Homogeneous linewidth and spectral diffusion in semiconductor nanocrystals

Semiconductor nanocrystals a few nanometers in extent exhibit unique electronic and optical properties. These properties depend on the size and dimensionality of the confinement potential. The optical absorption spectrum of nanocrystals generally features a zero-phonon line (ZPL). Its width indicates the coupling of the electron to the electromagnetic vacuum and to phonons. Electron-phonon interactions are one strong source of exciton decoherence. The ZPL linewidth yields the total decoherence rate.; This dissertation presents experimental studies of the decoherence rate in CdSe/ZnS core/shell quantum dots (QDs), nanorods, and PbS QDs. The spectroscopic work is accomplished by using high-resolution spectral-hole burning (SHB), which eliminates effects of inhomogeneous broadening due to nanocrystal size variations. SHB response-dependence on the measurement timescale is also used to probe spectral diffusion:random spectral shifts in the optical transition frequency due to a fluctuating local environment. These studies provide important information on decoherence processes in nanocrystals.; SHB response obtained from spherical CdSe/ZnS QDs exhibits a sharp ZPL and discrete acoustic phonon sidebands due to phonon-assisted transitions. The suppression of effects of spectral diffusion in the SHB measurement leads to a ZPL homogeneous linewidth of 1.5 GHz; corresponding to a decoherence rate of 0.75 GHz, which is more than one order-of-magnitude smaller than that observed previously, still far exceeding the expected radiative linewidth. The observed acoustic phonon sidebands are in agreement with a theoretical estimate of the confined phonon modes in nanocrystals. No ZPL, however, was observed in the SHB response of PbS QDs, reflecting the strong electron-phonon interaction in these nanocrystals.; The 0-D to 1-D transition is of particular interest, and is examined by comparing decoherence rates in QDs to those in nanorods. SHB response obtained from nanorods reveals a sharp ZPL along with a broad background of acoustic phonon sidebands. A decoherence rate of 4.4 GHz was observed, which is greater than that of spherical nanocrystals. Phonon-assisted exciton migration between localization sites in the one-dimensional confinement potential is proposed as a possible mechanism for the large homogeneous linewidth in the nanorods.