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- ItemStrain Engineered Electrically Pumped SiGeSn Microring Lasers on Si(Washington, DC : ACS, 2022) Marzban, Bahareh; Seidel, Lukas; Liu, Teren; Wu, Kui; Kiyek, Vivien; Zoellner, Marvin Hartwig; Ikonic, Zoran; Schulze, Joerg; Grützmacher, Detlev; Capellini, Giovanni; Oehme, Michael; Witzens, Jeremy; Buca, DanSiGeSn holds great promise for enabling fully group-IV integrated photonics operating at wavelengths extending in the mid-infrared range. Here, we demonstrate an electrically pumped GeSn microring laser based on SiGeSn/GeSn heterostructures. The ring shape allows for enhanced strain relaxation, leading to enhanced optical properties, and better guiding of the carriers into the optically active region. We have engineered a partial undercut of the ring to further promote strain relaxation while maintaining adequate heat sinking. Lasing is measured up to 90 K, with a 75 K T0. Scaling of the threshold current density as the inverse of the outer circumference is linked to optical losses at the etched surface, limiting device performance. Modeling is consistent with experiments across the range of explored inner and outer radii. These results will guide additional device optimization, aiming at improving electrical injection and using stressors to increase the bandgap directness of the active material.
- ItemSaturation of the anomalous Hall effect at high magnetic fields in altermagnetic RuO2(Melville, NY : AIP Publ., 2023) Tschirner, Teresa; Keßler, Philipp; Gonzalez Betancourt, Ruben Dario; Kotte, Tommy; Kriegner, Dominik; Büchner, Bernd; Dufouleur, Joseph; Kamp, Martin; Jovic, Vedran; Smejkal, Libor; Sinova, Jairo; Claessen, Ralph; Jungwirth, Tomas; Moser, Simon; Reichlova, Helena; Veyrat, LouisObservations of the anomalous Hall effect in RuO2 and MnTe have demonstrated unconventional time-reversal symmetry breaking in the electronic structure of a recently identified new class of compensated collinear magnets, dubbed altermagnets. While in MnTe, the unconventional anomalous Hall signal accompanied by a vanishing magnetization is observable at remanence, the anomalous Hall effect in RuO2 is excluded by symmetry for the Néel vector pointing along the zero-field [001] easy-axis. Guided by a symmetry analysis and ab initio calculations, a field-induced reorientation of the Néel vector from the easy-axis toward the [110] hard-axis was used to demonstrate the anomalous Hall signal in this altermagnet. We confirm the existence of an anomalous Hall effect in our RuO2 thin-film samples, whose set of magnetic and magneto-transport characteristics is consistent with the earlier report. By performing our measurements at extreme magnetic fields up to 68 T, we reach saturation of the anomalous Hall signal at a field Hc ≃ 55 T that was inaccessible in earlier studies but is consistent with the expected Néel-vector reorientation field.
- ItemGrowth of PdCoO2 films with controlled termination by molecular-beam epitaxy and determination of their electronic structure by angle-resolved photoemission spectroscopy(Melville, NY : AIP Publ., 2022) Song, Qi; Sun, Jiaxin; Parzyck, Christopher T.; Miao, Ludi; Xu, Qing; Hensling, Felix V. E.; Barone, Matthew R.; Hu, Cheng; Kim, Jinkwon; Faeth, Brendan D.; Paik, Hanjong; King, Phil D. C.; Shen, Kyle M.; Schlom, Darrell G.Utilizing the powerful combination of molecular-beam epitaxy (MBE) and angle-resolved photoemission spectroscopy (ARPES), we produce and study the effect of different terminating layers on the electronic structure of the metallic delafossite PdCoO2. Attempts to introduce unpaired electrons and synthesize new antiferromagnetic metals akin to the isostructural compound PdCrO2 have been made by replacing cobalt with iron in PdCoO2 films grown by MBE. Using ARPES, we observe similar bulk bands in these PdCoO2 films with Pd-, CoO2-, and FeO2-termination. Nevertheless, Pd- and CoO2-terminated films show a reduced intensity of surface states. Additionally, we are able to epitaxially stabilize PdFexCo1-xO2 films that show an anomaly in the derivative of the electrical resistance with respect to temperature at 20 K, but do not display pronounced magnetic order.
- ItemLasing by Template-Assisted Self-Assembled Quantum Dots(Weinheim : Wiley-VCH, 2023) Aftenieva, Olha; Sudzius, Markas; Prudnikau, Anatol; Adnan, Mohammad; Sarkar, Swagato; Lesnyak, Vladimir; Leo, Karl; Fery, Andreas; König, Tobias A.F.Miniaturized laser sources with low threshold power are required for integrated photonic devices. Photostable core/shell nanocrystals are well suited as gain material and their laser properties can be exploited by direct patterning as distributed feedback (DFB) lasers. Here, the 2nd-order DFB resonators tuned to the photoluminescence wavelength of the QDs are used. Soft lithography based on template-assisted colloidal self-assembly enables pattern resolution in the subwavelength range. Combined with the directional Langmuir–Blodgett arrangement, control of the waveguide layer thickness is further achieved. It is shown that a lasing threshold of 5.5 mJ cm−2 is reached by a direct printing method, which can be further reduced by a factor of ten (0.6 mJ cm−2) at an optimal waveguide thickness. Moreover, it is discussed how one can adjust the DFB geometries to any working wavelength. This colloidal approach offers prospects for applications in bioimaging, biomedical sensing, anti-counterfeiting, or displays.
- ItemPlasmonic Properties of Colloidal Assemblies(Weinheim : Wiley-VCH, 2021) Rossner, Christian; König, Tobias A.F.; Fery, AndreasThe assembly of metal nanoparticles into supracolloidal structures unlocks optical features, which can go beyond synergistic combinations of the properties of their primary building units. This is due to inter-particle plasmonic coupling effects, which give rise to emergent properties. The motivation for this progress report is twofold: First, it is described how simulation approaches can be used to predict and understand the optical properties of supracolloidal metal clusters. These simulations may form the basis for the rational design of plasmonic assembly architectures, based on the desired functional cluster properties, and they may also spark novel material designs. Second, selected scalable state-of-the-art preparative strategies based on synthetic polymers to guide the supracolloidal assembly are discussed. These routes also allow for equipping the assembly structures with adaptive properties, which in turn enables (inter-)active control over the cluster optical properties. © 2021 The Authors. Advanced Optical Materials published by Wiley-VCH GmbH
- ItemSpectrometer‐free Optical Hydrogen Sensing Based on Fano‐like Spatial Distribution of Transmission in a Metal−Insulator−Metal Plasmonic Doppler Grating(Weinheim : Wiley-VCH, 2021) Chen, Yi‐Ju; Lin, Fan‐Cheng; Singh, Ankit Kumar; Ouyang, Lei; Huang, Jer‐ShingOptical nanosensors are promising for hydrogen sensing because they are small, free from spark generation, and feasible for remote optical readout. Conventional optical nanosensors require broadband excitation and spectrometers, rendering the devices bulky and complex. An alternative is spatial intensity-based optical sensing, which only requires an imaging system and a smartly designed platform to report the spatial distribution of analytical optical signals. Here, a spatial intensity-based hydrogen sensing platform is presented based on Fano-like spatial distribution of the transmission in a Pd-Al2O3-Au metal-insulator-metal plasmonic Doppler grating (MIM-PDG). The MIM-PDG manifests the Fano resonance as an asymmetric spatial transmission intensity profile. The absorption of hydrogen changes the spatial Fano-like transmission profiles, which can be analyzed with a “spatial” Fano resonance model and the extracted Fano resonance parameters can be used to establish analytical calibration lines. While gratings sensitive to hydrogen absorption are suitable for hydrogen sensing, hydrogen insensitive gratings are also found, which provide an unperturbed reference signal and may find applications in nanophotonic devices that require a stable optical response under fluctuating hydrogen atmosphere. The MIM-PDG platform is a spectrometer-free and intensity-based optical sensor that requires only an imaging system, making it promising for cellphone-based optical sensing applications. © 2021 The Authors. Advanced Optical Materials published by Wiley-VCH GmbH.
- ItemTailored Disorder in Photonics: Learning from Nature(Weinheim : Wiley-VCH, 2021) Rothammer, Maximilian; Zollfrank, Cordt; Busch, Kurt; Freymann, Georg vonDisorder and photonics have long been seen as natural adversaries and designers of optical systems have often driven systems to perfection by minimizing deviations from the ideal design. Especially in the field of photonic crystals and metamaterials but also for optical circuits, disorder has been avoided as a nuisance for many years. However, starting from the very robust structural colors found in nature, scientists learn to analyze and tailor disorder to achieve functionalities beyond what is possible with perfectly ordered or ideal systems alone. This review article covers theoretical and materials aspects of tailored disorder as well as experimental results. Furthermore selected examples are highlighted in greater detail, for which the intentional use of disorder adds additional functionality or provides novel functionality impossible without disorder. © 2021 The Authors. Advanced Optical Materials published by Wiley-VCH GmbH
- ItemPhotoluminescence Mapping over Laser Pulse Fluence and Repetition Rate as a Fingerprint of Charge and Defect Dynamics in Perovskites(Weinheim : Wiley-VCH, 2023) Rao, Shraddha M.; Kiligaridis, Alexander; Yangui, Aymen; An, Qingzhi; Vaynzof, Yana; Scheblykin, Ivan G.Defects in metal halide perovskites (MHP) are photosensitive, making the observer effect unavoidable when laser spectroscopy methods are applied. Photoluminescence (PL) bleaching and enhancement under light soaking and recovery in dark are examples of the transient phenomena that are consequent to the creation and healing of defects. Depending on the initial sample composition, environment, and other factors, the defect nature and evolution can strongly vary, making spectroscopic data analysis prone to misinterpretations. Herein, the use of an automatically acquired dependence of PL quantum yield (PLQY) on the laser pulse repetition rate and pulse fluence as a unique fingerprint of both charge carrier dynamics and defect evolution is demonstrated. A simple visual comparison of such fingerprints allows for assessment of similarities and differences between MHP samples. The study illustrates this by examining methylammonium lead triiodide (MAPbI3) films with altered stoichiometry that just after preparation showed very pronounced defect dynamics at time scale from milliseconds to seconds, clearly distorting the PLQY fingerprint. Upon weeks of storage, the sample fingerprints evolve toward the standard stoichiometric MAPbI3 in terms of both charge carrier dynamics and defect stability. Automatic PLQY mapping can be used as a universal method for assessment of perovskite sample quality.
- ItemTiOx/Pt3Ti(111) surface-directed formation of electronically responsive supramolecular assemblies of tungsten oxide clusters(Frankfurt, M. : Beilstein-Institut zur Förderung der Chemischen Wissenschaften, 2021) Moors, Marco; An, Yun; Kuc, Agnieszka; Monakhov, Kirill YuHighly ordered titanium oxide films grown on a Pt3Ti(111) alloy surface were utilized for the controlled immobilization and tip-induced electric field-triggered electronic manipulation of nanoscopic W3O9 clusters. Depending on the operating conditions, two different stable oxide phases, z'-TiO x and w'-TiO x , were produced. These phases show a strong effect on the adsorption characteristics and reactivity of W3O9 clusters, which are formed as a result of thermal evaporation of WO3 powder on the complex TiO x /Pt3Ti(111) surfaces under ultra-high vacuum conditions. The physisorbed tritungsten nano-oxides were found as isolated single units located on the metallic attraction points or as supramolecular self-assemblies with a W3O9-capped hexagonal scaffold of W3O9 units. By applying scanning tunneling microscopy to the W3O9-(W3O9)6 structures, individual units underwent a tip-induced reduction to W3O8. At elevated temperatures, agglomeration and growth of large WO3 islands, which thickness is strongly limited to a maximum of two unit cells, were observed. The findings boost progress toward template-directed nucleation, growth, networking, and charge state manipulation of functional molecular nanostructures on surfaces using operando techniques.
- ItemAuger- and X-ray Photoelectron Spectroscopy at Metallic Li Material: Chemical Shifts Related to Sample Preparation, Gas Atmosphere, and Ion and Electron Beam Effects(Basel : MDPI, 2022) Oswald, SteffenLi-based batteries are a key element in reaching a sustainable energy economy in the near future. The understanding of the very complex electrochemical processes is necessary for the optimization of their performance. X-ray photoelectron spectroscopy (XPS) is an accepted method used to improve understanding around the chemical processes at the electrode surfaces. Nevertheless, its application is limited because the surfaces under investigation are mostly rough and inhomogeneous. Local elemental analysis, such as Auger electron spectroscopy (AES), could assist XPS to gain more insight into the chemical processes at the surfaces. In this paper, some challenges in using electron spectroscopy are discussed, such as binding energy (BE) referencing for the quantitative study of chemical shifts, gas atmospheric influences, or beam damage (including both AE and XP spectroscopy). Carefully prepared and surface-modified metallic lithium material is used as model surface, considering that Li is the key element for most battery applications.