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- ItemOptimizing the Geometry of Photoacoustically Active Gold Nanoparticles for Biomedical Imaging(Washington, DC : ACS, 2020) García-Álvarez, Rafaela; Chen, Lisa; Nedilko, Alexander; Sánchez-Iglesias, Ana; Rix, Anne; Lederle, Wiltrud; Pathak, Vertika; Lammers, Twan; von Plessen, Gero; Kostarelos, Kostas; Liz-Marzán, Luis M.; Kuehne, Alexander J.C.; Chigrin, Dmitry N.Photoacoustics is an upcoming modality for biomedical imaging, which promises minimal invasiveness at high penetration depths of several centimeters. For superior photoacoustic contrast, imaging probes with high photothermal conversion efficiency are required. Gold nanoparticles are among the best performing photoacoustic imaging probes. However, the geometry and size of the nanoparticles determine their photothermal efficiency. We present a systematic theoretical analysis to determine the optimum nanoparticle geometry with respect to photoacoustic efficiency in the near-infrared spectral range, for superior photoacoustic contrast. Theoretical predictions are illustrated by experimental results for two of the most promising nanoparticle geometries, namely, high aspect ratio gold nanorods and gold nanostars. Copyright © 2020 American Chemical Society.
- ItemBlind Super-Resolution Approach for Exploiting Illumination Variety in Optical-Lattice Illumination Microscopy(Washington, DC : ACS Publications, 2021) Samanta, Krishnendu; Sarkar, Swagato; Acuña, Sebastian; Joseph, Joby; Ahluwalia, Balpreet Singh; Agarwal, KrishnaOptical-lattice illumination patterns help in pushing high spatial frequency components of the sample into the optical transfer function of a collection microscope. However, exploiting these high-frequency components require precise knowledge of illumination if reconstruction approaches similar to structured illumination microscopy are employed. Here, we present an alternate blind reconstruction approach that can provide super-resolution without the requirement of extra frames. For this, the property of exploiting temporal fluctuations in the sample emissions using “multiple signal classification algorithm” is extended aptly toward using spatial fluctuation of phase-modulated lattice illuminations for super-resolution. The super-resolution ability is shown for sinusoidal and multiperiodic lattice with approximately 3- and 6-fold resolution enhancements, respectively, over the diffraction limit. © 2021 The Authors. Published by American Chemical Society
- 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.
- ItemOrigami-Inspired Shape Memory Folding Microactuator(Basel : MDPI, 2020) Seigner, Lena; Bezsmertna, Olha; Fähler, Sebastian; Tshikwand, Georgino; Wendler, Frank; Kohl, ManfredThis paper presents the design, fabrication and performance of origami-based folding microactuators based on a cold-rolled NiTi foil of 20 µm thickness showing the one-way shape memory effect. Origami refers to a variety of techniques of transforming planar sheets into three-dimensional (3D) structures by folding, which has been introduced in science and engineering for, e.g., assembly and robotics. Here, NiTi microactuators are interconnected to rigid sections (tiles) forming an initial planar system that self-folds into a set of predetermined 3D shapes upon heating. While this concept has been demonstrated at the macro scale, we intend to transfer this concept into microtechnology by combining state-of-the art methods of micromachining. NiTi foils are micromachined by laser cutting or photolithography to achieve double-beam structures allowing for direct Joule heating with an electrical current. A thermo-mechanical treatment is used for shape setting of as-received specimens to reach a maximum folding angle of 180°. The bending moments, bending radii and load-dependent folding angles upon Joule heating are evaluated. The shape setting process is particularly effective for small bending radii, which, however generates residual plastic strain. After shape setting, unloaded beam structures show recoverable bending deflection between 0° and 140° for a maximum heating power of 900 mW. By introducing additional loads to account for the effect of the tiles, the smooth folding characteristic evolves into a sharp transition, whereby full deflection up to 180° is reached. The achieved results are an important step towards the development of cooperative multistable microactuator systems for 3D self-assembly.
- ItemNonlinear Optical Characterization of CsPbBr3 Nanocrystals as a Novel Material for the Integration into Electro-Optic Modulators(Millersville, PA : Materials Research Forum LLC, 2020) Vitale, Francesco; De Matteis, Fabio; Casalboni, Mauro; Prosposito, Paolo; Steglich, Patrick; Ksianzou, Viachaslau; Breiler, Christian; Schrader, Sigurd; Paci, Barbara; Generosi, Amanda; Prosposito, PaoloThe present work is concerned with the investigation of the nonlinear optical response of green emissive CsPbBr3 nanocrystals, in the form of colloidal dispersions in toluene, synthesized via a room-temperature ligand-assisted supersaturation recrystallization (LASR) method. After carrying out a preliminary characterization via X-Ray Diffraction (XRD) and Absorption and Photoluminescence (PL) Spectroscopies, the optical nonlinearity of the as-obtained colloids is probed by means of a single-beam Z-scan setup. Results show that the material in question, within the sensitivity of the experimental apparatus, exhibits a nonlinear refractive index n2 that is the order of 10-15 cm2/W. Moreover, a three-photon absorption mechanism (3PA) is postulated, according to the fitting of the recorded Z-scan traces and the fundamental absorption threshold, which turns out to be off resonance with twice the energy of the laser radiation. A figure of merit is, then, calculated as an indicator of the quality of the CsPbBr3 nanocrystals as a candidate material for photonic devices, for instance, Kerr-like electro-optic modulators (EOMs).
- ItemIn-Situ Measurement of Fresh Produce Respiration Using a Modular Sensor-Based System(Basel : MDPI, 2020) Keshri, Nandita; Truppel, Ingo; Herppich, Werner B.; Geyer, Martin; Weltzien, Cornelia; Mahajan, Pramod VIn situ, continuous and real-time monitoring of respiration (R) and respiratory quotient (RQ) are crucial for identifying the optimal conditions for the long-term storage of fresh produce. This study reports the application of a gas sensor (RMS88) and a modular respirometer for in situ real-time monitoring of gas concentrations and respiration rates of strawberries during storage in a lab-scale controlled atmosphere chamber (190 L) and of Pinova apples in a commercial storage facility (170 t). The RMS88 consisted of wireless O2 (0% to 25%) and CO2 sensors (0% to 0.5% and 0% to 5%). The modular respirometer (3.3 L for strawberries and 7.4 L for apples) consisted of a leak-proof arrangement with a water-containing base plate and a glass jar on top. Gas concentrations were continuously recorded by the RMS88 at regular intervals of 1 min for strawberries and 5 min for apples and, in real-time, transferred to a terminal program to calculate respiration rates ( RO2 and RCO2 ) and RQ. Respiration measurement was done in cycles of flushing and measurement period. A respiration measurement cycle with a measurement period of 2 h up to 3 h was shown to be useful for strawberries under air at 10 °C. The start of anaerobic respiration of strawberries due to low O2 concentration (1%) could be recorded in real-time. RO2 and RCO2 of Pinova apples were recorded every 5 min during storage and mean values of 1.6 and 2.7 mL kg−1 h−1, respectively, were obtained when controlled atmosphere (CA) conditions (2% O2, 1.3% CO2 and 2 °C) were established. The modular respirometer was found to be useful for in situ real-time monitoring of respiration rate during storage of fresh produce and offers great potential to be incorporated into RQ-based dynamic CA storage system.
- ItemInfluence of core size and capping ligand of gold nanoparticles on the desorption/ionization efficiency of small biomolecules in AP‐SALDI‐MS(Hoboke, NJ : Wiley, 2020) Liu, Zhen; Zhang, Peng; Pyttlik, Andrea; Kraus, Tobias; Volmer, Dietrich A.Gold nanoparticles (AuNP) are frequently used in surface‐assisted laser desorption/ionization mass spectrometry (SALDI‐MS) for analysis of biomolecules because they exhibit suitable thermal and chemical properties as well as strong surface plasmonic effects. Moreover, the structures of AuNP can be controlled by well‐established synthesis protocols. This was important in the present work, which studied the influence of the nanoparticles’ structures on atmospheric pressure (AP)‐SALDI‐MS performance. A series of AuNP with different core sizes and capping ligands were investigated, to examine the desorption/ionization efficiency (DIE) under AP‐SALDI conditions. The results showed that both the AuNP core size as well as the nature of the surface ligand had a strong influence on DIE. DIE increased with the size of the AuNP and the hydrophobicity of the ligands. Chemical interactions between ligand and analytes also influenced DIE. Moreover, we discovered that removing the organic ligands from the deposited AuNP substrate layer by simple laser irradiation prior to LDI further amplified DIE values. The optimized AuNP were successfully used to analyze a wide arrange of different low molecular weight biomolecules as well as a crude pig brain extract, which readily demonstrated the ability of the technique to detect a wide range of lipid species within highly complex samples.
- 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.
- ItemRemarkable Mechanochromism in Blends of a π-Conjugated Polymer P3TEOT: The Role of Conformational Transitions and Aggregation(Weinheim : Wiley-VCH, 2020) Zessin, Johanna; Schnepf, Max; Oertel, Ulrich; Beryozkina, Tetyana; König, Tobias A.F.; Fery, Andreas; Mertig, Michael; Kiriy, AntonA novel mechanism for well-pronounced mechanochromism in blends of a π-conjugated polymer based on reversible conformational transitions of a chromophore rather than caused by its aggregation state, is exemplified. Particularly, a strong stretching-induced bathochromic shift of the light absorption, or hypsochromic shift of the emission, is found in blends of the water-soluble poly(3-tri(ethylene glycol)) (P3TEOT) embedded into the matrix of thermoplastic polyvinyl alcohol. This counterintuitive phenomenon is explained in terms of the concentration dependency of the P3TEOT's aggregation state, which in turn results in different molecular conformations and optical properties. A molecular flexibility, provided by low glass transition temperature of P3TEOT, and the fact that P3TEOT adopts an intermediate, moderately planar conformation in the solid state, are responsible for the unusual complex mechanochromic behavior. © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
- ItemWaveguide-Integrated Broadband Spectrometer Based on Tailored Disorder(Weinheim : Wiley-VCH Verlag, 2020) Hartmann, Wladick; Varytis, Paris; Gehring, Helge; Walter, Nicolai; Beutel, Fabian; Busch, Kurt; Pernice, WolframCompact, on-chip spectrometers exploiting tailored disorder for broadband light scattering enable high-resolution signal analysis while maintaining a small device footprint. Due to multiple scattering events of light in the disordered medium, the effective path length of the device is significantly enhanced. Here, on-chip spectrometers are realized for visible and near-infrared wavelengths by combining an efficient broadband fiber-to-chip coupling approach with a scattering area in a broadband transparent silicon nitride waveguiding structure. Air holes etched into a structured silicon nitride slab terminated with multiple waveguides enable multipath light scattering in a diffusive regime. Spectral-to-spatial mapping is performed by determining the transmission matrix at the waveguide outputs, which is then used to reconstruct the probe signals. Direct comparison with theoretical analyses shows that such devices can be used for high-resolution spectroscopy from the visible up to the telecom wavelength regime. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim