LASER-INDUCED INTRINSIC DEFECTS - SUBPICOSECOND STUDY OF TRAPPING KINETICS

We use a time resolved interferometric method to study the kinetics of carriers trapping and defects formation following electronic excitati on by high intensity ultrashort laser pulses in alkali halides. A syst ematic study of initial excitation density dependence of trapping kine tics, as well as the comparison of pure and doped samples, allows to i nvestigate in detail the mechanism of energy relaxation and carriers l ocalization accompanying the lattice rearrangement, The experimental r esults concern samples of pure and NO2- doped KBr, and of pure NaCl. W e observe faster trapping in KBrNO2- than in pure KBr. In NaCl, the tr apping rate increases with excitation density. These results show that in both materials the hole trapping is the primary process towards ra diation induced defects. Furthermore, in KBr our results give strong e vidence for the existence of an intermediate state with a short lifeti me in the relaxation pathway between the initial electron-hole pairs a nd the final F-H pairs. A simple model describing the modification of the refractive index and rate equations governing free and trapped ele ctron and holes populations are used to quantitatively interpret the e xperimental results. (C) 1998 Elsevier Science B.V. All rights reserve d.