Nonlinear Optical Spectroscopy of Single Walled Carbon Nanotubes.

Abstract

Single Walled Carbon Nanotubes (SWCNTs) have attracted immense attention for making molecular-scale transistors to replace silicon transistors in the post-CMOS era. Moreover, their direct bandgap nature, along with the quantized energy level structure due to reduced dimensionality makes them useful elements in chip-based photonic devices for sensing and communications. Despite the incredible amount of progress in the last few years, many aspects of the physical processes that are involved in the light-matter interaction for this system are not completely understood. This thesis reports results from optical experiments devoted to measuring small changes in the resonant energies of 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. These spectral shifts are detected by measuring the time-resolved nonlinear optical response of the SWCNTs. Experiments are conducted with two synchronized lasers whose colors can be independently tuned. The obtained probe energy dependent delay scans can be globally fitted to three timescales of 8.5, 19.4 and 60.1 ps, which also allows the extraction of the decay associated spectra (DAS). A model is proposed that incorporates excitation induced spectral shifts in the interaction of resonant light with SWCNTs and predicts the derivative-like spectral lineshapes observed in the DAS. The implication of this result for SWCNT based photonic devices is explored, and further experiments to test the hypothesis in this thesis are discussed.