Sub-Picosecond Laser Plasma Experiments on Dielectric Solids: Ultrafast Ionisation and Observation of Megagauss Magnetic Fields (English shop)

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The interaction of an intensive sub-picosecond laser pulse (1015{1020 W=cm2) with matter com-
prises many di erent phenomena. This mainly experimental work concentrates on the ultrafast
ionisation of a dielectric solid and the observation of megagauss magnetic elds in laser produced
plasmas:
Ultrafast Ionisation: The rapidly increasing electron density, caused by the leading edge of an in-
tensive laser pulse, surpasses the critical electron density very quickly, so that an initially
transparent glass plate becomes opaque. In this work a theoretical model of this interac-
tion is developed on the basis of optical eld ionisation (OFI). Computational simulations
show, that the transmissivity reduces by several orders of magnitude in less than 10 fs for
plasma producing laser pulses with 800 nm wavelength and 60 fs pulse-duration. This rapid
decrease in transmissivity is mainly due to the absorption of the incident laser beam and
not due to an increasing re ectivity. To verify these theoretical ndings a pump-probe
experiment was performed with a Titanium-Sapphire laser (800 nm; 1 mJ) at the Institute
for Optics and Quantum-Electronics at the Friedrich-Schiller-Universitat in Jena. In this
experiment laser pulses with a duration of 60 fs and 120 fs have been focused to an intensity
of approximately 1015 W=m2 to produce a plasma. A small part of these pulses was used
as a probe pulse to measure the time{frequency function of the plasma transmissivity. By
comparing the momenta of these time{frequency functions with the respective momenta
calculated from FROG measurements the theoretical predictions have been veri ed, so that
this switch can also be applied for characterising UV-pulses with a very short wavelength
in future experiments.
Megagauss Magnetic Fields: The sub-picosecond laser{plasma interaction can generate strong mag-
netic elds mainly due to radial thermal transport, pondermotorive forces and fast electron
currents. Particle in cell (PIC) simulations were performed, showing that the strongest
elds are azimuthal elds growing exponentially with increasing laser-intensity in the high
density region of the plasma. In the case of oblique incidence the strongest elds are present
at one side of the plasma only. These elds can be measured by means of the x-wave cut-o ,
the Faraday- and the Cotton-Mouton e ect, i.e. the polarisation dependent propagation of a
probe beam through the magnetised plasma. By using the high harmonics of the fundamen-
tal laser frequency, which are produced inside the plasma, one can overcome the diculties
resulting from the application of an external probe pulse due to its strong refraction at
the density gradient of the plasma. The experiment was performed with the CPA-beam
of VULCAN (1053 nm, 750 fs, 1020 W=cm2) at the Rutherford-Appleton Laboratory, UK.
The magnetic elds were observed with three VIS / UV polarimeters and two VUV / XUV
polarimeters. Based on the observation of x-wave cut-o s the peak magnetic eld was at
least 350(60) MG. Whilst the equations of the Cotton-Mouton e ect for the 5th harmonic
already suggested elds above 400(50) MG, the VUV / XUV polarimetry gave strong in-
dications of elds even beyond 700(100) MG. Up to 1020 W=cm2 no saturation of the eld
strength was found and no magnetic eld could be observed close to normal incidence. All
these results correspond well with the PIC simulations.