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Subject: search.filters.subject.530Institute: search.filters.UBSInstitut.4. Physikalisches InstitutStart date: 2018End date: 2019

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Now showing 1 - 7 of 7
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    Photo-excited dynamics in the excitonic insulator Ta2NiSe5
    (2018) Werdehausen, Daniel; Takayama, Tomohiro; Albrecht, Gelon; Lu, Yangfan; Takagi, Hidenori; Kaiser, Stefan
    The excitonic insulator is an intriguing correlated electron phase formed of condensed excitons. A promising candidate is the small band gap semiconductor Ta2NiSe5. Here we investigate the quasiparticle and coherent phonon dynamics in Ta2NiSe5 in a time resolved pump probe experiment. Using the models originally developed by Kabanov et al for superconductors (Kabanov et al 1999 Phys. Rev. B 59 1497), we show that the material’s intrinsic gap can be described as almost temperature independent for temperatures up to about 250 K to 275 K. This behavior supports the existence of the excitonic insulator state in Ta2NiSe5. The onset of an additional temperature dependent component to the gap above these temperatures suggests that the material is located in the BEC-BCS crossover regime. Furthermore, we show that this state is very stable against strong photoexcitation, which reveals that the free charge carriers are unable to effectively screen the attractive Coulomb interaction between electrons and holes, likely due to the quasi 1D structure of Ta2NiSe5.
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    Hybrid materials for nonlinear optics
    (2018) Albrecht, Gelon; Giessen, Harald (Prof. Dr.)
    The goals of this thesis are to find new and more efficient material systems as well as concepts for nonlinear optics on the nanoscale. Nonlinear optical effects are mainly limited in such systems by the low nonlinear susceptibility and low photo stability of the used materials. To improve the low nonlinear susceptibility, plasmonic materials have been used for several years. These systems use the near-field enhancement of the plasmonic resonance to increase the nonlinear conversion efficiency. The efficiency can additionally be increased by using the evanescent plasmonic near-field in the vicinity of the plasmonic nanostructure. Therefore, a highly nonlinear organic polymer is deposited on the plasmonic nanostructures, creating a hybrid organic plasmonic material. Several organic materials are particularly suited due to their high nonlinear susceptibility and their simple and reproducible handling. Combined with high photo stability, these are the key requirements for a suitable polymer. However, several tested polymers did not meet these requirements. Notably, the photo stability is too low. Furthermore, for the first time it could be unambiguously proven that these hybrid materials can be improved due to an increased overall nonlinear susceptibility. Many other concepts for hybrid materials only utilize the modified near-field distribution and cannot benefit from the surrounding nonlinear medium or cannot exclude this influence. The presented layout can easily be improved by replacing the used polymer with other existing polymers that exhibit larger nonlinear susceptibilities. The hybrid plasmonic structures use gold as plasmonic material. Even if it is more photo stable than polymers, gold does not withstand high illumination intensities due to its low dimensional stability. This is a major drawback since most applications require a stable plasmon resonance. To overcome this issue a simple but effective way to significantly increase the thermal stability as well as the photo stability of gold nanostructures is presented. The improved properties are due to an alumina protective coating. The alumina coating can be as thin as 4 nm maintaining access to the enhanced near-field of the plasmonic nanostructure. With this concept a platform for nonlinear optics and high temperature applications is available that is stable in air at temperatures up to 900°C and still has excellent optical properties. Moreover this system withstands laser intensities at least up to 10 GW/cm² , one order of magnitude more than usually used intensities for nonlinear spectroscopy on gold nanostructures. Finally, common and more uncommon plasmonic materials are surveyed to determine their linear and nonlinear optical properties. Furthermore, the thermal and chemical stability with and without a protective alumina coating is investigated. Based on the collected data silver, gold, copper, magnesium, and aluminum could be identified and confirmed to be suitable materials for nonlinear applications. Moreover, nickel, palladium, platinum, germanium, and YH2 are investigated for their plasmonic and thermal properties, however suitable nonlinear properties have not been observed. Based on this survey a comparison of the presented materials is possible, which surprisingly did not exist until this survey. Bi2Te2Se is investigated as an unusual plasmonic material that exhibits edge state plasmons. These edge state plasmons arise from the topological properties of the material. Up to now these edge state plasmons have only been observed via electron excitation. To reveal the predicted localized modes nanostructures are fabricated by several methods and dark field spectroscopy is applied. However, no optical plasmonic response could be identified, most likely due to the small scattering rate of the material.
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    Mid-infrared resonant nanostructures for in-vitro monitoring of polypeptides
    (2019) Semenyshyn, Rostyslav; Giessen, Harald (Prof. Dr.)
    Infrared vibrational spectroscopy is a technique based on the molecular vibrations, that is, the oscillation of individual atoms with respect to each other. Each of these vibrations has a characteristic resonance frequency which leads to the distinct vibrational fingerprint of a molecule and thus enables a label-free, non-destructive, and chemically specific detection of molecular species. The infrared absorption cross-sections, which characterize the optical interaction strength, are relatively small. This is of minor importance for conventional spectroscopy, where large ensembles of molecules can be measured and thus contribute to the overall signal. However, the small infrared absorption hampers detection of molecules at low concentrations, which is of large importance for medical diagnostics, for instance, where the determination of the secondary structures of proteins is crucial due to their role in many incurable diseases. A key to overcome this limitation is to utilize plasmonic nanostructures, which confine the electromagnetic radiation on the nanometer scale and allow higher overall absorption. In this dissertation, it is demonstrated that even a monolayer of proteins can be detected using mid-infrared resonant gold nanostructures. We use polypeptides as a model system and were able to investigate the secondary structure of molecular monolayers in-vitro. Applying different external stimuli, we are able to induce structural changes of polypeptides in aqueous environments. In addition to a mid-infrared resonant nanoantenna, nanoslits (or inverse antennas) can also enhance the optical response of polypeptides, which allowed us to detect the secondary structure of minicollagen monolayers. Both nanostructure designs provided the possibility even to monitor reversible conformational transitions of molecular monolayers. Scaling this approach down to a single nanostructure allows to detect only a few thousands of polypeptides in liquid environments. The demonstrated concept could lead to integrated chip-level technology for biological and even medical applications, where biosamples with minute concentrations are investigated. With further advances, it could be possible to scale the process to a few or single proteins and observe the structural changes of individual entities.
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    Adaptive method for quantitative estimation of glucose and fructose concentrations in aqueous solutions based on infrared nanoantenna optics
    (2019) Schuler, Benjamin; Kühner, Lucca; Hentschel, Mario; Giessen, Harald; Tarín, Cristina
    In life science and health research one observes a continuous need for new concepts and methods to detect and quantify the presence and concentration of certain biomolecules-preferably even in vivo or aqueous solutions. One prominent example, among many others, is the blood glucose level, which is highly important in the treatment of, e.g., diabetes mellitus. Detecting and, in particular, quantifying the amount of such molecular species in a complex sensing environment, such as human body fluids, constitutes a significant challenge. Surface-enhanced infrared absorption (SEIRA) spectroscopy has proven to be uniquely able to differentiate even very similar molecular species in very small concentrations. We are thus employing SEIRA to gather the vibrational response of aqueous glucose and fructose solutions in the mid-infrared spectral range with varying concentration levels down to 10 g/l. In contrast to previous work, we further demonstrate that it is possible to not only extract the presence of the analyte molecules but to determine the quantitative concentrations in a reliable and automated way. For this, a baseline correction method is applied to pre-process the measurement data in order to extract the characteristic vibrational information. Afterwards, a set of basis functions is fitted to capture the characteristic features of the two examined monosaccharides and a potential contribution of the solvent itself. The reconstruction of the actual concentration levels is then performed by superposition of the different basis functions to approximate the measured data. This software-based enhancement of the employed optical sensors leads to an accurate quantitative estimate of glucose and fructose concentrations in aqueous solutions.
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    3D printed stacked diffractive microlenses
    (2019) Thiele, Simon; Pruss, Christof; Herkommer, Alois; Giessen, Harald
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    Modeling of second-harmonic generation in periodic nanostructures by the Fourier modal method with matched coordinates
    (2018) Defrance, Josselin; Schäferling, Martin; Weiss, Thomas
    We present an advanced formulation of the Fourier modal method for analyzing the second-harmonic generation in multilayers of periodic arrays of nanostructures. In our method, we solve Maxwell’s equations in a curvilinear coordinate system, in which the interfaces are defined by surfaces of constant coordinates. Thus, we can apply the correct Fourier factorization rules as well as adaptive spatial resolution to nanostructures with complex cross sections. We extend here the factorization rules to the second-harmonic susceptibility tensor expressed in the curvilinear coordinates. The combination of adaptive curvilinear coordinates and factorization rules allows for efficient calculation of the second-harmonic intensity, as demonstrated for one- and two-dimensional periodic nanostructures.