Browsing by Subject "transient absorption"
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Item Development of a Multiphoton Photoacoustic Microscope(2012-08-15) Shelton, Ryan 1983-Cellular/subcellular imaging of biological tissue is an important tool for understanding disease mechanisms. Many current techniques for subcellular absorption contrast imaging, such as two-photon excited fluorescence (TPEF), require exogenous contrast agents to gain access to many naturally occurring biomolecules. Non-fluorescent biomolecules must have a fluorescent marker (tag) chemically bound in order to be observed by TPEF. Contrast agents and markers, while effective, are not an optimal solution because they can change the local environment in the biological system and require FDA approval for human use. Photoacoustic microscopy (PAM) is an imaging modality with high endogenous absorption contrast and penetration depth due to its ability to detect acoustic waves, which are attenuated much less than light in tissue. However, this technique suffers from poor axial resolution, precluding it from consideration for subcellular imaging. This manuscript describes the author's efforts to improve the axial resolution of traditional PAM by merging it with pump-probe spectroscopy. Pump-probe spectroscopy is a non-linear optical technique that exploits a physical process called transient absorption, providing spatial resolution equivalent to two-photon microscopy and access to molecular-specific traits, such as the ground state recovery time and transient absorption spectrum. These traits provide molecular contrast to the imaging technique, which is highly desirable in a complex, multi-chromophore biological system. In this manuscript, a novel technique called transient absorption ultrasonic microscopy (TAUM) is designed and characterized in detail. A second-generation TAUM system is also described, which improves speed and sensitivity of TAUM by up to 1000-fold. This system is validated by collecting volumes of red blood cells in blood smears and tissue samples. These results constitute the first time single cells have been fully resolved using a photoacoustic microscope. Finally, the TAUM system is modified to measure chromophore ground state recovery times. This technique is validated by measuring the recovery time of Rhodamine 6G, which matches well with published values of the fluorescence lifetime. Recovery times of oxidized and reduced forms of hemoglobin are also measured and shown to statistically differ from one another, suggesting the possibility of subcellular measurements of oxygen saturation in future iterations of TAUM.Item Energy Transfer Dynamics and Dopant Luminescence in Mn-Doped CdS/ZnS Core/Shell Nanocrystals(2012-11-13) Chen, Hsiang-YunMn-doped II-VI semiconductor nanocrystals exhibit bright dopant photoluminescence that has potential usefulness for light emitting devices, temperature sensing, and biological imaging. The bright luminescence comes from the 4T1?6A1 transition of the Mn2+ d electrons after the exciton-dopant energy transfer, which reroutes the exciton relaxation through trapping processes. The driving force of the energy transfer is the strong exchange coupling between the exciton and Mn2+ due to the confinement of exciton in the nanocrystal. The exciton-Mn spatial overlap affecting the exchange coupling strength is an important parameter that varies the energy transfer rate and the quantum yield of Mn luminescence. In this dissertation, this correlation is studied in radial doping location-controlled Mn-doped CdS/ZnS nanocrystals. Energy transfer rate was found decreasing when increasing the doping radius in the nanocrystals at the same core size and shell thickness and when increasing the size of the nanocrystals at a fixed doping radius. In addition to the exciton-Mn energy transfer discussed above, two consecutive exciton-Mn energy transfers can also occur if multiple excitons are generated before the relaxation of Mn (lifetime ~10^-4 - 10^-2 s). The consecutive exciton-Mn energy transfer can further excite the Mn2+ d electrons high in conduction band and results in the quenching of Mn luminescence. The highly excited electrons show higher photocatalytic efficiency than the electrons in undoped nanocrystals. Finally, the effect of local lattice strain on the local vibrational frequency and local thermal expansion was observed via the temperature-dependent Mn luminescence spectral linewidth and peak position in Mn-doped CdS/ZnS nanocrystals. The local lattice strain on the Mn2+ ions is varied using the large core/shell lattice mismatch (~7%) that creates a gradient of lattice strain at various radial locations. When doping the Mn2+ closer to the core/shell interface, the stronger lattice strain softens the vibrational frequency coupled to the 4T1?6A1 transition of Mn2+ (Mn luminescence) by ~50%. In addition, the lattice strain also increases the anharmonicity, resulting in larger local thermal expansion observed from the nearly an order larger thermal shift of the Mn luminescence compared to the Mn-doped ZnS nanocrystals without the core/shell lattice mismatch.