NONLINEAR-OPTICAL PROPERTIES OF SEMICONDUCTOR QUANTUM DOTS AND THEIR CORRELATION WITH THE PRECIPITATION STAGE

We present nanosecond hole-burning experiments for CdSxSe1-x and CdSe quantum dots grown in a glass matrix; we compare the results for different precipitation stages. For the first time to our knowledge a drastic sharpening of the spectral holes with a half-width as low as 10 meV has been found to be typical for the samples prepared in the nucleation and the normal growth stages, whereas structures grown in the coalescent stage show only broadband nonlinear absorption. The spectral width of the holes is highly sensitive to the illumination history, particularly in the case of small-size quantum dots. A coexistence of quantum dots from the coalescent and the noncoalescent growth regimes results in a structured nonlinear absorption spectrum from the superposition of the corresponding broadband and narrow-line nonlinear spectra; thus the actual electronic-level structure can be completely masked. The differences in the linewidth are attributed to different mechanisms of surface construction or reconstruction during the growth process or after strong laser exposure, leading to different surface properties such as localized states or spatial charge distributions, which in turn give rise to different mechanisms of line broadening.