Coherent transient phenomena of confined excitons in CuCl nanocrystals: non-Markovian dephasing and quantum beats

We have investigated coherent transient phenomena of confined excitons in C uCl nanocrystals with mean radii R in the range 2.7-23 nm embedded in glass by means of Femtosecond degenerated four-wave mixing (DFWM). In the transi ent DFWM signal for R < 5 nm, the non-Markovian behavior is observed, and t he analysis based on the stochastic model yields the reservoir correlation time of 4.4-8.5 ps. The size dependence of non-Markovian behavior is explai ned by an increase of exciton-acoustic phonon coupling strength and a chang e in the acoustic phonon density of states due to quantum confinement of ac oustic phonons. In the spectrally resolved DFWM signal, oscillating behavio rs have been observed. In large nanocrystals with R > 6.5 nm, quantum beats between the confined exciton sublevels have been observed, while for R < 6 .5 nm, we have observed quantum beats between the confined exciton and LO-p honon-assisted transitions due to the formation of exciton-LO-phonon comple xes. (C) 1999 Elsevier Science B.V. All rights reserved.