Temporal characterization of harmonic radiation generated by intense laser-plasma interaction

Attosecond physics has become one of the most thriving field of
science over the last decade. Although high-order harmonic
generation from gaseous media is widely used as the source of
attosecond pulses, a demand for more intense coherent extreme
ultraviolet (XUV) and soft x-ray (SXR) radiation sources is
growing. The process of high-order harmonic generation from plasma
surfaces has attracted a strong interest as a promising candidate
to meet this demand. Despite many theoretical predictions of the
possibilities to generate intense attosecond pulses, experimental
verifications are yet to come. The main theme of this thesis is to
characterize the temporal structure of the harmonics generated from
plasma surfaces. To achieve this goal, several preparatory
experiments are made first. The contrast of the laser pulse is one
of the most critical parameters for the harmonic generation process
and its improvement is demonstrated by using a plasma mirror. The
properties of the generated harmonics are studied thoroughly to
find the optimal condition for temporal characterization. These
experiments provide the groundwork for the autocorrelation
measurements of the pulse train. To characterize the temporal
structure of the generated harmonics, the technique of the volume
autocorrelation using two-photon ionization of helium is applied.
The measured autocorrelation traces reveal attosecond structures
within the XUV radiation generated from the plasma surfaces for the
first time. The observation of attosecond structures prove the
potential of this harmonic generation process as a source of
attosecond pulses. The process holds a promise to generate
attosecond pulses with unprecedented intensities, which will open
up a new regime of attosecond physics.