Energy relaxation in disordered insulators irradiated by ultrafast protons: a multiscale approach

Abstract

In this thesis, we probe energy relaxation of the electron-hole plasma in disordered insulators irradiated with a burst of ultrafast protons using a multiscale approach. We begin with a phenomenological extension of the two-temperature model, which characterises the relaxation using two time-scales; a rapid electron-phonon relaxation rate and a longer "chemical" relaxation rate linked to the decay of self-trapped excitons. This extended TTM correctly describes the unexpectedly long transient opacity observed in borosilicate glasses under proton irradiation. We also discuss the application of the model to other materials by performing sensitivity analysis tests. In the phenomenological model, we assume that the electron-hole plasma becomes homogeneous within the first few picoseconds. To validate this assumption, we present a finite element implementation of a semiconductor hydrodynamic model that simulates carrier transport through the material. Simulations of both a single track in BK7 glass and multiple tracks show that after 5ps the plasma remains highly inhomogeneous, with large gradients remaining in both the carrier density and carrier temperature. This indicates that plasma inhomogeneity must be taken into account for models of relaxation on the picosecond scale. Finally, we apply the semiconductor to nanostructured silica aerogels, using a hierarchical model for the aerogel structure. Experiments reveal that silica aerogels irradiated with protons display an unexpectedly long transient opacity that scales with the mass density of the aerogel. Simulation of the electron-hole plasma over 5ps indicates that the presence of the nanostructure serves to impede the diffusion of the electron-hole plasma, resulting in a longer timescale for relaxation.