?High harmonic generation is a well established technique to investigate the structure
and the inner dynamics of atoms and molecules. This thesis describes how the generating
field parameters can be manipulated to extend the limits imposed on the technique
by the use of traditional laser sources. In this field, with traditional source we mean
high intensity, linearly polarised laser pulses at 800 nm.
The first parameter to be investigated is the wavelength λ of the generating beam. The
unfavourable scaling of the high harmonic yield with λ seems to suggest that high harmonic
spectroscopy of atoms and molecules should be restricted to the wavelengths that
obviate this problem, and that therefore shorter wavelength should be used. But longer
wavelengths, in the mid infrared, present a great advantage respect to shorter ones. The
maximum harmonic order that we can obtain is proportional to the ionisation potential
of the target and to the wavelength times the intensity of the beam, so a higher number
of harmonic can be produced with a longer wavelength than with short, the intensity
being equal. This becomes incredibly valuable when the specie under investigation is a
molecule with low ionisation potential.
To produce high harmonics, a linearly polarised beam is required. If ellipticity is introduced
in the beam, the harmonic signal quickly fades out, as non-linearly polarisation in
monochromatic beams switches off the mechanism at the basis of high harmonic generation.
This is not true if the polarisation of the beam is changed through the introduction
of an additional laser beam, perpendicularly polarised respect to the fundamental. In
this thesis the additional degree of freedom that this second field implies is investigated
by combining the fundamental with its second harmonic and by controlling the relative
delay of the two with sub-cycle precision. The key result is that the addition of the second
harmonic gives access to the control of the harmonic amplitude and to the time at
which the high harmonics are emitted, by simply controlling the relative phase between
the two pulses.