SUBPICOSECOND STUDY OF CARRIER TRAPPING DYNAMICS IN WIDE-BAND-GAP CRYSTALS

Using a very sensitive time-resolved interferometric technique, we stu dy the laser induced carrier trapping dynamics in wide band-gap crysta ls 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 las er intensity in SiO2, while the trapping rate increases in NaCl with t he excitation density. We interpret this result as a direct evidence o f exciton trapping in the first case, and an electronic trapping follo wing a hole trapping in the second. This result is explained in terms of electron trajectories calculated with a simple Monte Carlo simulati on: the electrons can explore a large volume before being trapped in N aCl, not in SiO2. A temperature influence on the initial trapping proc ess is observed in KBr, not in NaCl and SiO2. Finally, we find no evid ence of STE formation in diamond. This result is in agreement with gen eral consideration about the STE's formation in terms of lattice elast icity and deformation potentials.