Subpicosecond study of carrier trapping dynamics in wide-band-gap crystals

Using a very sensitive time-resolved interferometric technique, we study the laser induced carrier trapping dynamics in wide band-gap crystals with 100 fs temporal resolution. The fast trapping of electrons in the band-gap is associated with the formation of self-trapped excitons (STE's). The STE's formation kinetics does not depend on the pump laser intensity in SiO2, while the trapping rate increases in NaCl with the excitation density. We interpret this result as a direct evidence of exciton trapping in the first case, and an electronic trapping following a hole trapping in the second. This result is explained in terms of electron trajectories calculated with a simple Monte Carlo simulation: the electrons can explore a large volume before being trapped in NaCl, not in SiO2. A temperature influence on the initial trapping process is observed in KBr, not in NaCl and SiO2. Finally, we find no evidence of STE formation in diamond. This result is in agreement with general consideration about the STE's formation in terms of lattice elasticity and deformation potentials.