Understanding the dynamics of excited carriers in wide band gap materials is a requirement to describe a broad range of physical mechanisms such as scintillator response, radiation induced damage of crystals, or laser-induced breakdown in optical materials and coatings. The difficulty arises from the competition between all the different relaxation channels: electron-phonon collisions, impact ionization, exciton and transient or permanent defects formation. Ultrashort laser pulses are ideal tool to investigate transparent materials since they allow to induce a large excitation density, and provide a temporal resolution high enough to track in real time the carrier relaxation.
Two results concerning material which are extremely important for numerous application, namely silica (SiO2) and sapphire (Al2O3), and using different techniques, will be presented.
First, in Al2O3, we have measured in a broad temporal range – from 30fs to 8 ns - the absorption induced by photo-excited carriers using time revolved absorption spectroscopy. By changing the intensity of the pump pulse, and thus the initial excitation density, we could measure the induced absorption on more than two orders of magnitude and demonstrate that the carrier relaxation dynamics exhibit a complex decay, and strongly depends on the initial density of excited carriers. We have developed a two steps model based on rate equations and taking into account the laser damping, which allows to fully reproduce the decay and the amplitude of the measured absorption. We demonstrate that in sapphire the electrons are mobile and can recombine with any hole. With this experiment and our modelling we can explain for instance the complex decay of luminescence observed when sapphire is irradiated with heavy ions or VUV photons.
In SiO2, an important problem related to optical breakdown is the impact ionization which can lead to avalanche: electron excited by an intense laser can gain high kinetic energy in the conduction band and collide with valence electron (impact ionization) thus multiplying the excited carrier density. By using a sequence of double pump pulse we could control independently the two key parameters: plasma density and temperature. Under appropriate conditions, using time resolved interferometry as a probe, we could directly observe for the first time an electronic avalanche induced by a laser pulse. Again a complete modeling, using multiple rate equation and taking into account the laser propagation,; allow to completely describe the experimental results.