Control of high harmonic generation by manipulation of field parameters

Abstract

?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.