischen Airglow vom Satelliten aus durchgeführt werden. In der hier vorliegenden Arbeit werden einige grundlegende Studien zur Infraschallausbreitung in der Atmosphäre sowie insbesondere zur Detektion von Infraschallsignaturen in Hydroxyl(OH)-Airglow-Messdaten vorgestellt. In der Arbeit wird unter anderem die Ausbreitung von Infraschall in der Atmosphäre anhand von Simulationsdaten untersucht. Hierbei wird in Anbetracht der verschiedenen, im Auswertungskapitel verwendeten, auf atmosphärischen Airglow beruhenden Messmethoden das Hauptaugenmerk auf die Höhe der OH-Schicht, welche ihr Zentrum im Mittel bei etwa 87 km Höhe hat, gelegt. Die Simulationsergebnisse werden in Bezug auf mögliche Satelliten-Messungen atmosphärischen OH-Airglows ausgewertet. Hierfür wird eine mögliche Flugbahn eines Satelliten simuliert und eine Studie zur Messung von möglichen Infraschallsignalen für eine exemplarische Schallquelle durchgeführt

Airborne remote sensing of cloud properties with the German research aircraft HALO

The new German research aircraft HALO (High Altitude Long range) can be equipped with a remote sensing payload to study cloud properties and water vapor profiles of the atmosphere. This package, first flown during the NARVAL (Next‐generation Aircraft Remote sensing for VALidation studies) mission in December 2013 and January 2014, consists of a cloud radar, microwave radiometers and a lidar system. HALO is a for atmospheric measurements modified Gulfstream G550 business jet with a maximum payload of about 3 tons, an endurance of more than 10 hours and a maximum ceiling of about 15 km. The HALO microwave package (HAMP) consists of the cloud radar and the microwave radiometers. The cloud radar is a nadir pointing Ka‐band radar (35 GHz) adapted from the METEX Mira‐36 radar. While the radar electronics resides in the cabin of the aircraft, the 1 m diameter antenna (0.6° beam width) is mounted inside the belly pod beneath the fuselage. The microwave radar radiometers build by Radiometer Physics (RPG) are operating at 26 frequencies in the K‐band (22 GHz, water‐vapor line), V‐band (58 GHz, O2 line), W‐band (90 GHz, window channel), F‐band (118 GHz, O2 line), and G‐band (183 GHz, water‐vapor line). The radiometers are mounted inside the belly‐pod in three containers with five nadir pointing antennas (beam width 2.7 to 5°). The lidar is the DLR WALES system with a water‐vapor DIAL (935 nm) and a HSRL (1064 and 532 nm) for cloud and aerosol properties. The focus of the NARVAL campaign was on the characterization of precipitation from shallow clouds in the North Atlantic trade wind zone (NARVAL‐South, December 2013) and on post‐frontal mesoscale precipitation systems over the North Atlantic near Iceland and Greenland (NARVAL‐North, January 2014). For intercomparison, additionally several patterns over ground stations like Jülich, Lindenberg, Leipzig, Mace‐Head, and Chilbolton were flown. Also a number of under‐passes of Cloudsat and Calipso were performed. A joint flight was also together with the French Falcon operating the 95 GHz cloud radar