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- ItemMultiple lobes in the far-field distribution of terahertz quantum-cascade lasers due to self-interference(New York : American Institute of Physics, 2016) Röben, B.; Wienold, M.; Schrottke, L.; Grahn, H.T.The far-field distribution of the emission intensity of terahertz (THz) quantumcascade lasers (QCLs) frequently exhibits multiple lobes instead of a single-lobed Gaussian distribution. We show that such multiple lobes can result from selfinterference related to the typically large beam divergence of THz QCLs and the presence of an inevitable cryogenic operation environment including optical windows. We develop a quantitative model to reproduce the multiple lobes. We also demonstrate how a single-lobed far-field distribution can be achieved.
- ItemNanoscopic tip sensors fabricated by gas phase etching of optical glass fibers(Heidelberg : Springer, 2012) Bierlich, J.; Kobelke, J.; Brand, D.; Kirsch, K.; Dellith, J.; Bartelt, H.Silica-based fiber tips are used in a variety of spectroscopic, micro- or nano-scopic optical sensor applications and photonic micro-devices. The miniaturization of optical sensor systems and the technical implementation using optical fibers can provide new sensor designs with improved properties and functionality for new applications. The selective-etching of specifically doped silica fibers is a promising method in order to form complex photonic micro structures at the end or within fibers such as tips and cavities in various shapes useful for the all-fiber sensor and imaging applications. In the present study, we investigated the preparation of geometrically predefined, nanoscaled fiber tips by taking advantage of the dopant concentration profiles of highly doped step-index fibers. For this purpose, a gas phase etching process using hydrofluoric acid (HF) vapor was applied. The shaping of the fiber tips was based on very different etching rates as a result of the doping characteristics of specific optical fibers. Technological studies on the influence of the etching gas atmosphere on the temporal tip shaping and the final geometry were performed using undoped and doped silica fibers. The influence of the doping characteristics was investigated in phosphorus-, germanium-, fluorine- and boron-doped glass fibers. Narrow exposed as well as protected internal fiber tips in various shapes and tip radiuses down to less than 15 nm were achieved and characterized geometrically and topologically. For investigations into surface plasmon resonance effects, the fiber tips were coated with nanometer-sized silver layers by means of vapour deposition and finally subjected to an annealing treatment.
- ItemSpatially modulated broad-area lasers for narrow lateral far-field divergence(Washington, DC : Soc., 2021) Zeghuzi, Anissa; Koester, Jan-Philipp; Radziunas, Mindaugas; Christopher, Heike; Wenzel, Hans; Knigge, AndreaA novel laser design is presented that combines a longitudinal-lateral gain-loss modulation with an additional phase tailoring achieved by etching rectangular trenches. At 100 A pulsed operation, simulations predict a far-field profile with 0.3° full width at half maximum (ΘFWHM=0.3∘) where a 0.4°-wide main lobe contains 40% of the emitted optical output power (Θ40%=0.4∘). While far-field measurements of these structured lasers emitting 10 ns long pulses with 35 W peak power confirm a substantial enhancement of radiation within the central 1∘ angular range, the measured far-field intensity outside of the obtained central peak remains high.
- ItemTemperature-dependent Raman investigation of rolled up InGaAs/GaAs microtubes(New York, NY [u.a.] : Springer, 2012) Rodriguez, R.D.; Sheremet, E.; Thurmer, D.J.; Lehmann, D.; Gordan, O.D.; Seidel, F.; Milekhin, A.; Schmidt, O.G.; Hietschold, M.; Zahn, D.R.T.Large arrays of multifunctional rolled-up semiconductors can be mass-produced with precisely controlled size and composition, making them of great technological interest for micro- and nano-scale device fabrication. The microtube behavior at different temperatures is a key factor towards further engineering their functionality, as well as for characterizing strain, defects, and temperature-dependent properties of the structures. For this purpose, we probe optical phonons of GaAs/InGaAs rolled-up microtubes using Raman spectroscopy on defect-rich (faulty) and defect-free microtubes. The microtubes are fabricated by selectively etching an AlAs sacrificial layer in order to release the strained InGaAs/GaAs bilayer, all grown by molecular beam epitaxy. Pristine microtubes show homogeneity of the GaAs and InGaAs peak positions and intensities along the tube, which indicates a defect-free rolling up process, while for a cone-like microtube, a downward shift of the GaAs LO phonon peak along the cone is observed. Formation of other type of defects, including partially unfolded microtubes, can also be related to a high Raman intensity of the TO phonon in GaAs. We argue that the appearance of the TO phonon mode is a consequence of further relaxation of the selection rules due to the defects on the tubes, which makes this phonon useful for failure detection/prediction in such rolled up systems. In order to systematically characterize the temperature stability of the rolled up microtubes, Raman spectra were acquired as a function of sample temperature up to 300°C. The reversibility of the changes in the Raman spectra of the tubes within this temperature range is demonstrated.
- ItemOptical properties of silicon nanowire arrays formed by metal-assisted chemical etching: Evidences for light localization effect(New York, NY [u.a.] : Springer, 2012) Osminkina, L.A.; Gonchar, K.A.; Marshov, V.S.; Bunkov, K.V.; Petrov, D.V.; Golovan, L.A.; Talkenberg, F.; Sivakov, V.A.; Timoshenko, V.Y.We study the structure and optical properties of arrays of silicon nanowires (SiNWs) with a mean diameter of approximately 100 nm and length of about 1-25 μm formed on crystalline silicon (c-Si) substrates by using metal-assisted chemical etching in hydrofluoric acid solutions. In the middle infrared spectral region, the reflectance and transmittance of the formed SiNW arrays can be described in the framework of an effective medium with the effective refractive index of about 1.3 (porosity, approximately 75%), while a strong light scattering for wavelength of 0.3 ÷ 1 μm results in a decrease of the total reflectance of 1%-5%, which cannot be described in the effective medium approximation. The Raman scattering intensity under excitation at approximately 1 μm increases strongly in the sample with SiNWs in comparison with that in c-Si substrate. This effect is related to an increase of the light-matter interaction time due to the strong scattering of the excitation light in SiNW array. The prepared SiNWs are discussed as a kind of 'black silicon', which can be formed in a large scale and can be used for photonic applications as well as in molecular sensing.
- ItemLaser-induced reactive microplasma for etching of fused silica(Berlin ; Heidelberg ; New York : Springer, 2020) Ehrhardt, Martin; Lorenz, Pierre; Han, Bing; Zimmer, KlausThe ultra-precise machining (UPM) of surfaces with contact-free, beam-based technologies enables the development of flexible and reliable fabrication methods by non-vacuum processes for future application in advanced industrial fields. Laser machining by laser ablation features limitations for ultra-precise machining due to the depth precision, the surface morphology, and laser-induced defect formation. Contrary to physically-based etching, chemical-based dry and wet processing offer high quality, low damage material removal. In order to take advantage of both principles, a combined laser-plasma process is introduced. Ultra-short laser pulses are used to induce a free-standing microplasma in a CF4 gas atmosphere due to an optical breakdown. CF4 gas, with a pressure of 800–900 mbar, is ionized only near the focal point and reactive species are generated therein. Reactive species of the laser-induced microplasma can interact with the surface atoms of the target material forming volatile products. The release of these products is enhanced by the pulsed, laser-induced plasma resulting in material etching. In the present study, SiO2 surfaces were etched with reactive species of CF4 microplasma generated by their laser-induced break down with 775 nm pulses of an fs-laser (150 fs) at a repetition rate of 1 kHz. The dependency of the depth, the width, and the morphology of the etching pits were analysed systematically against the process parameters used. In particular, a linear increase of the etching depth up to 10 µm was achieved. The etched surface appears smooth without visible cracks, defects, or LIPSS (Laser-induced periodic surface structures). © 2020, The Author(s).
- ItemLateral Selective SiGe Growth for Local Dislocation-Free SiGe-on-Insulator Virtual Substrate Fabrication(Pennington, NJ : ECS, 2023) Anand, K.; Schubert, M.A.; Corley-Wiciak, A.A.; Spirito, D.; Corley-Wiciak, C.; Klesse, W.M.; Mai, A.; Tillack, B.; Yamamoto, Y.Dislocation free local SiGe-on-insulator (SGOI) virtual substrate is fabricated using lateral selective SiGe growth by reduced pressure chemical vapor deposition. The lateral selective SiGe growth is performed around a ∼1.25 μm square Si (001) pillar in a cavity formed by HCl vapor phase etching of Si at 850 °C from side of SiO2/Si mesa structure on buried oxide. Smooth root mean square roughness of SiGe surface of 0.14 nm, which is determined by interface roughness between the sacrificially etched Si and the SiO2 cap, is obtained. Uniform Ge content of ∼40% in the laterally grown SiGe is observed. In the Si pillar, tensile strain of ∼0.65% is found which could be due to thermal expansion difference between SiO2 and Si. In the SiGe, tensile strain of ∼1.4% along 〈010〉 direction, which is higher compared to that along 〈110〉 direction, is observed. The tensile strain is induced from both [110] and [−110] directions. Threading dislocations in the SiGe are located only ∼400 nm from Si pillar and stacking faults are running towards 〈110〉 directions, resulting in the formation of a wide dislocation-free area in SiGe along 〈010〉 due to horizontal aspect ratio trapping.
- ItemThree-Dimensional Cobalt Hydroxide Hollow Cube/Vertical Nanosheets with High Desalination Capacity and Long-Term Performance Stability in Capacitive Deionization([Beijing] : China Association for Science and Technology, 2021) Xiong, Yuecheng; Yu, Fei; Arnold, Stefanie; Wang, Lei; Presser, Volker; Ren, Yifan; Ma, JieFaradaic electrode materials have significantly improved the performance of membrane capacitive deionization, which offers an opportunity to produce freshwater from seawater or brackish water in an energy-efficient way. However, Faradaic materials hold the drawbacks of slow desalination rate due to the intrinsic low ion diffusion kinetics and inferior stability arising from the volume expansion during ion intercalation, impeding the engineering application of capacitive deionization. Herein, a pseudocapacitive material with hollow architecture was prepared via template-etching method, namely, cuboid cobalt hydroxide, with fast desalination rate (3.3 mg (NaCl)·g-1 (h-Co(OH)2)·min-1 at 100 mA·g-1) and outstanding stability (90% capacity retention after 100 cycles). The hollow structure enables swift ion transport inside the material and keeps the electrode intact by alleviating the stress induced from volume expansion during the ion capture process, which is corroborated well by in situ electrochemical dilatometry and finite element simulation. Additionally, benefiting from the elimination of unreacted bulk material and vertical cobalt hydroxide nanosheets on the exterior surface, the synthesized material provides a high desalination capacity ( mg (NaCl)·g-1 (h-Co(OH)2) at 30 mA·g-1). This work provides a new strategy, constructing microscale hollow faradic configuration, to further boost the desalination performance of Faradaic materials.