This thesis presents experimental and theoretical results on the interaction of high-intensity
lasers with solid and gaseous targets. All the measurements that are described belong to
the optical region of the spectrum.
The interaction with solid targets has been investigated for two different intensity regimes.
Intensities of up to 10[21] Wcm-2 have been accessed on the VULCAN laser
system at the Rutherford Appleton Laboratory whereas the JETI laser system at the Institut
für Optik und Quantenelektronik in Jena allowed to reach intensities of up to 4x10[19]
Wcm-2 . For both regimes, the transport of relativistic electrons generated in the interactions
has been investigated through measurements of the optical radiation emitted from
the rear surface of the solid targets. Polarimetry and angular distribution measurements
indicate that the radiation presents a high degree of polarisation and is non-isotropically
emitted. It is, therefore, mainly attributed to transition radiation. A theoretical model
has been developed in order to interpret and validate the experimental observations. As a
result, for the high intensity regime variation of the signal strength of the transition radiation
with respect to the direction of observation is attributed to the presence of mm-scale
filaments.
The interaction with gaseous targets has been investigated at the Astra Gemini facility
at the Rutherford Appleton Laboratory, for peak intensities of up to 3x10[19] Wcm-2 in
a spot size of 20 [Mu]m FWHM. In this experiment the properties of the laser pulse were
studied after interaction with the targets. For this purpose, a second harmonic generation
FROG device was used. This allowed to determine both the pulse duration and the
temporal phase of the pulse, giving an insight on the dependence of the pulse properties
with respect to interaction length and electron number density. The experimental results
show that the nonlinear evolution of the pulse can lead to compression from 45 fs before
the interaction to a single pulse of below 20 fs duration, after propagating in the gaseous
medium.