DYNAMICS OF EXCITONS IN CUBR NANOCRYSTALS - SPECTRAL-HOLE BURNING ANDTRANSIENT 4-WAVE-MIXING MEASUREMENTS

CuBr nanocrystals (NC's) embedded in a borosilicate glass matrix have been studied by transmission pump-and-probe experiments with both spec tral and temporal resolution. Transient as well as persistent spectral hole-burning phenomena are observed in the excitonic absorption of sm all NC's (mean radius less than 4 nm) when picosecond and nanosecond d ye-laser pulses excite selectively NC's of a specific size at low temp eratures. The holes consist of a zero-phonon line (at the photon energ y of the excitation) and marked phonon sidebands. This structure is ex plained by a strong exciton-optical-phonon interaction. A value of abo ut 1 of the Huang-Rhys factor S is obtained from the Stokes shifts bet ween absorption and photoluminescence spectra. From the width of zero- phonon holes, a lower limit of 2.3 ps of the exciton dephasing time T- 2 is deduced. This value is compared to the one obtained by transient four-wave-mixing measurement (6.4 ps) at low excitation intensities. W hen the radii of NC's decrease, both types of spectral holes become br oader and the persistent spectral-hole burning is more efficient.