This thesis presents experimental and simulation results of the properties of the reflected and transmitted light for laser interactions with thin foils spanning the relativistic transparency and the sheath or radiation pressure acceleration regime. These diagnostics were used to obtain
a better understanding of the plasma properties and its temporal evolution as it transitioned from an opaque plasma to a relativistically transparent one.
For thin foils 100nm irradiated with intensities \SI{10^21}{\watt \per\centi\meter\squared} at normal incidence, the dominant ion acceleration mechanism transitions from radiation pressure acceleration to relativistic transparency as the electron density of the plasma decreases below the relativistic critical density.
This transition was diagnosed in the reflected and transmitted light both through the spatial pro les and the quantity of emitted energy, and through the pulse length and instantaneous frequency of the transmitted radiation. The limit of opacity was found to be \SI{25}{\nano \meter} for the given experimental conditions. Differences between linear and circular polarisation were quantified and a study on harmonic generation on the front and rear surface was also completed.
A temporal measurement of the coherent transition radiation emitted from targets >\SI{ 25}{\nano\meter} was made, measuring the lifetime of the hot electron bunch at \SI{37}{\femto\second}. These results
were supported by 2D PIC simulations which allowed for further information on the electron heating and density variations to be obtained.
Imaging the target's front surface during a high power laser plasma interaction is often difficult
due to experimental constraints and the high levels of
fluence. However, by capturing the
near field of the reflected light, an image of the front surface can be inferred through the
Fourier transform. In this thesis, the front surface of a target irradiated at 45 degrees with an intensity of \SI{3\times 10^21}{\watt\per\centi\meter\squared} was imaged, showing unexpected spatial variations between the first harmonic reflection and the second harmonic generation.