High Intensity Laser Laboratory (HILL)

      During the last decade ultra-high laser intensities have been used for investigating relativistic laser-plasma interaction, non-perturbative nonlinear optics and high-field atomic physics.

      High intensity (>1018 W/cm2) femtosecond laser systems are based on chirped-pulse amplified 1 μm Nd:glass and 0.8 μm Ti:sapphire lasers. These high intensities scale with large Iλ2 (where I = intensity, λ = wavelength), which is the typical scale for laser-plasma interaction. Laser-plasma interactions become highly non-isotropic, and the generation of fast electrons dominates the interaction of the intense laser radiation with matter. Ponderomotive effects and thus the energy of fast particles depend on Iλ2. At shorter wavelengths laser-matter interaction therefore allows for the generation of large light pressures while the energy of the individual particles in the plasmas remains at a moderate level.

      The short wavelength (0.248 μm) of the KrF laser allows efficient x-ray generation because the UV laser radiation can penetrate into the dense plasma. Intense femtosecond lasers lead to the generation of dense plasmas with particle densities larger than 1024 cm-3 and temperatures above 1 keV. A similar state of matter is found inside of astrophysical objects and therefore laser-matter interaction opens a field of studying the plasma state of astrophysical objects on a laboratory scale, however under controlled and reproducible conditions.

      Moreover, short-wavelength high-power lasers can meet the requirements of the concept of fast ignition, which can be included as hole boring through the coronal plasma, penetration to high density, and the generation of particles with MeV energy level. In the concept of fast ignition, the heating rate of the centre fuel is determined by the hot electrons flux, and by the critical density nc at which the laser energy is absorbed, with nc being proportional to λ-2. Thus the heating flux, as well as the coronal hole boring, is strongly favoured by short wavelength lasers.

      The properties of short wavelength rare-gas-halide excimer based short-pulse laser systems can be compared with that of solid-state lasers. The peak power of solid-state lasers is higher, but the focused intensity is comparable in both systems due to the superior beam homogeneity, better scalability and better focusability of excimer amplifiers. Due to the recent advances in solid-state technology the solid-state based systems have already reached the physical limits, however the potentials of short pulse excimer amplifiers have just been partially exploited.

      In 1994 the High Intensity Research Group was founded at the Department of Experimental Physics, JATE University. A high-intensity short-pulse hybrid dye / excimer laser system capable of generating 1018W/cm2 focused intensities at the UV range was built in our laboratory. This facility − called as High Intensity Laser Laboratory (HILL) − now incorporate two short-pulse excimer systems and operate as a users facility. The aim of the activity at the Department of Experimental Physics (DEP) is to explore some of these advantages and to further improve and upscale the extraction efficiency, the power and intensity of short-pulse excimer laser systems.

I. Laser Research and Development

      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 (2nd or 3rd 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.

II. Laser Plasma Activities

      Laser plasma activities in the High Intensity Laser Laboratory (HILL) is based on the home-made short-pulse excimer laser systems. At present the laser gives 15 mJ energy in a 600 fs laser pulse on the 248 nm (UV) wavelength. Simple lens-focusing of the laser beam results in intensity of 5*10 15 W/cm2. Focusing the diffraction-limited beam with an off-axis parabola mirror to a spot as small as 2mm diameter results a focused intensity of 5*1017 W/cm2 corresponding to electric field strength above 1010 V/cm. A special feature of the excimer-based system is the high contrast. As it is based on direct amplification (it is not a CPA-system) the only source of prepulse is the amplified spontaneous emission (ASE). As it has a long, 15ns pulse duration and it is not focused, its intensity in the focal plane is less than 107 W/cm2, thus the contrast is better than 10 10. Focusing the high intensity laser radiation onto solid surfaces or into gases, hot plasma of several million degrees temperatures is generated within less than 1 picosecond (10 -12 s) duration.

      High intensity plasma experiments are performed in a collaboration between the Department of Experimental Physics of the University of Szeged and the Department of Plasma Physics of the KFKI-Research Institute for Particle and Nuclear Physics

Research topics:

  • Investigation of high-harmonicsin laser-plasma interactions on solid-state targets.
  • Vacuum-ultraviolet (VUV, 10-200 nm) and EUV (1-30 nm) spectroscopy of laser plasmas.
  • Studying isochoric- and fast electron-heating of materials with x-ray spectroscopy int he keV range.
  • Efficient VUV generation in gas jet targets.
  • Laboratory astophysics: Radiative shock-waves in gas jet targets.
  • Investigation of damage under the influence of 283 nm laser radiation for fusion related materials (materials for the first wall of the future of inertial fusion energy reactor).
  • Recent publications