2,290 research outputs found
Characterization of the Nonlinear Refractive Index of Carbon Disulfide Over an Extended Spectral and Temporal Range
The intensity dependent refractive index change of a medium is frequently described in terms of the product n₂ · I where n₂ is the nonlinear refractive index and I the light intensity. The nonlinear refractive index is often treated as constant which is a reasonable assumption if the light interacts only with bound electrons. In the case of carbon disulfide (CS₂) however, nuclear motions contribute to n₂. These motions occur on the sub picosecond time scale and thus become especially relevant for ultrashort laser pulses. The neat liquid CS₂ is studied because it exhibits a large nonlinear refractive index in comparison to other liquids. Therefore, it is employed in optical switching, optical limiting, and beam filamentation applications. This thesis presents effective n₂ values for Gaussian shaped linearly polarized pulses with central wavelength at [lambda]= 700nm. A theoretical model describing the time evolution of the material response is applied to distinguish between the instantaneous electronic, the ultrafast nuclear and the slow nuclear origins of the nonlinear refractive index. Moreover, the tensor nature of the material response function is studied by means of circularly polarized light. The relative magnitudes of bound electronic and nuclear contributions to n₂ are experimentally determined. Eventually, the dispersion of the instantaneous electronic response is measured in the spectral range between 390nm and 1064nm
A new generation of high-power, waveform controlled, few-cycle light sources
Mit der zunehmenden Verbreitung von in Spitzen- und Durchschnittsleistung skalierbaren Femtosekundenlasern, insbesondere von modengekoppelten Dünnscheibenoszillatoren, entstand auch die Nachfrage nach ebenso leistungsskalierbaren Methoden der Pulskompression, Träger-Einhüllenden-Phasenstabilisierung und Frequenzumwandlung. Diese Techniken werden routinemäßig bei ultraschnellen Lasern mit kleineren Durchschnittsleistungen angewandt, zum Beispiel den Titan:Saphir-basierten. Jedoch mussten sie für die stärksten Femtosekundenlichtquellen mit etwa 100 W Durchschnittsleitung und 10 MW Spitzenleistung neu erfunden werden.
Diese Dissertation zeigt, wie die von einem modengekoppelten Dünnscheibenoszillator mit 45 W Durchschnittsleistung emittierten Pulse zum ersten Mal auf eine Dauer von nur wenigen optischen Zyklen komprimiert worden sind. Die lediglich 7.7 fs kurzen Pulse wurden durch zwei sequentielle spektralen Verbreiterungs- und Kompressionsstufen mit gechirpten Spiegeln erzielt.
Selbige Lichtquelle war auch der erste Dünnscheibenoszillator, sowie gleichzeitig der erste Laseroszillator mit mehr als 10 W Durchschnittsleistung, von dessen emittierten Pulszügen die Träger-Einhüllenden-Phase stabilisiert wurde. Zwei Stabilisierungsschemata werden vorgestellt: Das erste beruht auf der Modulation der Leistungsverluste des Lichts in der Laserkavität durch einen akustooptischen Modulator. Der Ansatz resultierte in einem verbleibenden Phasenrauschen von 125mrad, gemessen im Regelkreis, sowie 270 mrad, außerhalb des Regelkreises gemessen. Die zweite Methode verwendete ein zusätzliches Netzgerät zur Modulation des Pumplaserdiodenstroms. Hierbei wurde ein verbleibendes, im Regelkreis gemessenes Phasenrauschen von 390mrad erreicht.
Während die Methoden zur Stabilisierung der Träger-Einhüllenden-Phase leistungsskalierbar sind, wird die Skalierbarkeit des ursprünglichen Ansatzes zur Kompression der Oszillatorpulse auf nur wenige optische Zyklen durch die Zerstörschwelle von photonischen Kristallfasern mit festem Kern eingeschränkt. Die Dissertation beschreibt detailliert die Limitierungen der Fasern bezüglich maximal durchlässiger Spitzenleistungen sowie erreichbarer spektraler Verbreitungsfaktoren. Weiterhin wird ein alternativer Ansatz demonstriert. Dieser beruht auf spektraler Verbreiterung in Hohlkernfasern mit einer
Kagomé-artigen photonischen Kristallstruktur. Ein zweistufiges Verbreiterungs- und Kompressionsexperiment erzielte Pulsdauern von nur 9.1fs. Allerdings zeigte sich auch eine signifikante Überhöhung des Intensitätsrauschens der ultrakurzen Pulse im Vergleich zu denen, die direkt vom Laseroszillator emittiert wurden.
Daher wurde spektrales Verbreitern in Kristallfenstern untersucht. Durch das Ausnutzen des optischen Kerr-Effekts wurden Spektren erzeugt, deren Bandbreite Pulsdauern von 15 fs erlaubt. Dieses Ergebnis motiviert die ausschließliche Anwendung von Kristallfenstern in der spektralen Verbreiterung von kurzen Pulsen. Simulationsergebnisse für die Lichtpropagation durch eine Reihe von Kerr-Medien versprechen eine gute Effizienz dieser Methode. Darüber hinaus wurde die Kompression von 190fs Pulsen mit einer Durchschnittsleistung von 90 W zu 30 fs und einer Durchschnittsleistung von 70 W experimentell demonstriert. Hierzu wurden selbst-defokussierende BBO-Kristalle verwendet.
Die umfassende Studie über spektrale Verbreiterungs- und Pulskompressionstechniken ist
wegbereitend für die Erzeugung von kurzen Pulsen mit Dauern von wenigen optischen Zyklen bei Spitzenleistungen von einigen hundert Megawatt und Durchschnittsleitungen von mehr als einhundert Watt.
Schließlich befasst sich die Dissertation mit der Aufgabe die breitbandigen, leistungsstarken Spektren in einen Wellenlängenbereich zu übertragen, der eine riesige Anzahl an charakteristischen molekularen Absorptionen beherbergt - dem mittleren Infrarot. Die Umwandlung des Nahinfrarots zu kleineren Frequenzen durch optische parametrische Verstärkung resultierte in Strahlung mit bis zu 5W bei 4,1 Mikrometer und 1.3 W bei 8,5 Mikrometer, was einer Durchschnittsleistungssteigerung von einer Größenordnung gegenüber anderen kompakten Femtosekundenlichtquellen mit über 5 Mikrometer Wellenlänge entspricht.
Zusätzlich zu den Leistungsmessungen werden Durchstimmbarkeit und Superkontinuumserzeugung durch aufeinanderfolgende quadratische Nichtlinearitäten demonstriert. Dies führte insgesamt zu einer spektralen Abdeckung der Wellenlängen von 1,6 bis 11 Mikrometer mit Leistungsdichten, welche 1 µW/cm−1 im gesamten Bereich übersteigen.
Die demonstrierten Methoden zur Pulskompression sowie Träger-Einhüllenden-Phasenstabilisierung werden als grundlegende Techniken für die weitere Entwicklung einer neuen Generation von Hochleistungslichtquellen dienen. Durch Wellenform-kontrollierte Pulse, die nur wenige optische Zyklen dauern, können extrem nichtlineare Effekte bei bisher noch nicht erreichten Durchschnittsleistungen und Wiederholraten erzeugt werden. Die mehrere Oktaven überspannende Femtosekundenlichtquelle im mittleren Infrarot verspricht einzigarte Anwendungen beim Detektieren von molekularen Fingerabdrücken, insbesondere durch die Möglichkeiten zur Zurückwandlung der Frequenzen in den Nahinfrarotbereich oder die Nutzung der Frequenzkammspektroskopie.With the advent of peak and average power scalable femtosecond lasers, in particular mode-locked thin-disk oscillators, the need for equally scalable pulse compression, carrier-envelope-phase stabilization and frequency conversion schemes arose. These techniques have been routinely applied to lower average power ultrafast lasers, for instance the widely used Ti:sapphire based ones. But they had to be reinvented for cutting-edge femtosecond sources with 100 W level average and 10 MW level peak powers.
This dissertation presents how pulses emitted from a 45 W average power mode-locked thin-disk oscillator were compressed for the first time to a duration of only a few optical cycles. Pulses as short as 7.7 fs were attained from two sequential spectral broadening and chirped mirror pulse compression stages.
The same light source was also the first thin-disk oscillator, and simultaneously the first oscillator with an average power of more than 10 W, which was carrier-envelope-phase stabilized. Two stabilization methods are presented: The first one utilized intracavity loss modulation by means of an acousto-optic modulator. This resulted in 125 mrad in-loop and 270 mrad out-of-loop residual phase noise. The second one employed pump diode current modulation by means of an auxiliary power supply. This approach yielded a 390 mrad residual in-loop phase noise.
Whereas the presented carrier-envelope-phase stabilization schemes are power-scalable, the scalability of the initial few-cycle pulse generation approach is restricted by the damage threshold of solid-core photonic crystal fiber. The thesis reports in detail on the limitations of these fibers with respect to maximally transmittable peak powers and attainable spectral broadening factors. Moreover, an alternative approach utilizing hollow-core Kagomé-type photonic crystal fibers is demonstrated. A double-stage broadening and compression setup yielded pulse durations of only 9.1 fs, but also showed a significant intensity noise increase in comparison to the thin-disk oscillator output.
Therefore, spectral broadening in bulk crystals was studied. By exploiting the optical Kerr effect, spectra with Fourier transform-limits of 15 fs were achieved, opening the perspective for all solid-state spectral broadening in bulk material. Simulation results for a sequence of thin Kerr media predict a good power efficiency of the method. Furthermore, an experimental realization of pulse compression from 190 fs pulses with 90 W average power to 30 fs pulses with 70 W average power in self-defocusing BBO crystals is reported. The presented comprehensive study on spectral broadening and pulse compression techniques paves the way to few-cycle pulse generation at hundreds of MW peak power and hundreds of Watts average power.
Eventually, the dissertation addresses the issue of transferring broadband, powerful spectra to a wavelength region with a huge variety of characteristic molecular absorptions - the mid-infrared. Frequency down-conversion via optical parametric amplification resulted in radiation with up to 5 W at 4.1 microns and 1.3 W at 8.5 microns, corresponding to an order-of-magnitude average power increase for compact femtosecond light sources operating at wavelengths longer than 5 microns.
In addition to the power measurements, both wavelength tunability and supercontinuum generation by means of cascaded quadratic nonlinearities are reported, resulting in overall spectral coverage from 1.6 to 11 microns with power spectral densities exceeding 1 µW/cm−1 over the entire range.
The pulse compression and carrier-envelope-phase stabilization schemes demonstrated in this dissertation will serve as fundamental techniques for the further development of a new generation of waveform-controlled few-cycle pulse lasers which are capable of triggering extreme nonlinear effects at unprecedented average powers and repetition rates. The multi-octave spanning, mid-infrared femtosecond source offers exciting opportunities for molecular fingerprinting, in particular by means of frequency up-conversion and field-sensitive techniques as well as frequency comb spectroscopy
Principles of Green Data Mining
This paper develops a set of principles for green data mining, related to the key stages of business un- derstanding, data understanding, data preparation, modeling, evaluation, and deployment. The principles are grounded in a review of the Cross Industry Stand- ard Process for Data mining (CRISP-DM) model and relevant literature on data mining methods and Green IT. We describe how data scientists can contribute to designing environmentally friendly data mining pro- cesses, for instance, by using green energy, choosing between make-or-buy, exploiting approaches to data reduction based on business understanding or pure statistics, or choosing energy friendly models
Pharmacophore Models Derived From Molecular Dynamics Simulations of Protein-Ligand Complexes: A Case Study.
A single, merged pharmacophore hypothesis is derived combining 2000 pharmacophore models obtained during a 20 ns molecular dynamics simulation of a protein-ligand complex with one pharmacophore model derived from the initial PDB structure. This merged pharmacophore model contains all features that are present during the simulation and statistical information about the dynamics of the pharmacophore features. Based on the dynamics of the pharmacophore features we derive two distinctive feature patterns resulting in two different pharmacophore models for the analyzed system – the first model consists of features that are obtained from the PDB structure and the second uses two features that can only be derived from the molecular dynamics simulation. Both models can distinguish between active and decoy molecules in virtual screening. Our approach represents an objective way to add/remove features in pharmacophore models and can be of interest for the investigation of any naturally occurring system that relies on ligand-receptor interactions for its biological activity
Reuse, Reduce, Support: Design Principles for Green Data Mining
This paper reports on a design science research (DSR) study that develops design principles for “green” – more environmentally sustainable – data mining processes. Grounded in the Cross Industry Standard Process for Data Mining (CRISP-DM) and on a review of relevant literature on data mining methods, Green IT, and Green IS, the study identifies eight design principles that fall into the three categories of reuse, reduce, and support. The paper develops an evaluation strategy and provides empirical evidence for the principles’ utility. It suggests that the results can inform the development of a more general approach towards Green Data Science and provide a suitable lens to study sustainable computing
Efficient High-Power Ultrashort Pulse Compression in Self-Defocusing Bulk Media
Peak and average power scalability is the key feature of advancing femtosecond laser technology. Today, near-infrared light sources are capable of providing hundreds of Watts of average power. These sources, however, scarcely deliver pulses shorter than 100fs which are, for instance, highly beneficial for frequency conversion to the extreme ultraviolet or to the mid-infrared. Therefore, the development of power scalable pulse compression schemes is still an ongoing quest. This article presents the compression of 90 W average power, 190 fs pulses to 70 W, 30 fs. An increase in peak power from 18 MW to 60 MW is achieved. The compression scheme is based on cascaded phase-mismatched quadratic nonlinearities in BBO crystals. In addition to the experimental results, simulations are presented which compare spatially resolved spectra of pulses spectrally broadened in self-focusing and self-defocusing media, respectively. It is demonstrated that balancing self-defocusing and Gaussian beam convergence results in an efficient, power-scalable spectral broadening mechanism in bulk material
A Squid-Based Beam Current Monitor For FAIR
A Cryogenic Current Comparator (CCC) wasdeveloped for the upcoming FAIR-Project, providing anon-destructive online monitoring of the beam current inthe nA-range. The CCC was optimized for a lowestpossible noise-limited current resolution together with ahigh system bandwidth. Therefore, the low temperatureproperties of ferromagnetic core materials used in thepick-up coil were investigated and differentSuperconducting Quantum Interference Device (SQUID)-systems were tested.In this contribution we present results of the completedCryogenic Current Comparator for FAIR working in alaboratory environment, regarding the improvements inresolution and bandwidth due to the use of suitableferromagnetic core materials and optimized SQUIDsystemcomponents
Imagine e Shapari - Software gráficos no Ensino e Aprendizagem de Matemática
O uso de software para abordar alguns conceitos de Matemática, segundo anossa experiência, tem se mostrado eficiente. Isso porque é uma maneira atraente detratar a Matemática, tão temida na escola e muito pouco apreciada pelos alunos. Nesseartigo, vamos apresentar algumas maneiras de trabalhar com dois softwares: Imagine eShapari. Esses dois são softwares gráficos, fáceis de usar, mas que possibilitam aabordagem de alguns conceitos matemáticos importantes, tais como: formasgeométricas, medidas, matrizes e transformações geométricas. Podemos criar atividadesnesses softwares que potencializem a utilização desses conceitos, de maneira lúdica einteressante, sem necessitar a memorização de fórmulas e regras. Outro ponto quevamos abordar é a forma diferenciada de avaliação que o uso de informática possibilita
Imagine e Shapari - Como esses softwares podem ser utilizados para trabalhar com conceitos matemáticos?
Mostraremos algumas maneiras de abordar conceitos matemáticos utilizando doissoftwares gráficos: Imagine e Shapari. O Imagine é uma nova versão do Software Logo,que utiliza a linguagem de programação Logo. Neste programa é possível programarobjetos a locomover-se em um plano. O Shapari é um software que faz transformaçõesgeométricas em figuras planas, tanto através de pares ordenados quanto com matrizes2x2.
Size dependent exciton dynamics in one-dimensional perylene bisimide aggregates
The size dependent exciton dynamics of one-dimensional aggregates of
substituted perylene bisimides are studied by ultrafast transient absorption
spectroscopy and kinetic Monte-Carlo simulations in dependence on the
temperature and the excitation density. For low temperatures the aggregates can
be treated as infinite chains and the dynamics is dominated by diffusion driven
exciton-exciton annihilation. With increasing temperature the aggregates
decompose into small fragments consisting of very few monomers. This scenario
is also supported by the time dependent anisotropy deduced from polarization
dependent experiments
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