Subpicosecond dynamic hole-burning spectroscopy of CdSe nanoclusters

The authors report a study of subpicosecond dynamic hole burning of 35-45A CdSe nanoclusters in solution at 300K. These well characterized samples exhibit a sharp excitonic absorption at 560nm. The evolution of the excitonic bleaching after 300fs excitation below the exciton resonance, (565-580 nm), was recorded with 350fs time resolution u;sing a continuum probe. A sharp hole, (50 cm-1), is observed at short delays (i.e., slight overlap of pump and probe), at the pump wavelength with a tail extending into the blue region of the spectrum. This feature results from coherent coupling terms in nonlinear susceptibility, it's width determined by the photophysical dynamics, (trapping, dephasing, etc.), of the system. The conclusion is support by calculation of the dynamic spectrum using the details of our excitation conditions. At longer delays the full exciton spectrum is bleached, the magnitude of which increases during relaxation of the free exciton into trap states, (1-5ps). This increase can be easily interpreted as resulting from the loss of exciton oscillator strength due to the large dipole fields, (∼104 v/m), created in the trapping process. A comparison of samples made under different synthetic conditions is reported