| This thesis is divided into two parts. The first part of this dissertation describes the motivation and background, theory and experimental methods of using linear and nonlinear optical techniques to probe the phase transitions of ferroelectric materials. In the second part, femtosecond time-resolved spectroscopy is applied to monitor the real-time response of the electronic structure and lattice vibrations to external perturbation.; With respect to ferroelectric materials, our research is motivated by a desire to understand the finite size effects of ferroelectricity in the nanometer size range. For this purpose we applied the optical second-harmonic generation (SHG) technique, a method which has already been proved to be a powerful probe of bulk ferroelectricities. Here for the first time, we demonstrate SHG as a tool to study finite size effects, with sensitivity down to an individual nanoscale object. Experimentally, we examined BaTi2O5 nanowires as a model system. Compared to the bulk, a much lower phase transition temperature is observed for single BaTi2O 5 nanowires with diameter around 50 nm. Explanations and future work are proposed.; The system of single-walled carbon nanotubes (SWNTs) has attracted much recent attention both as a model one-dimensional material and for its potential for diverse applications. In this thesis we present results of two fundamental studies of the properties of SWNTs probed by ultrafast laser pulses.; The first investigation is a measurement of the optical Stark effect (OSE) in SWNTs, that is, the shift in the optical transition energy induced by strong off-resonance light fields. This effect is explored experimentally using a femtosecond infrared pulse to induce the OSE and a femtosecond pulse resonant with an excitonic transition to probe the response. We observe a significant and instantaneous blue shift of the transition. The strength of the OSE is attributed to the enhanced Coulomb interactions present in these one-dimensional materials.; The second investigation uses time-resolved anti-Stokes Raman scattering for a direct determination of the lifetime of the zone-center (G mode) optical phonons in semiconducting SWNTs. For (6,5) nanotubes, a lifetime of just over 1 ps is obtained. In addition to its intrinsic interest as a measure of phonon-phonon coupling, this lifetime is important in assessing the role of non-equilibrium phonon populations in high-field transport phenomena. |
