One of the possible methods to generate coherent radiation
in the VUV and EUV spectral range is by high harmonics of a visible laser
beam. Efficient high harmonics generation of odd order have been observed
in different neutral gases.
Harmonics generated in laser-plasma interaction may offer an alternative,
more efficient method. As early as 1980, high harmonics were obtained from
laser plasma interactions with IR laser beams. In that case plasma profile
steepening resulted in a sharp density gradient, nonlinear oscillation of
the electrons crossing the critical surface as driven by the electric field
of the laser was the source of harmonics. The situation drastically
changed in recent years with the ultrashort laser pulses. In plasmas generated
on solid surfaces by ultrashort laser pulses the scalelength of the plasma
may remain less than the laser wavelength, thus no profile steepening is
necessary for favorable initial conditions.
Even and odd order harmonics were generated by a Ti:sapphire laser in
1995, and soon afterwards harmonics up to the 75th order of a Nd-laser were
obtained on the wavelength of 14.0nm. In our laboratory a small, table-top
KrF laser with nonrelativistic intensity generated harmonics to the wavelength
of 82.8 nm in 1996, and with an increased intensity of 3*1017
W/cm 2 up to 62 nm.
Experiments were carried out with a prepulse, thus modifying the
density gradient. Conversion to harmonics were optimized for a plasma
scalelength being one third of the laser wavelength. In the case of
low, 5*1015 W/cm2 intensities it was found that
harmonics propagate in the specular direction both for p- and surprisingly
for s-polarized radiation. This behaviour was explained by a single-electron
model which takes into account the evanescent electromagnetic field in the
skin layer of the overdense plasma. Present experiments aim to clear propagation-
and polarization-properties of harmonics up to the 4th or 5th order
in the vacuum ultraviolet.