Single Particle Spectroscopy: Ultrafast Studies and Absorption Technique Development

In this thesis, I present topics involved in single particle spectroscopy. Specifically, the topics of ultrafast transient absorption on semiconductor nanowires and the development of a polarization modulation technique are discussed. These two topics demonstrate the advantages of single particle spectroscopy as they pertain to the scientific process of experimentally determining the properties of nanostructures. The ultrafast transient absorption portion of this thesis describes experiments performed primarily on Cadmium Telluride (CdTe) nanowires and sparingly on Cadmium Selenide (CdSe) nanowires for comparison. The CdTe nanowires demonstrated a fast decay on the order of a several picoseconds. This decay process is attributed to the trapping of charge carriers at surface defects. The characteristics and time constants observed for this trapping process varied from wire to wire. The time constant for a given wire was also observed to change as different parts of the wire were probed. The variations observed were attributed to changes in the trapping energies and/or densities of surface states. The CdSe nanowire displayed no fast time dynamics in their transient absorption traces. The absence of the fast charge trapping in CdSe is consistent with the higher emission quantum yields observed in CdSe when compared to CdTe. These results demonstrate the information that can be gained from this technique. The second part of this thesis describes the development of a new single particle spectroscopy extension of the technique called polarization modulation microscopy (PMM). This technique utilizes a photo-elastic modulator (PEM) to rapidly modulate the polarization of a focused laser to study single anisotropic particles. A lock-in amplifier records the signal produced from the polarization modulated laser-particle interaction at twice the fundamental frequency of the PEM, which gives the difference between the extinction cross-sections for light parallel and perpendicular to the optically active axis of the nanostructure. Furthermore, PMM demonstrates the ability to determine the nanoparticle orientation on the sample slide. To demonstrate this PMM technique, studies preformed on gold nanorods are presented. The PMM's strengths and weaknesses are also compared to the photothermal heterodyne imagining (PHI) technique and the spatial modulation spectroscopy (SMS) technique.