| Femtosecond laser radiation, with its high peak power and time resolution, provides a powerful and versatile tool for optical spectroscopy. This thesis presents several spectroscopic techniques enabled by femtosecond lasers and their applications in characterizing and understanding novel nanostructured materials. There are two major themes in these studies. The first concerns the transport properties of free carriers within material systems exhibiting different degree of spatial confinement, in particular the markedly different behaviors observed in bulk material and nanoparticles. The second theme focuses on single-walled carbon nanotubes (SWNTs), including light emission and carrier dynamics in ensemble samples and Rayleigh spectroscopy of individual SWNTs.; The carrier transport properties are studied via Terahertz time-domain spectroscopy (THz TDS), a method exploiting ultrashort pulsed far-infrared radiation. We have investigated the transport of photo-excited carriers in bulk large bandgap crystals and in semiconductor nanoparticles. In a large bandgap insulator like sapphire (Al2O3), we found that the carriers were highly mobile and that their frequency-dependent complex conductivity could be described by the free electron model. The response of the charge carriers changes dramatically, however, in semiconductor nanoparticles. This behavior is a manifestation of the quantum confinement effect in nano-sized materials.; In the study of SWNTs, an important family of nanostructured materials, we have examined their light emission and carrier dynamics using ultrafast techniques like time-resolved fluorescence and transient absorption. These measurements elucidate the different dynamical pathways for carriers in semiconducting carbon nanotubes---namely, radiative recombination, defect trapping, and Auger recombination---and the rates of these competing channels. To further characterize SWNTs at the single tube level, we have developed a novel microscopy technique based on Rayleigh scattering. The approach employs supercontinuum radiation generated by passing femtosecond laser pulses through microstructured fiber to study suspended SWNTs, and has yielded the electronic structures of individual nanotubes. |
