Optical Properties of Single Cadmium Selenide Nanowires: Breaking Through the Limitations of Ensemble Measurements

In this thesis the optical properties of colloidal CdSe nanowires (NWs) are elucidated at the singe wire level. Using single molecule fluorescence microscopy we have established that dielectric contrast controls their optical response to polarized light. Specifically, single CdSe wire excitation polarization anisotropies (̏_exc) were studied as a function of NW radius, excitation wavelength, and dielectric environment. Experiments reveal that the strongest dependence of ̏_exc-values is on a NW's surroundings in agreement with a classical dielectric contrast model. These results, however, only hold true for wires with radii less then the wavelength of light. In the limit of optically thick wires a transition from strong dielectric contrast influences to the onset of bulk-like behavior occurs. Namely, ̏_exc-values decrease concurrently with the emergence of a sizable wavelength dependence when NW radii become comparable to the wavelength of light. As a consequence, pronounced ̏_exc rolloffs are observed at short wavelengths in the visible. We quantitatively explain the wavelength sensitivities by modeling the NW as an absorbing dielectric cylinder under plane wave excitation. A comparison of predicted ̏_exc-values to experimental numbers shows good agreement and confirms the existence of wavelength-dependent ̏_exc-values in optically thick wires. In addition, absorption cross section values (̏®Õ_abs) of single CdSe nanowires have been measured by photothermal heterodyne imaging (PHI). Specifically, PHI signals from isolated gold nanoparticles (NPs) with known absorption cross sections were compared to those of individual CdSe NWs excited at 532 nm. Based on our analysis an average CdSe NW absorption cross section value of ̏®Õ_abs = (3.17 Ìâå± 0.44) x 10^-11 cm²/Ì_å_m was determined. This agrees well with the theoretical value obtained using a classical Poynting vector field analysis (̏®Õ_abs = 5.00 x 10^-11 cm²/Ì_å_m) and also with earlier ensemble estimates. The sizable absorption cross section extracted from our PHI measurements ultimately suggested that direct single NW absorption studies were possible. As a result, we conducted spatial modulation spectroscopy on single CdSe NWs to directly measure their extinction throughout the visible. Our data reveal up to four resolvable transitions, a significant single NW Stokes shift, and transitions that saturate with increasing excitation intensity. The ability to resolve single wire resonances has allowed us to quantitatively study their excited state size progression. We have done so within the context of a six band effective mass model that includes the enhanced exciton binding energies and dielectric confinement associated with one dimensional nanostructures. The observed experimental resonances agree well with the excitonic transitions predicted by the model.