Investigation of high-harmonics in laser-plasma interactions

       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.