An ultrafast optical parametric laser for driving high energy density science

This thesis describes the development of a multi-mJ, few-cycle, absolute-phase controlled laser system based on optical parametric chirped pulse amplification (OPCPA) operating at a kHz repetition rate. A laser system with these specifications will provide a table-top platform to enable a broad range of experiments in demanding research areas, including laser electron acceleration and the creation of exotic highenergy density plasmas from solid targets. The approach of the work is a combination of both experimental effort and numerical simulations used to guide and aid interpretation of laboratory studies. The non-collinear parametric gain stages of the laser have been optimised using detailed numerical simulations. A comparison is given on phase matching conditions in BBO and LBO crystals along with a novel nonlinear material BiBO. The production of 600 μJ pulses with a bandwidth that supports a transform limited temporal duration of 8.5 fs is presented in a three stage BBO based design. An all optical, low-jitter synchronisation scheme for the OPCPA pump and signal pulses has been designed and implemented by use of solitonic wavelength shifting in a photonic crystal fiber (PCF). Commercially available fibers with various core sizes have been assessed. The propagation of few-cycle pulses in the PCF has been studied by numerically solving the generalised Schr¨odinger equation with the splitstep Fourier method. An OPA pump laser with excellent spatial and temporal qualities has been developed. Amplification of the PCF output at 1053 nm is achieved in a regenerative diode pumped Nd:YLF amplifier and a multipass power amplifier. Self-phase modulation and gain narrowing is greatly reduced using a customised 500 μm low-finesse etalon in the regenerative amplifier cavity. Spectral modulation was found to increase both frequency doubling and parametric amplification efficiency and stability. The construction of an alternative 10 Hz, high-energy pump beam line is also presented.Open Acces

Time-resolved ionization measurements with intense ultrashort XUV and X-ray free-electron laser pulses

Modern free-electron lasers (FEL) operating in XUV (extreme ultraviolet) or X-ray range allow an access to novel research areas. An example is the ultrafast ionization of a solid by an intense femtosecond FEL pulse in XUV which consequently leads to a change of the complex index of refraction on an ultrashort timescale. The photoionization and subsequent impact ionization resulting in electronic and atomic dynamics are modeled with our hybrid code XTANT(X-ray thermal and non-thermal transitions) and a Monte Carlo code XCASCADE(X-ray-induced electron cascades). The simulations predict the temporal kinetics of FEL-induced electron cascades and thus yield temporally and spatially resolved information on the induced changes of the optical properties. In a series of experiments at FERMI and LCLS, single shot measurements with spatio-temporal encoding of the ionization process have been performed by a correlation of the FEL pump pulse with an optical femtosecond probe pulse. An excellent agreement between the experiment and the simulation has been found. We also show that such kind of experiments forms the basis for pulse duration and arrival time jitter monitoring as currently under development for XUV-FEL