Dynamics of excitons in CuBr nanocrystals: Spectral-hole burning and transient four-wave-mixing measurements

CuBr nanocrystals (NC’s) embedded in a borosilicate glass matrix have been studied by transmission pump-and-probe experiments with both spectral and temporal resolution. Transient as well as persistent spectral hole-burning phenomena are observed in the excitonic absorption of small NC’s (mean radius less than 4 nm) when picosecond and nanosecond dye-laser pulses excite selectively NC’s of a specific size at low temperatures. The holes consist of a zero-phonon line (at the photon energy of the excitation) and marked phonon sidebands. This structure is explained by a strong exciton–optical-phonon interaction. A value of about 1 of the Huang-Rhys factor $S$ is obtained from the Stokes shifts between 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. When the radii of NC’s decrease, both types of spectral holes become broader and the persistent spectral-hole burning is more efficient.