Exciton dephasing and absorption line shape in semiconductor quantum dots

The homogeneous broadening of exciton absorption spectral lines in semiconductor quantum dots (QDs) in the strong confinement regime is studied theoretically. It is shown that the term linear in nuclear displacements in the difference of the phonon Hamiltonians of the ground and optically excited states does not lead to the zero-phonon line (ZPL) broadening. The ZPL width is contributed by the term quadratic in nuclear displacements associated with short-living optical phonons. This contribution is estimated for CdSe nanocrystals (NCs) and found to be much less than the linewidth observed in recent experiments. We conclude that the experimentally observed linewidth is due to the longitudinal lifetime associated with the exciton relaxation to the dark state. The shape of spectral wings originating from the exciton interaction with long-living acoustic phonons is studied at various temperatures for a CdSe NC embedded in a glass matrix.