Design und Aufbau eines Hochenergielasersystems

An der Gesellschaft für Schwerionenforschung mbH in Darmstadt (GSI) wird von den Arbeitsbereichen Atomphysik und Plasmaphysik gegenwärtig der Hochintensitätslaser PHELIX (Petawatt-High-Energy-Laser-for-Heavy-Ion-Experiments) aufgebaut. In Verbindung mit dem Schwerionenbeschleuniger eröffnen sich neue Möglichkeiten für die Erforschung von Licht-Materie Wechselwirkungen. Für diese Experimente werden etwa für die Diagnostik (z.B. Interferometrie) solch laserinduzierter Plasmen weitere Strahlen benötigt. Um die experimentellen Möglichkeiten am Strahlplatz Z6 zu verbessern, wurde im Rahmen dieser Diplomarbeit das bestehende Nd-YAG/Nd-Glas NHELIX-Lasersystem erweitert.Die Erweiterung bestand in erster Linie in der Implementierung eines zweiten Laserfrontends. Der von der Firma GEOLA gekaufte Infrarot-Laser erzeugt einen Puls mit 0,5ns Halbwertsbreite und einer Ausgangsenergie von 150mJ. Er besitzt damit eine 10mal höhere Ausgangsleistung als das bisher verwendete Frontend der Firma CONTINUUM. Dessen Puls hat eine Länge von 15ns (FWHM) und eine Energie von ebenfalls 150 mJ bei gleicher Wellenlänge von 1064nm. Das neue NHELIX-Lasersystem liefert nun zwei unabhängige Laserpulse mit einer Wellenlänge von 1064nm und unterschiedlichen Pulslängen, die gleichzeitig betrieben und zeitlich gegeneinander bis auf theoretisch 1 ns genau verschoben werden können.Die Implementierung erforderte ein vollständig erneuertes Design der Strahlführung. Dazu wurden vorbereitende Simulationen der Strahlpropagation mit dem Simulationsprogramm MIRO durchgeführt, das der GSI im Rahmen einer Kooperation mit dem Forschungszentrum ”Laboratoire pour l‘Utilisation des Lasers Intenses“ (LULI) in Paris zu Verfügung gestellt wurde. Nach Abschluss der Planungsphase wurden die benötigten optischen und mechanischen Komponenten bestellt bzw. in Auftrag gegeben und begonnen, das neue System aufzubauen. Das neue Design enthält Elemente, die in dieser Art und Weise vorher noch nicht im System vorhanden waren. In diesem Zusammenhang sind ein Doppelpass sowie ein Dreifachpass mit den vorhandenen Nd-Glas Verstärkerstäben zu nennen und ein Strahlabschnitt, in dem die Strahlen beider Frontends gleichzeitig gemeinsam durch eine Verstärkerkette geführt werden. Es wurde hierzu notwendig, die Polarisation der Strahlen zu kontrollieren.In der folgenden Arbeit stellt ein ausführlicher einleitender Theorieteil alle verwendeten optischen Elemente vor. Daraufhin wird das neue Design beschrieben und auf die Besonderheiten des Systems eingegangen, sowie eine Charakterisierung der Laserstrahlen vorgestellt. Zum Abschluss wurde das Strahlverhalten in der Doppelpassverstärkung untersucht und die Veränderung des Verstärkungskoeffizienten des doppelt durchlaufenen Verstärkers mit dem einfachen Durchgang verglichen

Tracing few-femtosecond photodissociation dynamics on molecular oxygen with a single-color pump-probe scheme in the VUV

Molecular wave-packet dynamics in oxygen are studied in the time domain, using a single-color VUVpump–VUV-probe scheme. 17-fs VUV pulses, centered at 161 nm are generated via high-order harmonicgeneration driven by an intense 800-nm pulse leading to VUV pulse energies that reach 1.1 μJ per pulse. Anall-reflective interferometric pump-probe scheme is used for studying the delay dependence of the molecularoxygen ion signal with simultaneous nonresonant photoionization of krypton as a precise timing-reference.Access to the excited dissociative state lifetime is provided by the resulting delay-dependent O$_{2}^{+}$ signal,ultimately limited by the molecular ionization window. The ability to use a two-photon VUV probe providesthe delay-dependent detection of O$^+$ as an additional observable, extending the dissociation observationwindow

Weak-field few-femtosecond VUV photodissociation dynamics of water isotopologues

We present a joint experimental and theoretical study of the VUV-induced dynamics of $H_2O$ and its deuterated isotopologues in the first excited state $(A^1B_1)$ utilizing a VUV-pump VUV-probe scheme combined with $ab$ initio classical trajectory calculations. 16-fs VUV pulses centered at 161 nm created by fifth-order harmonic generation are employed for single-shot pump-probe measurements. Combined with a precise determination of the VUV pulses' temporal profile, they provide the necessary temporal resolution to elucidate sub-10-fs dissociation dynamics in the 1+1 photon ionization time window. Ionization with a single VUV photon complements established strong-field ionization schemes by disclosing the molecular dynamics under perturbative conditions. Kinetic isotope effects derived from the pump-probe experiment are found to be in agreement with our by ab initio classical trajectory calculations, taking into account photoionization cross sections for the ground and first excited state of the water cation

Femtosecond dynamics of correlated many-body states in C60\mathrm{C_{60}} fullerenes

In this joint theoretical and experimental work we investigate the population and decay dynamics of excited states of the C$_{60}$ molecule by time-resolved two-photon photoemission. We map out how the thermally excited vibrational degrees of freedom lead to a transient redistribution of the photo-excited states. This includes the super-atom molecular orbitals (SAMOs), which are of great interest currently. The measured lifetimes are in line with full-fledged first-principle calculations

Femtosecond dynamics of correlated many-body states in C60\mathrm{C_{60}} fullerenes

Fullerene complexes may play a key role in the design of future molecular electronics and nanostructured devices with potential applications in light harvesting using organic solar cells. Charge and energy flow in these systems is mediated by many-body effects. We studied the structure and dynamics of laser-induced multi-electron excitations in isolated C60 by two-photon photoionization as a function of excitation wavelength using a tunable fs UV laser and developed a corresponding theoretical framework on the basis of ab initio calculations. The measured resonance line width gives direct information on the excited state lifetime. From the spectral deconvolution we derive a lower limit for purely electronic relaxation on the order of ${\tau }_{\mathrm{el}}={10}_{-3}^{+5}$ fs. Energy dissipation towards nuclear degrees of freedom is studied with time-resolved techniques. The evaluation of the nonlinear autocorrelation trace gives a characteristic time constant of ${\tau }_{\mathrm{vib}}=400\pm 100$ fs for the exponential decay. In line with the experiment, the observed transient dynamics is explained theoretically by nonadiabatic (vibronic) couplings involving the correlated electronic, the nuclear degrees of freedom (accounting for the Herzberg–Teller coupling), and their interplay

A high-harmonic generation source for seeding a free-electron laser at 38 nm

Direct seeding with a high-harmonic generation (HHG) source can improve the spectral, temporal, and coherence properties of a free-electron laser (FEL) and shall reduce intensity and arrival-time fluctuations. In the seeding experiment sFLASH at the extreme ultraviolet FEL in Hamburg FLASH, which operates in the self-amplified spontaneous emission mode (SASE), the 21st harmonic of an 800 nm laser is refocused into a dedicated seeding undulator. For seeding, the external light field has to overcome the noise level of SASE; therefore, an efficient coupling between seed pulse and electron bunch is mandatory. Thus, an HHG beam with a proper divergence, width, beam quality, Rayleigh length, pointing stability, single-shot pulse energy, and stability in the 21st harmonic is needed. Here, we present the setup of the HHG source that seeds sFLASH at 38.1 nm, the optimization procedures, and the necessary diagnostics

Control of FEL Radiation by Tailoring the Seed Pulses

Seeded free-electron lasers (FELs) produce intense, ultrashort and fully coherent X-ray pulses. These seeded FEL pulses depend on the initial seed properties. Therefore, controlling the seed laser allows tailoring the FEL radiation for phase-sensitive experiments. In this contribution, we present detailed simulation studies to characterize the FELprocess and to predict the operation performance of seeded pulses. In addition, we show experimental data on the temporal characterization of the seeded FEL pulses performed at the sFLASH experiment in Hamburg

Single-Shot Timing Measurement of Extreme-Ultraviolet Free-Electron Laser Pulses

Arrival time fluctuations of extreme-ultraviolet (EUV) pulses from the free-electron laser in Hamburg (FLASH) are measured single-pulse resolved at the experimental end-station. To this end, they are non-collinearly superimposed in space and time with visible femtosecond laser pulses on a GaAs substrate. The EUV irradiation induces changes of the reflectivity for the visible pulse. The temporal delay between the two light pulses is directly encoded in the spatial position of the reflectivity change which is captured with a CCD camera. For each single shot, the relative EUV/visible arrival-time can be measured with about 40 fs rms accuracy. The method constitutes a novel route for an improvement of future pump–probe experiments at short-wavelength free-electron lasers (FELs) by a pulse-wise correction with simultaneously measured arrival times of individual EUV pulses