Nonlinear Optical Spectroscopy of Single Walled Carbon Nanotubes
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This thesis reports results from optical experiments devoted to measuring small changes in the resonant energies of Single Walled Carbon Nanotubes (SWCNTs). These changes are attributed to nanomechanical effects resulting from an interaction between the optically excited excitonic states with the underlying lattice of the SWCNTs. Theoretical calculations in the literature have predicted that strong exciton-phonon coupling in SWCNTs causes a differential change in the SWCNT diameter due to optically excited population. It is inferred that such a structural change in the SWCNT leads to excitation induced shifts in the resonant energy for successive excitations on the SWCNT. This work demonstrates how time-resolved nonlinear optical spectroscopy techniques serve as a powerful tool to detect such spectral shifts. The spectral shifts are detected using two color differential transmission measurements, which allow the extraction of the decay associated spectra (DAS). Further, a model is proposed, that incorporates excitation induced spectral shifts in the interaction of resonant light with SWCNTs, and predicts the spectral lineshapes observed in the DAS. Finally, the implication of this result for SWCNT based photonic devices is explored, and further experiments to test the proposed hypothesis are discussed.