2021, Arshad, Muhammad Assad, Hartung, Alexander, Pratiwi, Arni Candra, Jäger, Matthias

We report on the observation of a new phenomenon occurring in a fiber ring laser. This phenomenon is about the transition from an initially bidirectional emission of a reciprocal fiber ring laser to a unidirectional emission at a certain pump power threshold. In addition, the final direction is not predefined but appears to be randomly chosen every time the threshold is exceeded. Therefore, we term this new phenomenon direction instability. Furthermore, we provide a first discussion of how the instability threshold is influenced by the length and the loss of the cavity. We show that the threshold follows a power times length scaling, indicating a nonlinear origin.

2022, Pistiki, Aikaterini, Ryabchykov, Oleg, Bocklitz, Thomas W., Rösch, Petra, Popp, Jürgen

Deep-UV resonance Raman spectroscopy (UVRR) allows the classification of bacterial species with high accuracy and is a promising tool to be developed for clinical application. For this attempt, the optimization of the wavenumber calibration is required to correct the overtime changes of the Raman setup. In the present study, different polymers were investigated as potential calibration agents. The ones with many sharp bands within the spectral range 400–1900 cm−1 were selected and used for wavenumber calibration of bacterial spectra. Classification models were built using a training cross-validation dataset that was then evaluated with an independent test dataset obtained after 4 months. Without calibration, the training cross-validation dataset provided an accuracy for differentiation above 99 % that dropped to 51.2 % after test evaluation. Applying the test evaluation with PET and Teflon calibration allowed correct assignment of all spectra of Gram-positive isolates. Calibration with PS and PEI leads to misclassifications that could be overcome with majority voting. Concerning the very closely related and similar in genome and cell biochemistry Enterobacteriaceae species, all spectra of the training cross-validation dataset were correctly classified but were misclassified in test evaluation. These results show the importance of selecting the most suitable calibration agent in the classification of bacterial species and help in the optimization of the deep-UVRR technique.

2020, Korinth, F., Mondol, A.S., Stiebing, C., Schie, I.W., Krafft, C., Popp, J.

Shifted excitation Raman difference spectroscopy (SERDS) is a background correction method for Raman spectroscopy. Here, the difference spectra were directly used as input for SERDS-based classification after an optimization procedure to correct for photobleaching of the autofluorescence. Further processing included a principal component analysis to compensate for the reduced signal to noise ratio of the difference spectra and subsequent classification by linear discriminant analysis. As a case study 6,028 Raman spectra of single pollen originating from plants of eight different genera and four different growth habits were automatically recorded at excitation wavelengths 784 and 786 nm using a high-throughput screening Raman system. Different pollen were distinguished according to their growth habit, i.e. tree versus non-tree with an accuracy of 95.9%. Furthermore, all pollen were separated according to their genus, providing also insight into similarities based on their families. Classification results were compared using spectra reconstructed from the differences and raw spectra after state-of-art baseline correction as input. Similar sensitivities, specificities, accuracies and precisions were found for all spectra with moderately background. Advantages of SERDS are expected in scenarios where Raman spectra are affected by variations due to detector etaloning, ambient light, and high background.

2020, Vegesna, S.V., Bhat, V.J., Bürger, D., Dellith, J., Skorupa, I., Schmidt, O.G., Schmidt, H.

A novel small signal equivalent circuit model is proposed in the inversion regime of metal/(ZnO, ZnMnO, and ZnCoO) semiconductor/Si3N4 insulator/p-Si semiconductor (MSIS) structures to describe the distinctive nonlinear frequency dependent capacitance (C-F) and conductance (G-F) behaviour in the frequency range from 50 Hz to 1 MHz. We modelled the fully depleted ZnO thin films to extract the static dielectric constant (εr) of ZnO, ZnMnO, and ZnCoO. The extracted enhancement of static dielectric constant in magnetic n-type conducting ZnCoO (εr ≥ 13.0) and ZnMnO (εr ≥ 25.8) in comparison to unmagnetic ZnO (εr = 8.3–9.3) is related to the electrical polarizability of donor-type bound magnetic polarons (BMP) in the several hundred GHz range (120 GHz for CdMnTe). The formation of donor-BMP is enabled in n-type conducting, magnetic ZnO by the s-d exchange interaction between the electron spin of positively charged oxygen vacancies Vo+ in the BMP center and the electron spins of substitutional Mn2+ and Co2+ ions in ZnMnO and ZnCoO, respectively. The BMP radius scales with the Bohr radius which is proportional to the static dielectric constant. Here we show how BMP overlap can be realized in magnetic n-ZnO by increasing its static dielectric constant and guide researchers in the field of transparent spintronics towards ferromagnetism in magnetic, n-ZnO.

2019, Wünsche, Martin, Fuchs, Silvio, Weber, Thomas, Nathanael, Jan, Abel, Johann J., Reinhard, Julius, Wiesner, Felix, Hübner, Uwe, Skruszewicz, Slawomir J., Paulus, Gerhard G., Rödel, Christian

We present a modular extreme ultraviolet (XUV) spectrometer system optimized for a broad spectral range of 12-41 nm (30-99 eV) with a high spectral resolution of λ/Δλ 784 ± 89. The spectrometer system has several operation modes for (1) XUV beam inspection, (2) angular spectral analysis, and (3) imaging spectroscopy. These options allow for a versatile use in high harmonic spectroscopy and XUV beam analysis. The high performance of the spectrometer is demonstrated using a novel cross-sectional imaging method called XUV coherence tomography. © 2019 Author(s).

2020, Wegner, Th., Geiger, B., Foest, R., Jansen van Vuuren, A., Winters, V. R., Biedermann, C., Burhenn, R., Buttenschön, B., Cseh, G., Joda, I., Kocsis, G., Kunkel, F., Quade, A., Schäfer, J., Schmitz, O., Szepesi, T.

Coated glass targets are a key component of the Wendelstein 7-X laser blow-off system that is used for impurity transport studies. The preparation and analysis of these glass targets as well as their performance is examined in this paper. The glass targets have a high laser damage threshold and are coated via physical vapor deposition with μm thick films. In addition, nm-thin layers of Ti are used as an interface layer for improved ablation efficiency and reduced coating stress. Hence, the metallic or ceramic coating has a lateral homogeneity within 2% and contaminants less than 5%, being optimal for laser ablation processing. With this method, a short (few ms) and well defined pulse of impurities with about 1017 particles can be injected close to the last closed flux surface of Wendelstein 7-X. In particular, a significant amount of atoms with a velocity of about 1 km/s enters the plasma within 1 ms. The atoms are followed by a negligible concentration of slower clusters and macro-particles. This qualifies the use of the targets and applied laser settings for impurity transport studies with the laser blow-off system in Wendelstein 7-X. © 2020 Author(s).

2015, Alebrahim, M.A., Krafft, C., Popp, J., El-Khateeb, Mohammad Y.

The structure and chemical features of the human dentin enamel junction (DEJ) were characterized using Raman spectroscopic imaging. Slices were prepared from 10 German, and 10 Turkish teeth. Raman images were collected at 785 nm excitation and the average Raman spectra were calculated for analysis. Univariate and multivariate spectral analysis were applied for investigation. Raman images were obtained based on the intensity ratios of CH at 1450 cm-1 (matrix) to phosphate at 960 cm-1 (mineral), and carbonate to phosphate (1070/960) ratios. Different algorithms (HCA, K-means cluster and VCA) also used to study the DEJ. The obtained results showed that the width of DEJ is about 5 pm related to univariate method while it varies from 6 to 12 μm based on multivariate spectral technique. Both spectral analyses showed increasing in carbonate content inside the DEJ compared to the dentin, and the amide I (collagen) peak in dentin spectra is higher than DEJ spectra peak.

2019, Nathanael, Jan, Wünsche, Martin, Fuchs, Silvio, Weber, Thomas, Abel, Johann J., Reinhard, Julius, Wiesner, Felix, Hübner, Uwe, Skruszewicz, Slawomir J., Paulus, Gerhard G., Rödel, Christian

We present a laboratory beamline dedicated to nanoscale subsurface imaging using extreme ultraviolet coherence tomography (XCT). In this setup, broad-bandwidth extreme ultraviolet (XUV) radiation is generated by a laser-driven high-harmonic source. The beamline is able to handle a spectral range of 30-130 eV and a beam divergence of 10 mrad (full width at half maximum). The XUV radiation is focused on the sample under investigation, and the broadband reflectivity is measured using an XUV spectrometer. For the given spectral window, the XCT beamline is particularly suited to investigate silicon-based nanostructured samples. Cross-sectional imaging of layered nanometer-scale samples can be routinely performed using the laboratory-scale XCT beamline. A depth resolution of 16 nm has been achieved using the spectral range of 36-98 eV which represents a 33% increase in resolution due to the broader spectral range compared to previous work. © 2019 Author(s).

2018, Schmidl, Gabriele, Jia, Guobin, Gawlik, Annett, Kreusch, Jonathan, Schmidl, Frank, Dellith, Jan, Dathe, André, Lin, Zhan-Hong, Huang, Jer-Shing, Plentz, Jonathan

We report on the fabrication of spherical Au spheres by pulsed laser treatment using a KrF excimer laser (248 nm, 25 ns) under ambient conditions as a fast and high throughput fabrication technique. The presented experiments were realized using initial Au layers of 100 nm thickness deposited on optically transparent and low cost Borofloat glass or single-crystalline SrTiO3 substrates, respectively. High (111)-orientation and smoothness (RMS ≈ 1 nm) are the properties of the deposited Au layers before laser treatment. After laser treatment, spheres with size distribution ranging from hundreds of nanometers up to several micrometers were produced. Single-particle scattering spectra with distinct plasmonic resonance peaks are presented to reveal the critical role of optimal irradiation parameters in the process of laser induced particle self-assembly. The variation of irradiation parameters like fluence and number of laser pulses influences the melting, dewetting and solidification process of the Au layers and thus the formation of extremely well shaped spherical particles. The gold layers on Borofloat glass and SrTiO3 are found to show a slightly different behavior under laser treatment. We also discuss the effect of substrates.We report on the fabrication of spherical Au spheres by pulsed laser treatment using a KrF excimer laser (248 nm, 25 ns) under ambient conditions as a fast and high throughput fabrication technique. The presented experiments were realized using initial Au layers of 100 nm thickness deposited on optically transparent and low cost Borofloat glass or single-crystalline SrTiO3 substrates, respectively. High (111)-orientation and smoothness (RMS ≈ 1 nm) are the properties of the deposited Au layers before laser treatment. After laser treatment, spheres with size distribution ranging from hundreds of nanometers up to several micrometers were produced. Single-particle scattering spectra with distinct plasmonic resonance peaks are presented to reveal the critical role of optimal irradiation parameters in the process of laser induced particle self-assembly. The variation of irradiation parameters like fluence and number of laser pulses influences the melting, dewetting and solidification process of the Au layers and thus the formation of extremely well shaped spherical particles. The gold layers on Borofloat glass and SrTiO3 are found to show a slightly different behavior under laser treatment. We also discuss the effect of substrates.

2018, Baghdasaryan, Tigran, Geernaert, Thomas, Chah, Karima, Caucheteur, Christophe, Schuster, Kay, Kobelke, Jens, Thienpont, Hugo, Berghmans, Francis

It is common belief that photonic crystals behave similarly to isotropic and transparent media only when their feature sizes are much smaller than the wavelength of light. Here, we counter that belief and we report on photonic crystals that are transparent for anomalously high normalized frequencies up to 0.9, where the crystal’s feature sizes are comparable with the free space wavelength. Using traditional photonic band theory, we demonstrate that the isofrequency curves can be circular in the region above the first stop band for triangular lattice photonic crystals. In addition, by simulating how efficiently a tightly focused Gaussian beam propagates through the photonic crystal slab, we judge on the photonic crystal’s transparency rather than on isotropy only. Using this approach, we identified a wide range of photonic crystal parameters that provide anomalous transparency. Our findings indicate the possibility to scale up the features of photonic crystals and to extend their operational wavelength range for applications including optical cloaking and graded index guiding. We applied our result in the domain of femtosecond laser micromachining, by demonstrating what we believe to be the first point-by-point grating inscribed in a multi-ring photonic crystal fiber.