Erzeugung harmonischer hoher Ordnung an steilen Plasma-Dichtegradienten

In der vorliegenden Arbeit wird die Erzeugung Harmonischer hoher Ordnung während der Wechselwirkung hochintensiver, ultrakurzer Laserimpulse mit Festkörperoberflächen untersucht. Hierbei werden neben den experimentellen Untersuchungen auch numerische PIC-Simulationen vorgestellt, die nicht nur die experimentellen Ergebnisse bestätigen, sondern auch wesentlich zum physikalischen Verständnis der Harmonischen-Erzeugung beitragen. Die Ordnung der Harmonischen-Emission ist bei der Anregung mit p-polarisierten Laserimpulsen in dem während der Arbeit zugänglichen Intensitätsbereich (1) als auch mit der 2. Harmonischen im nicht relativistischen Intensitätsbereich (a0<0.3) nachgewiesen werden. Entscheidend für den Nachweis ist der Einsatz von Festkörpern, die unter der Annahme der vollständigen Ionisation, in verschiedenen Elektronendichten resultieren. Die experimentellen Untersuchungen bei der Anregung mit der p-polarisierten 2. Harmonischen können mit Hilfe von numerischen PIC-Simulationen qualitativ und quantitativ reproduziert werden. Entscheidend für die Übereinstimmung ist die Annahme einer sehr kurzen Skalenlänge des Plasmas (L/l =O(0.01)), die den Elektronendichte-Verlauf des Oberflächenplasmas am Plasma-Vakuum-Übergang charakterisiert. Die numerischen Simulationen zeigen, dass der Mechanismus der Harmonischen-Erzeugung auf der Anregung von Elektronendichte-Oszillationen in dem inhomogenen Bereich des Plasma-Vakuum Übergangs beruht. Die Frequenzen dieser Oszillationen entsprechen wiederum ganzzahligen Vielfachen der Laserfrequenz. Die numerischen Simulationen berechnen für größere Skalenlängen (L/l >0.1) Harmonischen-Spektren, deren Ordnung nicht durch die Plasmafrequenz begrenzt wird. In diesem Fall basiert die Erzeugung Harmonischer hoher Ordnung durch die anharmonische Oszillation der kritischen Elektronendichte, deren Frequenzspektrum wiederum aus ganzzahligen Harmonischen der Laserfrequenz besteht. Beide Bereiche werden durch einen Dritten abgetrennt, in dem die berechneten Spektren eine Erzeugung von höheren Harmonischen ausschließen. Mit Hilfe eines künstlichen Vorimpulses, dessen Verzögerungszeit bezüglich des hochintensiven Laserimpulses variiert werden kann, wird die Abhängigkeit der Harmonischen-Erzeugung von der Skalenlänge experimentell untersucht. Ein Übergang in den Bereich der Harmonischen-Erzeugung ohne die Einschränkung durch die Plasmafrequenz kann nicht verifiziert werden. Lediglich ein schneller Abfall des Harmonischen-Signals unabhängig von der Ordnung wird beobachtet . Eine mögliche Ursache für diese Diskrepanz ist die Ausbildung von räumlichen Störungen innerhalb des Oberflächenplasmas. Hinweise auf diese Störungen liefern Untersuchungen bezüglich der reflektieren, bzw. vom Oberflächenplasma emittierten, räumlichen Energieverteilung. Die Experimente zur Skalenlängenabhängigkeit der Harmonischen-Erzeugung führen zu einem weiteren interessanten Gebiet der Laser-Plasma-Wechselwirkung. Für sehr kurze Skalenlängen L/l»1 und schwach relativistische Intensitäten (a0=0.6) wird die Emission der 3/2-Harmonischen beobachtet, die unmittelbar an die Entwicklung von parametrischen Instabilitäten während der Laser-Plasma-Wechselwirkung gekoppelt ist. Für Skalenlängen L/l»1 erfolgt die Emission kollimiert in zwei verschiedene Richtungen »25° und »75° bei einem Einfallswinkel von q=38°. Für größere Skalenlängen wird die 3/2-Harmonische in den gesamten vor der Festkörperprobe befindlichen Halbraum emittiert. Die bestimmten Spektren sind sehr breit und lassen auf ein noch sehr viel breiteres Plasmonen-Spektrum schließen. Im Bereich der kollimierten Emission decken die über mehrere Laserimpulse gemittelten Spektren eine Abhängigkeit von der Emissionrichtung aus. Erklärt werden können die gemessenen Spektren durch die Entwicklung einer hybriden Plasma-Instabilität, bestehend aus Zwei-Plasmonen-Zerfall und Stimulierter-Raman-Streuung , sowie durch die räumliche Trennung der parametrischen Plasma-Instabilitäten und der nachfolgenden Summenfrequenz-Mischung

Trapping colloids near chemical stripes via critical Casimir forces

We study theoretically and experimentally the solvent-mediated critical Casimir force acting on colloidal particles immersed in a binary liquid mixture of water and 2,6-lutidine and close to substrates which are chemically patterned with periodically alternating stripes of antagonistic adsorption preferences. These patterns are experimentally realized via microcontact printing. Upon approaching the critical demixing point of the solvent, normal and lateral critical Casimir forces generate laterally confining effective potentials for the colloids. We analyze in detail the rich behavior of the spherical colloids close to such substrates. For all patterned substrates we investigated, our measurements of these effective potentials agree with the corresponding theoretical predictions. Since both the directions and the strengths of the critical Casimir forces can be tuned by minute temperature changes, this provides a new mechanism for controlling colloids as model systems, opening encouraging perspectives for applications.Comment: Invited contribution to Molecular Physics Special Issue on Bob Evans' 65th birthda

Monitoring Early Response to Anti-Angiogenic Therapy: Diffusion-Weighted Magnetic Resonance Imaging and Volume Measurements in Colon Carcinoma Xenografts

Objectives: To evaluate the use of diffusion-weighted MRI (DW-MRI) and volume measurements for early monitoring of antiangiogenic therapy in an experimental tumor model. Materials and Methods: 23 athymic nude rats, bearing human colon carcinoma xenografts (HT-29) were examined before and after 6 days of treatment with regorafenib (n=12) or placebo (n=11) in a clinical 3-Tesla MRI. For DW-MRI, a single-shot EPI sequence with 9 b-values (10-800 s/mm(2)) was used. The apparent diffusion coefficient (ADC) was calculated voxelwise and its median value over a region of interest, covering the entire tumor, was defined as the tumor ADC. Tumor volume was determined using T2-weighted images. ADC and volume changes between first and second measurement were evaluated as classifiers by a receiver-operator-characteristic (ROC) analysis individually and combined using Fisher's linear discriminant analysis (FLDA). Results: All ADCs and volumes are stated as median +/- standard deviation. Tumor ADC increased significantly in the therapy group (0.76 +/- 0.09x10(-3) mm(2)/s to 0.90 +/- 0.12x10(-3) mm(2)/s;p<0.001), with significantly higher changes of tumor ADC than in the control group (0.10 +/- 0.11x10(-3) mm(2)/s vs. 0.03 +/- 0.09x10(-3) mm(2)/s;p = 0.027). Tumor volume increased significantly in both groups (therapy: 347.8 +/- 449.1 to 405.3 +/- 823.6 mm(3);p = 0.034;control: 219.7 +/- 79.5 to 443.7 +/- 141.5 mm(3);p<0.001), however, the therapy group showed significantly reduced tumor growth (33.30 +/- 47.30% vs. 96.43 +/- 31.66%;p<0.001). Area under the curve and accuracy of the ADC-based ROC analysis were 0.773 and 78.3%;and for the volume change 0.886 and 82.6%. The FLDA approach yielded an AUC of 0.985 and an accuracy of 95.7%. Conclusions: Regorafenib therapy significantly increased tumor ADC after 6 days of treatment and also significantly reduced tumor growth. However, ROC analyses using each parameter individually revealed a lack of accuracy in discriminating between therapy and control group. The combination of both parameters using FLDA substantially improved diagnostic accuracy, thus highlighting the potential of multi-parameter MRI as an imaging biomarker for non-invasive early tumor therapy monitoring

Hair Cortisol and Perceived Stress-Predictors for the Onset of Tics?:A European Longitudinal Study on High-Risk Children

Some retrospective studies suggest that psychosocial stressors trigger the onset of tics. This study examined prospective hypothalamic-pituitary-adrenal (HPA) axis activity and perceived stress prior to tic onset. In the present study, 259 children at high risk for developing tics were assessed for hair cortisol concentration (HCC) and parent-on-child-reported perceived stress four-monthly over a three-year period. We used (i) generalised additive modelling (GAM) to investigate the time effects on HCC (hair samples n = 765) and perceived stress (questionnaires n = 1019) prior to tic onset and (ii) binary logistic regression to predict tic onset in a smaller subsample with at least three consecutive assessments (six to nine months before, two to five months before, and at tic onset). GAM results indicated a non-linear increasing course of HCC in children who developed tics, and a steady HCC course in those without tics, as well as a linear-increasing course of perceived stress in both groups. Logistic regression showed that with a higher HCC in hair samples collected in a range of two to five months before tic onset (which refers to cortisol exposure in a range of four to eight months), the relative likelihood of tic onset rose. Our study suggests increased stress prior to tic onset, as evidenced by higher HCC several months before tic onset. </p

Salt-induced changes of colloidal interactions in critical mixtures

We report on salt-dependent interaction potentials of a single charged particle suspended in a binary liquid mixture above a charged wall. For symmetric boundary conditions (BC) we observe attractive particle-wall interaction forces which are similar to critical Casimir forces previously observed in salt-free mixtures. However, in case of antisymmetric BC we find a temperature-dependent crossover from attractive to repulsive forces which is in strong contrast to salt-free conditions. Additionally performed small-angle x-ray scattering experiments demonstrate that the bulk critical fluctuations are not affected by the addition of salt. This suggests that the observed crossover can not be attributed alone to critical Casimir forces. Instead our experiments point towards a possible coupling between the ionic distributions and the concentration profiles in the binary mixture which then affects the interaction potentials in such systems.Comment: 5 pages, 4 Figure

Vena Cava Thrombus in Patients with Wilms Tumor

(1) Background: Vena cava thrombus (VCT) is rare in Wilms tumor (WT) (4–10%). The aim of this study is to identify factors for an outcome to improve treatment for better survival. (2) Methods: 148/3015 patients with WT (aged < 18 years) and VCT, prospectively enrolled over a period of 32 years (1989–2020) by the German Society for Pediatric Oncology and Hematology (SIOP-9/GPOH, SIOP-93-01/GPOH and SIOP-2001/GPOH), are retrospectively analyzed to describe clinical features, response to preoperative chemotherapy (PC) (142 patients) and surgical interventions and to evaluate risk factors for overall survival (OS). (3) Results: 14 VCT regressed completely with PC and another 12 in parts. The thrombus was completely removed in 111 (85.4%), incompletely in 16 (12.3%), and not removed in 3 (2.3%). The type of removal is unknown in four patients. Patients without VCT have a significantly (p < 0.001) better OS (97.8%) than those with VCT (90.1%). OS after complete resection is (89.9%), after incomplete (93.8%) and with no resection (100%). Patients with anaplasia or stage IV without complete remission (CR) after PC had a significantly worse OS compared to the remaining patients with VCT (77.1% vs. 94.4%; p = 0.002). (4) Conclusions: As a result of our study, two risk factors for poor outcomes in WT patients with VCT emerge: diffuse anaplasia and metastatic disease, especially those with non-CR after PC

Hair cortisol-a stress marker in children and adolescents with chronic tic disorders? A large European cross-sectional study

Background: There is clear evidence that tic disorders (TDs) are associated with psychosocial stress as well as emotional and behavioral problems. Studies have shown that individuals with TDs have higher acute physiological stress responses to external, single stressors (as reflected by saliva cortisol). The aim of the present study was to examine a physiological marker of longer-term stress (as reflected by hair cortisol concentration) in children and adolescents with TDs and unaffected siblings of individuals with TDs. Methods: Two samples of a European cohort were included in this study. In the COURSE sample, 412 children and adolescents aged 3-16 years with a chronic TD including Tourette syndrome according to DSM IV-TR criteria were included. The ONSET sample included 131 3-10 years old siblings of individuals with TDs, who themselves had no tics. Differences in hair cortisol concentration (HCC) between the two samples were examined. Within the COURSE sample, relations of HCC with tic severity and perceived psychosocial stress as well as potential effects and interaction effects of comorbid emotional and behavioral problems and psychotropic medication on HCC were investigated. Results: There were no differences in HCC between the two samples. In participants with TDs, there were no associations between HCC and tic severity or perceived psychosocial stress. No main effects of sex, psychotropic medication status and comorbid emotional and behavioral problems on HCC were found in participants with TDs. Conclusion: A link between HCC and TDs is not supported by the present results

3D-printed facet-attached optical elements for connecting VCSEL and photodiodes to fiber arrays and multi-core fibers

Multicore optical fibers and ribbons based on fiber arrays allow for massively parallel transmission of signals via spatially separated channels, thereby offering attractive bandwidth scaling with linearly increasing technical effort. However, low-loss coupling of light between fiber arrays or multicore fibers and standard linear arrays of vertical-cavity surface-emitting lasers (VCSEL) or photodiodes (PD) still represents a challenge. In this paper, we demonstrate that 3D-printed facet-attached microlenses (FaML) offer an attractive path for connecting multimode fiber arrays as well as individual cores of multimode multicore fibers to standard arrays of VCSEL or PD. The freeform coupling elements are printed in situ with high precision on the device and fiber facets by high-resolution multi-photon lithography. We demonstrate coupling losses down to 0.35 dB along with lateral 1 dB alignment tolerances in excess of 10 μm, allowing to leverage fast passive assembly techniques that rely on industry-standard machine vision. To the best of our knowledge, our experiments represent the first demonstration of a coupling interface that connects individual cores of a multicore fiber to VCSEL or PD arranged in a standard linear array without the need for additional fiber-based or waveguide-based fan-out structures. Using this approach, we build a 3 × 25 Gbit/s transceiver assembly which fits into a small form-factor pluggable module and which fulfills many performance metrics specified in the IEEE 802.3 standard

3D-printed facet-attached optical elements for connecting VCSEL and photodiodes to fiber arrays and multi-core fibers

Multicore optical fibers and ribbons based on fiber arrays allow for massively parallel transmission of signals via spatially separated channels, thereby offering attractive bandwidth scaling with linearly increasing technical effort. However, low-loss coupling of light between fiber arrays or multicore fibers and standard linear arrays of vertical-cavity surface-emitting lasers (VCSEL) or photodiodes (PD) still represents a challenge. In this paper, we demonstrate that 3D-printed facet-attached microlenses (FaML) offer an attractive path for connecting multimode fiber arrays as well as individual cores of multimode multicore fibers to standard arrays of VCSEL or PD. The freeform coupling elements are printed in situ with high precision on the device and fiber facets by high-resolution multi-photon lithography. We demonstrate coupling losses down to 0.35 dB along with lateral 1 dB alignment tolerances in excess of 10 μm, allowing to leverage fast passive assembly techniques that rely on industry-standard machine vision. To the best of our knowledge, our experiments represent the first demonstration of a coupling interface that connects individual cores of a multicore fiber to VCSEL or PD arranged in a standard linear array without the need for additional fiber-based or waveguide-based fan-out structures. Using this approach, we build a 3 × 25 Gbit/s transceiver assembly which fits into a small form-factor pluggable module and which fulfills many performance metrics specified in the IEEE 802.3 standard