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.