Excimer-based high-brightness laser systems are the only sources to amplify radiation in the deep ultraviolet (UV) part of the spectrum. Such systems apply excimer modules as power amplifiers for frequency converted pulses of mainly IR or visible short-pulse lasers. Moreover, UV sources offer important advantages over IR laser systems. One of them is the excellent focusability of the laser beam, inversely scaling with the wavelength. Spatial concentration of the energy is of crucial importance for the generation of extremely high focused intensities. Another fundamental advantage is a much better contrast (signal-to-noise ratio) of the pulses. In high field physics experiments, where the light-matter interaction is studied at relativistic intensities, it is of fundamental importance, that clean optical pulses are available. Thinking in terms of a focused intensity of 10¹⁹-10²⁰ W/cm² on target, a background radiation (noise) of 10^-4-10^-5 times the peak intensity would already generate a hot plasma, thus preventing the possibility of making such experiments under controlled conditions. The above mentioned intensity contrast is typical of highly optimized high-power IR solid state laser systems. On the contrary, excimer amplifiers use frequency converted seed pulses which are cleaned up through the nonlinear upconversion process (2^nd or 3^rd harmonic generation). The only remaining source of noise is the amplified spontaneous emission (ASE) of the amplifier, having a much larger divergence than the amplified short pulse, thus resulting in a practically negligible contribution in the far field (intensity contrast ≈ 10^-10).

       To exploit the full potential offered by UV femtosecond lasers, extension of the existing table-top discharge pumped technology is desirable. Using the earlier developed off-axis amplification scheme the parameters of the short pulse amplification can be kept close to the optimum. Because of the limited energy storage time of excimers, it is only the momentarily stored energy which can be extracted by a short pulse. Since the stored energy is proportional to the active volume, development of excimer amplifiers of large excited volume is one main research project. Since the homogeneity of many amplifiers is far from ideal a novel method for the homogenization of intensity distribution laser beams has been developed. Unlike conventional methods, the new scheme is capable of filtering beams of limited pointing stability. With this scheme frequency conversion or temporal filtering of the pulse can be realized together with significant improvement of the beam quality. Besides the good focusability temporal properties of the laser pulse are equally important. The spectral and temporal properties of a pulse are connected by Fourier transformation. If the spectral distribution of the laser pulse is filtered the temporal profile of the laser pulse can be smoothened. A frequency doubling method has been developed, which introduces smooth Gaussian-like temporal profiles by spectral filtering of the input beam.

      In the past, only very few attempts were made to amplify short (subpicosecond) optical pulses in e-beam pumped excimer modules. The relatively moderate success of these approaches were due to a missing intermediate stage between the applied front end system and the final power amplifier, and the lack of an appropriate optical scheme allowing efficient energy extraction out of the amplifier modules.

      The worldwide first compact high-brightness UV laser system was developed and built in the frame of cooperation between DEP and Laser Laboratorium Göttingen (LLG). In contrast to other common high-power lasers operating in the infrared, the system developed by the DEP and the LLG produces deep ultraviolet femtosecond laser pulses by amplification of frequency doubled short pulses.

      In short pulse UV lasers the key components of the system are excimer modules used as amplifiers in dual-wavelength schemes, where high quality short pulses are generated in the long-wavelength region and then shifted into the ultraviolet through frequency doubling or tripling. For physical and technical reasons, among excimers, the KrF mixture suits best for short pulse amplification.

      In a row of experimental studies, the operation of the entire system was carefully characterized and optimized.

      We have studied the spatial evolution of the chirp and the pulse duration of an originally positively chirped pulse when it passes through a dispersive element and a subsequent image system. We demonstrated, that spatial evolution of the pulse duration allows us to simulate the conventional chirped-pulse amplification technique in a much simpler way, only by inserting an amplifier into the beam where the pulse duration is stretched. In this way the amplifier and the target can be separated only by a lens or a mirror without the need of separate pulse stretchers and compressors. This arrangement is ideally suited for travelling wave excitation (TWE) of targets.

      We also studied intensity-dependent loss mechanisms occurring in window materials at 248 nm. It was found that the loss is mainly due to light scattering and absorption. Absorption in fused silica is primarily caused by two-photon absorption, while in CaF₂, LiF, and MgF₂ the combined effect of color-center formation and three-photon absorption must be considered. We have demonstrated that taking proper care of the directional properties of the beam, the high-power table-top laser system can generate focused intensities of as high as 10¹⁹W/cm². In excimer amplifiers – due to the short storage time of the active medium – successive replenishment of the momentarily stored energy is the only way to have access to the whole stored energy. This can be done by optical multiplexing. We have developed a new multiplexing technique which allows automatic (phase-locked) synchronization of the partial beams, therefore ideally suited for multiplexing of femtosecond pulses.

      Presently two target chambers are available for studies of laser-plasma interactions. These studies are performed in cooperation of the Plasma Physics Department of KFKI Research Institute of Particle and Nuclear Physics. Former experiments were performed at moderate (2x10¹⁵ W/cm²) intensity when the beam was focused by an f/10 optics. Recently 10¹⁸ W/cm² intensity was achieved in a 1 μm diameter focal spot where focusing of the beam was performed by an off-axis paraboloid.