Semiconductor nanocrystals are important systems for the study of three-dimensional quantum confinement. Our observed acoustic modes are confined vibrations which result purely from the effect of size. Our direct time domain observation that the coupling is significant in InAs and that substantial motion of the acoustic wavepacket takes place on the excited state strongly suggests that coupling to such modes can facilitate relaxation routes, resulting in size dependent broadening of the exciton line. The coupling strength is stronger in InAs as compared to CdSe, consistent with the material parameters.
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