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.
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
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.
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.
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
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.
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 ≈
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.
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.
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.
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